DISCUSSION OF FUNDAMENTALS. A hydraulic system can be controlled either manually or automatically:

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1 Unit 1 Introduction to Electrical Control of Hydraulic Systems UNIT OBJECTIVE When you have completed this unit, you will be able to identify the components used for electrical control of the Hydraulics Trainer and to safely operate the trainer. DISCUSSION OF FUNDAMENTALS In a hydraulic system, fluid power provides the muscles or power to do work, while a control part provides the brain to command system operation. Control of a hydraulic system may range from the simple starting and stopping of the system to controlling extension and retraction of several cylinders in a completely automated factory. A hydraulic system can be controlled either manually or automatically: C C Manual control: system operation is sequenced and commanded by an operator that decides each action to take. Automatic control: system operation is sequenced and commanded by a controller that decides each action to take. Automatic control can be accomplished by means of: a. electrical signals (electrical control); b. compressed air (pneumatic control); c. mechanical link (mechanical control). Manual control is good for system operations which do not require constant repetition. An earth-moving truck such as those used in construction, farming, and mining is a common example of a machine requiring manual control. Since the operator must constantly change the position to where the shovel digs and the depth at which the shovel digs, automatic control could not be used because the sequence of operations is not repetitive. In systems requiring the repetition of a series of operations, however, it would be inefficient to manually shift the hydraulic valves each time the direction of oil flow needs to be changed. As an example, Figure 1-1 shows manual and automatic (electrical) operation of a hydraulic drilling system. 1-1

2 Introduction to Electrical Control of Hydraulic System Figure 1-1. Manual and automatic (electrical) controls of a hydraulic drilling system. 1-2

3 Introduction to Electrical Control of Hydraulic System In Figure 1-1 (a), the part to drill is positioned by hand on the drilling machine. A directional control valve is then shifted manually to extend the drill cylinder. When the part is drilled, the directional control valve is shifted in the opposite direction to retract the drill cylinder. Then the drilled part is removed and a new part is positioned on the machine. Each step of the drilling sequence must be initiated by the operator, based on a visual observation that the previous step has been completed. In Figure 1-1 (b), the only thing the operator has to do is to start the system by pressing the START pushbutton. This causes the controller to activate the solenoid of directional valve 1 to extend the feed cylinder and push a part under the drill. When the feed cylinder is extended, it activates a photoelectric switch, PE1. PE1 sends a signal to the controller that the part has been pushed into position. This causes the controller to deactivate the solenoid of directional valve 1 to retract the feed cylinder. Once this cylinder is retracted, it activates photoelectric switch PE2. This sends a signal to the controller causing it to energize the solenoid of directional valve 2 to extend the drill cylinder. When the drill cylinder has extended far enough to drill the part, it activates photoelectric switch PE3. This sends a signal to the controller, causing it to deactivate the solenoid of directional valve 2 to retract the drill cylinder. Once this cylinder is retracted, it activates photoelectric switch PE4. This causes the controller to commence a new sequence of operations by activating the solenoid of directional valve 1 to extend the feed cylinder. The sequence of operations will repeat until the system is manually stopped or a malfunction occurs. As you can see, electrical control adds flexibility, enhanced performance, and safety to systems involving a series of interrelated operations. 1-3

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5 Exercise 1-1 Familiarization with the Equipment EXERCISE OBJECTIVE C C To identify the components used for electrical control of the Lab-Volt Hydraulics Trainer; To describe the function of each of the following parts of an electrical control circuit: input element, controller, and actuating mechanism. DISCUSSION Basic principles of electrical control Electrical control is by far the most popular type of automatic control used for industrial hydraulic applications. As Figure 1-2 shows, an electrical control circuit consists of the following parts: 1) Input element(s) 2) Controller 3) Actuating mechanism(s) Figure 1-2. Breakdown of an electrical control circuit. An input element is a device that provides an electrical signal to indicate that a hydraulic actuator (cylinder or motor) has reached a specific position, or that it is time 1-5

6 Familiarization with the Equipment to start the sequence of operations. Examples of input elements are limit switches, pushbutton switches, and relay contacts. The signal issued from an input element is called input signal because it is sent to the input of a controller. A controller is a device that decides what action to take based on the signals sent to it from the input element(s). The controller may be a set of electromechanical relays, a programmable logic controller (PLC), or a computer. The signal issued from the controller is called control signal because it is used to control the motion of a hydraulic actuator through an actuating mechanism. An actuating mechanism is a device that provides oil flow to a hydraulic actuator according to the control signal sent to it from the controller. Examples of actuating mechanisms are hydraulic solenoid-operated valves and electro-hydraulic servo valves. Indicating devices such as pilot lamps and meters do not make part of the control circuit because they have no effect on the control process. Electrical control offers a high flexibility because operation of the hydraulic system can be changed only by modifying the logic of the controller instead of modifying the hydraulic circuitry itself. On high-pressure applications, however, electrical control may become complex and costly because the actuating mechanisms (solenoid-operated hydraulic valves) must be pilot-operated. This course will show you how to control the Lab-Volt Hydraulics Trainer with a type of electrical control called electromechanical relay control. With this type of control, the controller is a series of relay contacts achieving the proper logic to move the actuators in a specific sequence. Procedure summary In this exercise, you will identify the components used for electrical control of the Lab-Volt Hydraulics Trainer. You will then classify these components as input element, controller element, or actuating mechanism. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE Identifying the components used for electrical control 1. Figure 1-3 shows the components used for electrical control of the Hydraulics Trainer. et these components from their storage location and identify each component by writing its part number (P/N) in Figure

