123 Chapter 5 Design of Control Mechanism of Variable Suspension System 5.1: Introduction: Objective of the Mechanism: In this section, Design, control and working of the control mechanism for varying the suspension parameters is discussed. This mechanism is changing the effective damping constant by varying the number of orifice openings on the piston when the sprung mass varies and so the effective stiffness as discussed earlier. To achieve the control actuation, the microcontroller is programmed in such a way that it will decide the number of orifice to be opened for a desired value of damping factor as per sprung mass supported. An exercise is carried out with the developed mechanism on the test rig. Observations are recorded and compared for the ride comfort point of view. 5.2: Working of Control Mechanism: Figure 5.1 shows automatic actuation mechanism of the system. Flow chart explaining the working of the mechanism is illustrated as follows. The variable stiffness suspension system will be fitted in the vehicle along with a load cell, LPC2138 microcontroller, stepper motor and a mechanism for opening and closing orifice fixed hole. The system will use load cell to measure the corresponding value of mass. According to the mass supported as the sprung mass, analog signal will be generated by the load cell. Accordingly, this signal is conditioned with the help of signal conditioner. The analog signal is then given to LPC2138 microcontroller which will convert this analog value into digital value. The advantage of LPC2138 microcontroller over 8051 microcontroller is that, it has inbuilt analog to digital (A/D) converter. The digital value will decide how much degree rotation will be required by the stepper motor shaft. Accordingly stepper motor will be provided with the number of pulses. The motor will be driven with the help of stepper motor drive. A constant supply of 24 V will be provided by means of Switched Mode Power Supply circuit (SMPS). The 24 V supply will be provided to stepper motor drive. Whenever one pulse is sent by the controller, stepper motor will turn by 1.8. On the stepper motor shaft, one hollow
124 shaft has been placed with the help of rough screw. One gear is mounted on this shaft, which is having 50 teeth and having module of 2. Both gears having equal number of teeth and module i.e. 50 and 2 respectively, which will result in total degrees of turn achieved as 7.2. The output shaft which is mounted on the orifice opening and closing mechanism, will eventually turn the shaft by 7.2 for one pulse generated by the controller. Load cell Signal conditioning Analog to digital converter LPC 2138 ARM 7 Microcontroller Stepper motor drive 24 V UART SMPS Stepper Gearing mechanism Orifice opening and closing mechanism Figure 5.1: Flow Chart of Control Actuation Mechanism. To achieve the automatic control actuation, the microcontroller will be programmed in such a way that it will decide the number of pulses provided to stepper motor as per mass supported i.e. a sprung mass. 5.3: Various Components of the Control Mechanism: Various components of the mechanism are required for Automatic Control Mechanism of Variable Suspension System.
125 5.3.1: Actuator for Control Mechanism: Actuator is an energy converter, converting electrical energy into mechanical force. An actuator converts a control signal into a physical movement or action. Most electronic signals from a microprocessor are at a very low power and voltage level. This signal is normally interpreted by an additional element commonly known as a driver or output module. It converts the low-level digital signal to an appropriate level and type to drive the output actuator, for example a stepper motor (Bishop, et al, 2002). Some of researchers used the middle-sized passenger vehicle equipped with four electrorheological (ER) shock absorbers out of which two for front parts and two for rear parts road test is undertaken under various road conditions (Choi, et al, 2008 ) 5.3.2: Stepper Motor: A stepper motor is a device that converts a DC voltage pulse train into a proportional mechanical rotation of its shaft. A stepper motor (or step motor) divides a full rotation into a number of equal steps. In essence, stepper motors are a discrete version of the synchronous motor. The discrete motion of the stepper motor makes it ideally suited for use with a digitally based control system such as a microcontroller (Bishop, et al, 2002). Types of Stepper Motor: There are three basic types of stepper motor, viz. a) Variable Reluctance: This type of stepper motor has a soft iron multi-toothed rotor with a wound stator. The number of teeth on the rotor and stator, together with the winding configuration and excitation determines the step angle. This type of stepper motor provides small to medium sized step angles and is capable of operation at high stepping rates (Hillier et al., 2004). b) Permanent Magnet (PM): The rotor used in the PM type stepper motor consists of a circular permanent magnet mounted onto the shaft. PM stepper motors give a large step angle ranging from 45 to 120. The stator has two pairs