7 Familiarization with the Equipment Figure 1-3. Components used for electrical control of the Hydraulics Trainer. 1-7

8 Familiarization with the Equipment 2. Examine the 24-V DC Power Supply. When turned on, this device converts the 120-V AC line voltage into a 24-V DC voltage that is used to power the electrical control circuit. The 24-V DC voltage is supplied between the red and black terminals (also called banana jacks) located on top of the DC Power Supply. Look at the information silkscreened next to the jacks. What color is the jack corresponding to the positive (+) terminal of the DC Power Supply? To the negative (!) terminal? 3. Examine the Dual-Pushbutton Stations. Pushbuttons allow an operator to manually start and stop a sequence of operations. Each pushbutton has an internal pair of conducting parts called contact. Pressing a pushbutton causes its contact to close or open, which sends an electrical signal to the controller. Below each pushbutton is a silkscreened symbol indicating the state (open or closed) of the pushbutton contact when the pushbutton is not pressed, or in the NORMAL (deactivated) state. Based on the silkscreened symbols, what color are the normally open pushbuttons? The normally closed pushbuttons? 4. Draw in Figure 1-4 the symbols for normally open (N.O.) and normally closed (N.C.) pushbutton contacts, as silkscreened on a Pushbutton Station. Figure 1-4. Symbols for pushbutton contacts. 5. Examine the Limit-Switch Assembly. This device consists of two mechanical limit switches that are used to sense the position of a cylinder rod. As the cylinder rod travels across a switch, it pushes against the roller, depressing 1-8

9 Familiarization with the Equipment the lever arm. This activates the switch, which sends an electrical signal to the controller. Activate one of the switches by depressing the roller lever with a finger. Does the switch make a clicking noise as it activates? Yes No 6. Look at the symbol silkscreened on a mechanical limit switch. The switch has a pair of N.O. and N.C. contacts that are controlled by a common arm. When the switch is deactivated, the arm contacts the N.C. (red) terminal, forming a N.C. contact. When the switch is activated, the arm switches to the N.O. (black) terminal and closes the N.O. contact which was open in the deactivated (normal) condition. Upon deactivation of the switch, the arm is returned to its initial position by an internal spring. The switch is called a single-pole double-throw (SPDT) switch because a single armature switches back and forth between a N.O. and a N.C. terminal. In Figure 1-5, draw the symbol for the SPDT contacts of a mechanical limit switch, as silkscreened on the switch. Identify the COMMON, N.O., and N.C. terminals on your drawing. Figure 1-5. Symbol for the SPDT contacts of a mechanical limit switch. 7. Examine the Magnetic Proximity Switches. This type of switch is used to sense the position of the piston inside a cylinder. It is designed to clamp onto a cylinder equipped with a special magnetic piston, as is the case of the cylinders supplied with your Hydraulics Trainer. When the magnetic piston moves within proximity of the switch, its magnetic field activates the Magnetic Proximity Switch, which sends an electrical signal to the controller. The + and! terminals on top of the switch are used to power the sensing cell inside the switch. The three other terminals provide access to a pair of N.O. and N.C. contacts. The switch is of SPDT type because a single switch arm switches back and forth between a N.O. and a N.C. terminal. 1-9

10 Familiarization with the Equipment In Figure 1-6, draw the symbol for the SPDT contacts of a Magnetic Proximity Switch, as silkscreened on the switch. Identify the COMMON, N.O., and N.C. terminals on your drawing. Figure 1-6. Symbols for the SPDT contacts of the Magnetic Proximity Switch. 8. Mount a Magnetic Proximity Switch on one of the trainer Cylinders. To do so, loosen the set screw on the proximity switch until the clamp is loose enough to slip over the cylinder tie rod. Position the switch at the cap end or at the rod end of the cylinder. Then tighten the set screw until the clamp is attached firmly to the cylinder tie rod. Could the Magnetic Proximity Switch be positioned to indicate when the piston passes virtually any point in its stroke? Yes No 9. Examine the Diffuse Reflective Photoelectric Switch. This switch is used to sense the position of a cylinder rod. It consists of a light source, a receiver, and a pair of N.O. and N.C. contacts. When powered by a 24-V DC voltage, the light source projects a beam of infrared light. When the cylinder rod enters the beam, light reflects off the rod back to the receiver, causing the switch contacts to turn on. The + and! terminals on top of the switch are used to power the infrared light source. The three other terminals provide access to the switch N.O. and N.C. contacts. The switch is of SPDT type because a single switch arm switches back and forth between the N.O. and N.C. terminals. In Figure 1-7, draw the symbol for the SPDT contacts of the Diffuse Reflective Photoelectric Switch, as silkscreened on the switch. Identify the COMMON, N.O., and N.C. terminals on your drawing. 1-10

11 Familiarization with the Equipment Figure 1-7. Symbols for the SPDT contacts of the Diffuse Reflective Photoelectric Switch. 10. Examine the Pressure Switch. This switch is used to sense the pressure in a hydraulic circuit. It has a hydraulic port which is to be connected into the hydraulic circuit like a pressure gage. When the circuit pressure reaches a preset level, the Pressure Switch is activated, which sends an electrical signal to the controller. Look at the symbol silkscreened on top of the switch. The Pressure Switch is of SPDT type because a single switch arm switches back and forth between a N.O. and a N.C. terminal. In Figure 1-8, draw the symbol for the SPDT contacts of the Pressure Switch. Identify the COMMON, N.O., and N.C. terminals on your drawing. Figure 1-8. Symbols for the SPDT contacts of the Pressure Switch. 11. Examine the Pilot-Lamp Stations. These devices indicate the condition (activated or deactivated) of an associated device. Each lamp is connected to a pair of banana jacks allowing connection of that lamp into a circuit. In Figure 1-9, draw the symbol for a pilot lamp as silkscreened between a pair of jacks. 1-11

12 Familiarization with the Equipment Figure 1-9. Pilot lamp symbol. 12. Examine the Relay and Time-Delay Relay / Counter. A relay is an electromechanical component used in electrical control circuits as a controller element. It sends control signals to an actuating mechanism to control the motion of an actuator, based on the signals sent to it from an input element. One or more relays can be connected in various combinations to achieve proper logic to move several actuators in a specific sequence. 13. Look at the symbols silkscreened on the Relays. Each Relay consists of a coil, CR, controlling three sets of N.O. and N.C. contacts. Coil CR is to be connected to an input element such as a limit switch. The N.O. and N.C. contacts can be connected to actuating mechanism(s) such as valve solenoids, or to other relay coils to perform various logic functions. When the input element applies a 24-V DC voltage across coil CR, the coil energizes and shifts its associated contacts to their opposite state. N.O. contacts close and N.C. contacts open. When the 24-V DC voltage is removed from coil CR, the coil de-energizes and its associated contacts are returned to their normal state by a spring. Based on the silkscreened symbols, what type of relay contact (N.O./N.C.) is connected between the following pairs of terminals: 1-2, 4-5, and 7-8? Between the following pairs: 2-3, 5-6, and 8-9? 14. Examine the symbols silkscreened on the Time-Delay Relay / Counter. This device can be programmed for either timing or counting function by configuring the thumbwheel switches on top of the unit accordingly. When the preset time or count set on the thumbwheel switches is reached, the Time-Delay Relay / Counter shifts its two sets of contacts to the activated state. 1-12

13 Familiarization with the Equipment Based on the silkscreened information, are the symbols for N.C. and N.O. contacts of the Time-Delay Relay / Counter similar to those for N.O. and N.C contacts of the Relays? Yes No 15. Examine the single-solenoid operated, 4-way, 2-position, spring-return Directional Valve. This valve is an actuating mechanism providing fluid flow to a hydraulic actuator such as a cylinder or motor. It is operated by an electrical solenoid. This solenoid must be connected to a controller output usually a relay contact. When the solenoid is energized by the controller, it pushes the valve spool to the straight-arrows condition, causing the actuator to move in one direction. When the solenoid is de-energized, an internal spring returns the valve spool to the normal, crossed-arrows condition, causing the actuator to move in the other direction. In Figure 1-10, draw the symbol for the single-solenoid operated, 4-way, 2-position, spring-return Directional Valve, as engraved on the manufacturer name plate located on top of the valve. Figure Symbol for the single-solenoid operated, 4-way, 2-position, spring-return Directional Valve. 16. Examine the double-solenoid operated, 4-way, 3 position, spring-centered, tandem-center Directional Valve. This valve is operated by two separate solenoids that shift the spool to the straight-arrows and crossed-arrows conditions. Each solenoid must be connected to a controller output usually a relay contact. When neither solenoid is energized by the controller, the spool is kept in the center position by centering springs. The valve has a tandem center condition, which means that the pressure (P) and return (T) ports are connected when the valve is centered. 1-13

14 Familiarization with the Equipment In Figure 1-11, draw the symbol for the double-solenoid operated, 4-way, 3-position, spring-centered, tandem-center Directional Valve, as engraved on the manufacturer name plate located on top of the valve. Figure Symbol for a double-solenoid operated, 4-way, 3-position, spring-centered, tandemcenter Directional Valve. 17. Examine the electrical leads. These wires are used to carry electrical signals from one component of the electrical control circuit to another. They can be connected to any of the banana jacks on the electrical components of the Hydraulics Trainer. The leads can be stack connected. Practice connecting and disconnecting the leads and stack connect the lead ends as shown in Figure Then, disconnect and store the electrical leads. Figure Connecting and stacking leads. 1-14

15 Familiarization with the Equipment 18. Based on what you have learned in this exercise, classify the components used for electrical control of the Hydraulics Trainer as input element, controller element, or actuating mechanism by checking the appropriate box in Table 1-1. COMPONENT INPUT ELEMENT CONTROL ELEMENT ACTUATIN MECHANISM Double-solenoid operated, 4-way, 3-position, tandem-center Directional Valve Time-Delay Relay / Counter Limit Switch Magnetic Proximity Switch Relays Pressure Switch Single-solenoid operated, 4-way, 2-position, Directional Valve Diffuse Reflective Photoelectric Switch Pushbutton Table 1-1. Classifying the components used for electrical control of the Hydraulics Trainer. CONCLUSION In this exercise, you were introduced to the components used for electrical control of the Hydraulics Trainer. You classified these components as input element, controller element, or actuating mechanism. REVIEW QUESTIONS 1. Name the three parts of an electrical control circuit. 1-15

16 Familiarization with the Equipment 2. What is the function of an input element? 3. Name the two types of relays used as controller elements in the Hydraulics Trainer. 4. Name the part of an electrical control circuit that provides oil flow to the actuator(s) according to the control signals received from the controller. 5. What is meant by contact when speaking of an input switch or a control relay? 6. What device provides the 24-V DC voltage required to power the electrical components of the Hydraulics Trainer? 1-16

17 Unit Test 1. Which one of the following is not a part of an electrical control circuit? a. Input element; b. Controller; c. Meter; d. Actuating mechanism. 2. An input element is a device that a. decides what action to take based on the signals sent to it from the actuating mechanisms. b. decides what action to take based on the signals sent to it from the controller. c. provides an electrical signal to indicate that a hydraulic actuator has reached a specific position or that it is time to start an operation. d. provides oil flow to a hydraulic actuator according to the control signals sent to it from the controller. 3. Which one of the following is not an input element? a. Relay contact; b. Pushbutton; c. Pilot lamp; d. Mechanical limit switch. 4. What type of controller element is used for electrical control of the Lab-Volt Hydraulics Trainer? a. Set of electromechanical relays; b. Programmable logic controller (PLC); c. Computer; d. Multimeter. 5. Which part of an electrical control circuit provides oil flow to a hydraulic actuator according to the control signals sent to it from the controller? a. Input element; b. Actuating mechanism; c. Electromechanical relay; d. Metering device. 6. Which one of the following is an actuating mechanism? a. Single-solenoid operated directional valve; b. Double-solenoid operated directional valve; c. Flow control valve; d. Both a. and b. 1-17

18 Unit Test (cont'd) 7. Switch and relay contacts are symbolized a. as normally open. b. as normally closed. c. in their normal (deactivated) position. d. in their activated position. 8. A normally open (N.O.) switch contact is a contact that a. opens when the switch is activated. b. closes when the switch is activated. c. closes when the switch is deactivated. d. Both a. and c. 9. A normally closed (N.C.) relay contact is a contact that a. opens when the relay coil is activated. b. closes when the relay coil is deactivated. c. opens when the relay coil is deactivated. d. Both a. and b. 10. Which one of the following converts the 120-V AC line voltage into a 24-V DC voltage that is used to power the electrical components of the Lab-Volt Hydraulics Trainer? a. Limit-Switch Assembly; b. Relay; c. Electrical lead; d. 24-V DC Power Supply. 1-18

19 Unit 2 Electrical Control Principles UNIT OBJECTIVE When you have completed this unit, you will be able to measure the voltage, resistance, and current in an electrical circuit. You will also be able to read and draw simple ladder diagrams. A simple electrically controlled hydraulic system will be assembled and operated. DISCUSSION OF FUNDAMENTALS In order to understand how electrical control circuits work, it is important that you first become familiar with the three parameters associated with basic electricity: voltage, resistance, and current. Exercise 2-1 defines these parameters and describes the relationship between them. It also shows how to measure each parameter in an electrical DC circuit. Electrical control circuits are represented on paper by ladder diagrams. Ladder diagrams use a different set of symbols and rules than hydraulic schematics, but their function is the same: to show how components are connected and how the circuit operates. Exercise 2-2 explains how a ladder diagram works and how it relates to the hydraulic circuitry. It also lists the rules for drawing ladder diagrams. Exercise 2-3 introduces students to a basic electrically controlled hydraulic system called one-cycle reciprocation system. One-cycle reciprocation systems extend and retract a cylinder one time after an operator presses a START pushbutton. They are often used on machines where an operator must position the workpiece by hand before actuating the work cylinder. 2-1

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21 Exercise 2-1 Basic Electricity EXERCISE OBJECTIVE To measure voltage, resistance, and current in an electrical control circuit; To test the operation of an electrical control circuit; To be aware of the safety rules to follow when using electrical equipment to control a hydraulic system. DISCUSSION Fundamentals Electricity is a form of energy used for lighting, heating, or providing control and power for machines. It is produced by the flow of tiny particles of matter called electrons through a conducting material. Examples of conducting materials are iron, copper, and aluminium. Electrical components such as wires, lamps, and solenoids are made of conducting material and so allow electrons to pass through them. To produce a flow of electrons, the electrical component must be connected to a source of electromotive force that pushes the electrons through the component. This source may be either a generator or a battery. As an example, Figure 2-1 shows a battery pushing electrons through electrical wires to energize a solenoid. As a result, a magnetic field is created around the solenoid. Figure 2-1. Simple electrical circuit. The electromotive force exerted by a source is called voltage. The magnitude of the voltage is measured in volts (V). The instrument used to measure voltage is called a voltmeter. 2-3

22 Basic Electricity There is always an opposition to the flow of electrons through an electrical component. This opposition to electron flow is called resistance. Resistance is measured in ohms (Ω). The instrument used to measure resistance is called an ohmmeter. The result of electrons flowing through an electrical component is called current. The magnitude of the current is measured in amperes (A). One ampere is equal to the motion of 6.24 x electrons past a cross section in 1 second. The instrument used to measure current is called an ammeter. Ohm s law The magnitude of the current flowing through an electrical component is equal to the voltage drop across the component, in volts, divided by the resistance of the component, in ohms. This is called the Ohm s law. Written as an equation, it becomes: If, for example, the voltage drop across the solenoid in Figure 2-1 is 20 V and the resistance of the solenoid is 10 Ω, then the magnitude of the current flowing through the solenoid is 2 A. The Ohm s law can be reformulated to calculate either voltage drop, resistance, or current when the other two variables are known. Electrical power The capability of an electrical source to move electrons through a circuit is called electrical power. Electrical power is measured in watts (W). The amount of power generated by an electrical source is equal to the voltage supplied by this source multiplied by the current flowing through the circuit. In equation form: Some of the electrical power generated by the source is dissipated as heat by each component in the circuit due to the resistance, or opposition to the current flow, of the components. The rest of the power is consumed by an electrical device called a load to perform a useful work such as producing light (lamp), providing rotary motion (motor), or moving a plunger (solenoid). The amount of power consumed by a load is equal to the voltage drop across this load multiplied by the current flowing through it. It is also equal to the square of the current flowing through the load multiplied by the resistance of the load. In equation form: 2-4

23 Basic Electricity If, for example, the current flowing through the solenoid in Figure 2-1 is 2 A and the resistance of the solenoid is 5 Ω, then the power consumed by the solenoid is 20 W. Types of electric current Current flow through an electrical circuit may be one of two types: direct current or alternating current. Direct current (DC) is the type of current produced by batteries and DC power supplies. This type of current flows in only one direction: from the positive (+) terminal of the battery or power supply towards the negative (!) terminal. The DC Power Supply provided with your Hydraulics Trainer, for example, produces a DC current. Note: In DC circuits, the convention used for current flow says that current flows from the positive (+) terminal of the DC source towards the negative (!) terminal, even though the electrons actually flow from the negative terminal towards the positive terminal. Alternating current (AC) is the type of current supplied to most houses and plants. This type of current changes direction (polarity) many times each second. Examples of devices that produce AC current are alternators and AC generators. Figure 2-2 shows the symbols used to represent DC and AC power sources in electrical diagrams. Figure 2-2. Symbols used to represent DC and AC power sources in electrical diagrams. Closed and open circuits Figure 2-3 shows a simple DC circuit. This circuit includes a 24-V DC power supply, a normally open (N.O.) pushbutton, and a directional valve solenoid. The pushbutton allows an operator to control the flow of current through the circuit. 2-5

24 Basic Electricity Figure 2-3. A pushbutton controls the flow of current through the circuit. When the operator presses the pushbutton, the contact in the pushbutton goes from open to closed, which creates a complete conducting path, starting at the positive (+) terminal of the power supply, through the pushbutton contact, the solenoid, and back to the negative (!) terminal of the power supply, as Figure 2-3 a) shows. This permits the current to flow through the circuit. The circuit is said to be closed. As a result, solenoid SOL-A is energized. When the operator releases the pushbutton, the pushbutton contact goes from closed to open, which breaks the continuity of the conducting path and stops the flow of current, as Figure 2-3 b) shows. The circuit is said to be open. As a result, solenoid SOL-A is de-energized. Measuring voltage drop, resistance, and current As previously mentioned, voltage is measured with a voltmeter, resistance is measured with an ohmmeter, and current is measured with an ammeter. These meters are available as separate units, but they are usually found combined in a single enclosure called multimeter. Figure 2-4 shows how to measure voltage drop, resistance, and current in a DC circuit. Either a multimeter or a separate meter may be used. To measure the voltage drop across a component, connect a voltmeter or multimeter placed in voltmeter mode across the component terminals, as Figure 2-4 a) shows. Then turn on the power supply. To measure the resistance of a component, make sure the power supply is turned off, then disconnect the component from the circuit. This may require you to open one or more circuit connections. Connect an ohmmeter or multimeter placed in ohmmeter mode across the component terminals, as Figure 2-4 b) 2-6

25 Basic Electricity shows. The ohmmeter has its own internal power source (battery) that supplies a current used to test the resistance of the component. To measure the current flowing through a component, make sure the power supply is turned off, then connect an ammeter or multimeter placed in ammeter mode in series with the component, as Figure 2-4 c) shows. Then, turn on the power supply. Note: Series means that all the current will flow through the component and the rest of the circuit when the power supply is turned on. Figure 2-4. Measuring voltage, resistance, and current in a DC circuit. When using meters in DC circuits, it is essential to hook them up to the circuit according to the proper polarity, since they will not read properly if connected backwards and may be damaged. This means that the positive terminal (red probe) of the meter must be connected to the positive side of the component under test, and the negative terminal (black probe) of the meter to the negative side of the component. The positive side of a component is the side that is nearest to the positive terminal of the power supply on the electrical diagram of the circuit. The voltage on the positive side of a component is always higher than the voltage on its negative side. Safety rules Observe the following safety rules when using electrical equipment to control a hydraulic system: a. Always make sure that the electrical power supply is OFF when connecting or disconnecting electrical leads or components. b. Never leave any electrical lead unconnected. This could cause you to receive an electrical shock when you touch the unconnected end of a lead while the electrical power supply is on. This could also cause a short circuit to occur when the unconnected end of a lead touches a metal surface. 2-7

26 Basic Electricity c. Make sure the power switch on the electrical power supply is set to the OFF position before connecting the power supply line cord. d. When connecting an electrical circuit, make sure the contact terminals are free of dirt, oil, and water. Dirt and oil are insulators and do not allow a good connection to be made. Water is a conductor and might make a connection where it is not wanted. It is also a good idea to review the safety rules regarding manual control of a hydraulic system, as these rules still apply. These rules are located in the DISCUSSION section of Exercise 1-1 in the Hydraulics, Fundamentals manual. Procedure summary In the first part of the exercise, you will measure voltage, resistance, and current in an electrical control circuit. You will use the measured values to calculate the power consumed by a solenoid. In the second part of the exercise, you will test the operation of the electrical control circuit connected in the first part of the exercise. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE Measuring voltage, current, and resistance in an electrical control circuit 1. Connect the electrical control circuit shown in Figure Make sure the power switch on the 24-V DC Power Supply is set to the OFF position. Plug the line cord of the 24-V DC Power Supply into AC outlet. 3. Turn on the 24-V DC Power Supply by setting its power switch to the I position. The red indicator lamp inside the power switch should light to indicate that a 24-V DC voltage is now supplied between the red (+) and black (!) banana jacks of the DC Power Supply. 2-8

27 Basic Electricity Figure 2-5. Electrical control circuit to connect. 4. et the multimeter from its storage location. Connect the black probe of the multimeter to the common terminal of the multimeter, and the red probe to the multi-purpose terminal. 5. Measure the voltage supplied by the DC Power Supply. To do so, set the multimeter selector to read DC volts. Then, connect the red probe of the multimeter to the + terminal of the DC Power Supply and the black probe to the! terminal, as Figure 2-6 shows. Record below the voltage reading in volts on your multimeter. Note: If the multimeter displays a negative (!) voltage, it is wired incorrectly. Check polarity and switch the multimeter probes. Supply voltage = V 2-9

28 Basic Electricity Figure 2-6. Measuring the supply voltage. 6. Measure the voltage drop across solenoid SOL-A of the Directional Valve by performing the following steps: Connect the red probe of the multimeter to the positive (+) side of the solenoid, and the black probe to the negative (!) side. The + side of the solenoid is the side that is nearest to the + side of the DC Power Supply on the electrical diagram of the circuit (see Figure 2-5). Press pushbutton PB1 on the Pushbutton Station to allow the current to flow through the circuit and observe the voltage reading in volts on the multimeter. Record this voltage in Table 2-1 under VOLTAE DROP. VOLTAE DROP (V) RESISTANCE (Ω) CURRENT (A) Table 2-1. Solenoid data. 7. Disconnect the multimeter probes from the solenoid of the Directional Valve. 8. Measure the resistance of the solenoid of the Directional Valve by performing the following steps: Turn off the 24-V DC Power Supply by setting its power switch to the O position. Disconnect the solenoid of the Directional Valve from the electrical circuit. To do so, remove the lead connecting the + side of the solenoid 2-10

29 Basic Electricity to the! side of pushbutton PB1, and the lead connecting the! side of the solenoid to the! terminal of the 24-V DC Power Supply. Set the multimeter to read ohms. Then, connect the red probe of the multimeter to a terminal of the solenoid of the Directional Valve and the black probe to the other terminal. Observe the resistance reading in ohms on the multimeter. Record this resistance in Table 2-1 under RESISTANCE. Note: Never connect an ohmmeter or multimeter in ohmmeter mode into a circuit while the DC Power Supply is on. To do so could permanently damage the meter. 9. Disconnect the multimeter probes from the solenoid of the Directional Valve. 10. Measure the current flowing through the solenoid of the Directional Valve by performing the following steps: Reconnect the! side of the solenoid of the Directional Valve to the! terminal of the 24-V DC Power Supply, using an electrical lead. Set the multimeter to read a DC current. Then, connect the red probe of the multimeter to the! side of pushbutton PB1, and the black probe to the + side of the solenoid of the Directional Valve. Turn on the 24-V DC Power Supply. Press pushbutton PB1 to allow the current to flow through the solenoid of the Directional Valve and observe the current reading in milliamperes (ma) on the multimeter. Record this current in Table 2-1 under CURRENT. 11. Release pushbutton PB1. According to the current reading on the multimeter, does the current flow through the solenoid when the pushbutton is in its normal (released) condition? Why? 12. Turn off the 24-V DC Power Supply and the multimeter. Reconnect the! side of pushbutton PB1 to the + side of the solenoid of the Directional Valve using an electrical lead. 13. Based on the voltage drop and resistance recorded in Table 2-1, calculate the current flowing through the solenoid of the Directional Valve using the Ohm s law. 2-11

30 Basic Electricity 14. Compare the current value calculated in step 13 with the current value recorded in Table 2-1. Can the Ohm s law be used to determine the current through a component when the voltage drop and resistance of this component are known? Yes No 15. How would the current flow through the solenoid change if the voltage drop across the solenoid were doubled and the solenoid resistance held the same? 16. Calculate the amount of power consumed by the solenoid of the Directional Valve, based on the voltage drop and current recorded in Table Calculate the amount of power consumed by the solenoid of the Directional Valve, based on the current and resistance recorded in Table Compare the power value calculated in step 17 with the power value calculated in step 16. Can the consumed power be calculated using two different methods? Yes No Testing the operation of an electrical control circuit 19. Connect the electrically controlled hydraulic system shown in Figure 2-7. This will allow you to test the operation of the electrical control circuit used in the first part of the exercise. Note: To identify the ports on the solenoid-operated Directional Valve, refer to the letters silkscreened on the valve subplate next to the ports. 2-12

31 Basic Electricity Figure 2-7. Electrically controlled hydraulic system. 20. Turn on the 24-V DC Power Supply. 2-13

32 Basic Electricity 21. Before starting the Power Unit, perform the following start-up procedure: a. Make sure the hydraulic hoses are firmly connected. b. Check the level of the oil in the Power Unit reservoir. Oil should cover, but not be over, the black line above the temperature/oil level indicator on the Power Unit. Add oil if required. c. Put on safety glasses. d. Make sure the power switch on the Power Unit is set to the OFF position. e. Plug the Power Unit line cord into an AC outlet. f. Open the Relief Valve completely by turning its adjustment knob fully counterclockwise. 22. Turn on the Power Unit by setting its power switch to ON. With the solenoid of the Directional Valve in the deenergized condition, the Directional Valve is in the normal, crossed-arrows condition and the pumped oil is blocked at port B of this valve. As a result, the pumped oil is now being forced through the Relief Valve. Pressure auge A now reads the minimum pressure setting of the Relief Valve. Increase the pressure setting of the Relief Valve to 2000 kpa (290 psi). To do so, turn this valve adjustment knob clockwise until Pressure auge A reads 2000 kpa (290 psi). 23. While observing the reading of Pressure auge A, press pushbutton PB1 to energize the solenoid of the Directional Valve and shift this valve to the straight-arrows position. What happens to the reading of Pressure auge A? Why? 24. Release pushbutton PB1 to de-energize the solenoid of the Directional Valve. What happens to the reading of Pressure auge A? Why? 25. Turn off the Power Unit. Open the Relief Valve completely by turning its adjustment knob fully counterclockwise. 2-14

33 Basic Electricity 26. Turn off the 24-V DC Power Supply. 27. Disconnect all hoses and electrical leads and wipe off any hydraulic oil residue. Return all hoses and leads to their storage rack. 28. Remove all electrical and hydraulic components from the Work Surface and wipe off any hydraulic oil residue. Return all components to their storage location. 29. Clean up any hydraulic oil from the floor and the trainer. Properly dispose of any towels and rags used to clean up oil. CONCLUSION In this exercise, you learned how to measure voltage, resistance and current in an electrical circuit. You measured the voltage drop across a component by connecting a multimeter in voltmeter mode across the component terminals, with the power supply turned on. You measured the resistance of a component by disconnecting it from the circuit and by connecting a multimeter in ohmmeter mode across the component terminals. Finally, you measured the current flow through a component by connecting a multimeter in ammeter mode in series with the component. You learned that when the voltage drop and resistance of a component are known, the Ohm s law can be used to calculate the current through this component. You also learned two methods of calculating the power consumed by a load. REVIEW QUESTIONS 1. What is the difference between direct current (DC) and alternating current (AC)? 2. What are the units of measurement for voltage, resistance, current, and power? 2-15

34 Basic Electricity 3. What is the mathematical relationship between current, voltage drop, and resistance? 4. In a DC circuit, what is meant by the positive (+) side of a component? 5. Describe the method used to measure the current flowing through a component. 6. What are the two formulas for calculating the power consumed by an electrical component? 7. Can an ohmmeter or multimeter in ohmmeter mode be used to measure the resistance of an energized solenoid? Why? 2-16

35 Exercise 2-2 Ladder Diagrams EXERCISE OBJECTIVE To explain how a ladder diagram relates to the hydraulic circuitry; To assemble and operate basic ladder diagrams; To learn the rules for drawing ladder diagrams; To describe the operation of an electromechanical control relay. DISCUSSION The electrical control circuits you have seen until now were represented by pictorialtype schematic diagrams. There are other methods of drawing schematic diagrams, but ladder diagrams is the most popular method and probably the simplest and easiest. Ladder diagrams graphically show which switches must be closed or open to allow the current to flow to an output load. Figure 2-8 shows the general appearance of a ladder diagram. The vertical lines on the left and right sides of the diagram represent the terminals of the power supply. The left line is the hot (+) terminal, and the right line, the ground (!) terminal. The horizontal lines are called rungs. Each rung basically consists of an input element, an output load, and electrical wires joining these two devices. Input elements, as pushbuttons, switches, and relay contacts, are located on the left side of the rung. Output loads, as lamps, valve solenoids, and relay coils, are located on the right side of the rung. 2-17

36 Ladder Diagrams Figure 2-8. Basic ladder diagram. When the input element on a rung is closed, it forms a continuous path, or closed circuit, to the output load, allowing the current to flow from the positive (+) terminal of the power supply to energize the output load. In Figure 2-8, for example, pressing pushbutton PB-1 causes normally open (N.O.) switch contact PB1 in rung 1 to go closed and lamp L1 to turn on. When the pushbutton is released, contact PB1 returns to its normal state, which is open, causing lamp L1 to turn off; pressing pushbutton PB2 causes normally closed (N.C.) switch contact PB2 in rung 2 to go open and lamp L2 to turn off. When pushbutton PB2 is released, contact PB2 returns to its normal state, which is closed, causing lamp L2 to turn on. Series and parallel logic Two or more input elements can be connected on a rung in series or parallel to form AND and OR logic, as Figure 2-9 shows. Rung 1 of the ladder diagram is an example of series (AND) logic. Both switch contacts must close in order for lamp L1 to turn on. Rung 2 of the ladder diagram is an example of parallel (OR) logic. Only one of the switch contacts has to close in order for lamp L2 to turn on. 2-18

37 Ladder Diagrams Figure 2-9. Series (AND) and parallel (OR) logic. Several input elements can be connected in various combinations of series (AND) and parallel (OR) to perform additional logic functions. Rules for drawing ladder diagrams 1. The ladder diagram must show only electrical control devices such as switches, relay coils, and solenoids. Directional valves, cylinders, and other hydraulic devices never appear on a ladder diagram. These devices are drawn on a hydraulic diagram. 2. Output loads such as lamps, relay coils, and valve solenoids must be drawn on the right side of the ladder diagram, with one terminal connected directly to the! terminal of the power supply. Load devices should never be connected directly to the + terminal of the power supply. There may be two or more output loads on the same rung. In that case, the loads must be connected in parallel. Loads must never be connected in series on the same rung. Insofar as possible, output loads used to perform the control logic such as relay coils should be drawn in the upper rungs of the ladder. Output loads used for hydraulic control such as valve solenoids should be drawn in the lower rungs of the ladder. This will make it easier for another person to understand how the ladder works. 3. Input elements such as pushbuttons, switches, and relay contacts must be drawn on the left side of the ladder diagram. They should never be connected directly to the! terminal of the power supply. There must be at least one input element per rung. 2-19

38 Ladder Diagrams 4. All ladder rungs must be numbered. 5. Each device in the ladder diagram must be identified with a representative abbreviation. For example, PB is an abbreviation for pushbutton, and CR is an abbreviation for relay coil. Contacts operated by a relay coil must be identified with the same abbreviation as the coil which operates them. For example, contacts operated by relay coil CR1 are labelled CR1-A, CR1-B, CR1-C, etc. 6. All components must be drawn on horizontal lines (rungs). They must not be connected on the vertical lines, which represent the power supply terminals. Electromechanical control relays Electromechanical control relays are widely used in ladder diagrams to perform complex logic functions. An electromechanical control relay consists of a solenoid coil, a magnetic core, an armature, and one or more sets of normally open (N.O.) and normally closed (N.C.) contacts. When a current flows through the relay coil, the magnetic core and the armature are both magnetized and attract each other, causing the armature to move towards the core. This switches the relay contacts to their activated state. N.O. contacts go closed, while N.C. contacts go open. When the current is removed from the relay coil, the armature is moved back to its original position by a spring, which returns the relay contacts to their normal state. Figure 2-10 shows the Relay supplied with your Hydraulics Trainer. It contains a solenoid coil and three sets of N.O. and N.C. contacts. The Relay is of the triple-pole, double-throw (3PDT) type, because it simultaneously switches three conducting parts back and forth between two positions. 2-20

39 Ladder Diagrams Figure PDT Relay. Procedure summary In the first part of the exercise, you will test a basic ladder diagram. In the second part of the exercise, you will test ladder diagrams using series (AND) and parallel (OR) logic. In the third part of the exercise, you will test a ladder diagram using a control relay. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. 2-21

40 Ladder Diagrams PROCEDURE Basic ladder diagram 1. Make sure the power switch of the 24-V DC Power Supply is set to the OFF position. Connect the circuit shown in Figure Figure Testing the operation of a basic ladder diagram. 2. Turn on the 24-V DC Power Supply by setting its power switch to the I position. 3. Does lamp L1 light? Yes No 4. Press pushbutton PB1. What happens to lamp L1? Why? Explain by referring to the ladder diagram in Figure

41 Ladder Diagrams 5. Does lamp L2 light? Yes No 6. Press pushbutton PB2. What happens to lamp L2? Why? Explain by referring to the ladder diagram in Figure Turn off the 24-V DC Power Supply by setting its switch to the O position. Ladder diagrams using series (AND) and parallel (OR) logic 8. Connect the series (AND) logic circuit shown in Figure Figure Testing a ladder diagram using series (AND) logic. 9. Turn on the 24-V DC Power Supply. 2-23

42 Ladder Diagrams 10. Press pushbutton PB1. Does lamp L1 turn on? Yes No 11. Press pushbutton PB2. Does lamp L1 turn on? Yes No 12. Press both pushbuttons PB1 and PB2. Does lamp L1 turn on? Why? Explain by referring to the ladder diagram in Figure In a ladder rung containing two switch contacts in series, what is the condition required for the output load to energize? 14. Turn off the 24-V DC Power Supply. Connect the parallel (OR) logic circuit shown in Figure Figure Testing a ladder diagram using parallel (OR) logic. 2-24

43 Ladder Diagrams 15. Turn on the 24-V DC Power Supply. Press pushbutton PB1. Does lamp L1 turn on? Yes No 16. Release pushbutton PB1. Press pushbutton PB2. Does lamp L1 turn on? Yes No 17. In a ladder rung containing two switch contacts in parallel, what is the condition required for the output load to energize? 18. Turn off the 24-V DC Power Supply. Ladder diagram using a control relay 19. Connect the circuit shown in Figure Notice that coil CR of the Relay is not polarized, which means that it does not matter which coil terminal is connected to pushbutton PB1 and which coil terminal is connected to the! terminal of the 24-V DC Power Supply. Either way, the coil will still energize and shift the relay contacts to their opposite state. Figure Testing a ladder diagram using a control relay. 2-25