126 of independent windings AA1 and BB1 as shown in Figure 5.2, through which current from the driver circuit may pass in either direction and this will turn the rotor by 90 degree steps. When current passes through BB1 the magnetic laws of attraction and repulsion align the rotor with the active poles of the stator. Complete rotation is obtained by applying to the motor four electrical pulses of suitable polarity as shown in Figure 5.3. Direction of rotation depends on the polarity of the stator during the first pulse. Motors of this type normally have step angles of 7.5 0 120 0 (Hillier et al., 2004). Figure 5.2: Permanent-Magnet Stepper Motor (After Hillier et al., 2004). Figure 5.3: Rotation of Permanent-Magnet Stepper Motor (After Hillier et al., 2004). c) Hybrid Type: The hybrid stepper motor is a combination of Variable Reluctance and Permanent Magnet. Typically the stator has eight salient poles,
127 which are energized by a two-phase winding. The rotor consists of a cylindrical magnet, which is axially magnetized. The step angle depends on the method of construction and is generally in the range 0.9 5. The most popular step angle is 1.8. It also has a relatively high torque and can operate at high stepping rates (Hillier et al., 2004). In case of bipolar motors, windings are better utilized for the application purposes. As well as bipolar hybrid stepper motor gives 50% more torque as compared to unipolar motor. Hence, bipolar hybrid type of stepper motor as shown in Figure 5.4 is selected for the application. Bipolar hybrid stepper motor having output torque of 20 kg-cm (2 Nm) is selected for driving the orifice opening and closing cover piece. Figure 5.4: Stepper Motor
128 Specifications of Selected Stepper Motor: Step angle: 1.8º (The step angle is the angle which the motor shaft rotates with one control pulse from the micro controller.) Rated voltage: 3.2 V (Voltage value at which motor starts rotating due to applied value of voltage.) Rated Current: 2.8 A (Current value at which motor starts rotating due to electromagnetic field.) Holding Torque: 20 kg-cm (The maximum torque that can be applied to an energized stationary motor without causing spindle rotation.) Detent Torque: 0.68 kg-cm (This is the amount of torque that the motor produces when it is not energized. No current is flowing through the windings.) Weight: 0.7 kg. 5.4: Stepper Motor Drive: Figure 5.5: Stepper Motor Drive (ARM7 Microcontroller)
129 As stepper motor is driven with the help of LPC2138 microcontroller its output voltage needs to be precise of 24 V. This voltage pulse signal needs to be applied to the stepper motor with micro-stepping as well. Hence, RMCS-1102 stepper motor drive is selected as shown in Figure 5.5. This stepper motor drive has peak coil current value ranging from 0.5 A to 5 A. Hence, it is possible to drive higher torque stepper motors whose rated current value is up to 5 A. 5.4.1: Specifications of Selected Stepper Motor Drive: Supply voltage: 12 to 50 V (It is the range of the voltage supplied from SMPS to the drive) Phase Current: 0.5 to 5 A (It is the value of current applied at each stepper motor coil. Hence applicable for high torque capacity motor) PUL and DIR voltage: 2.5 to 7 V DC (This is the voltage signal achieved from micro controller.) Table 5.1 and Table 5.2 showed below gives the connections for the terminals of drive. Table 5.1: Power and Motor Terminal Connections Sr. No. Connections Stepper Motor Drive 1 Motor Coil Phase A+ A + 2 Motor Coil Phase A- A - 3 Motor Coil Phase B+ B + 4 Motor Coil Phase B- B - 5 SMPS +V +V 6 SMPS -V GND Table 5.2: Pulse and Direction Input Connections Sr. No. Connections Stepper Motor Drive 1 + Direction from micro controller DIR+ 2 - Direction from micro controller DIR- 3 + Pulse from micro controller PUL+ 4 - Pulse from micro controller PUL-
130 Apart from this, for motor coil current setting of 2.8 A, current has been selected as per rated current of the stepper motor. Switch setting has made accordingly for 2.8 A current and 400 Steps/Rev. 5.5: Switched Mode Power Supply: Switched Mode Power Supply, also called as SMPS is used for converting AC voltage into continuous 24 V output supply. As per current rating selected for stepper motor, SMPS draws current which need to stepped down with the help of stepper motor drive. 5.6: LPC 2138 Microcontroller: LPC 2138 microcontroller as shown in Figure 5.6 is 64 pin, 32 bit ARM7 micro controller with high speed application. The greatest advantage of this micro controller is that, it has inbuilt analog to digital (A/D) converter. Apart from this, it has 32 Kbyte on chip static random access memory (RAM) and 512 Kbyte on chip flash program memory. Hence, it will store the input value of sprung mass as well as programming code easily. Figure 5.6: LPC 2138 ARM7 Microcontroller
131 Device pins are controlled dynamically with the help of general purpose input/output (GPIO) registers. The pins can be triggered with the help of programming (.hex) file. Its measurement range is from 0 to 3.3 V. It has 2 Universal Asynchronous Receiver/Transmitter (UARTs) which enable user to interface with the micro controller easily. The UART registers the input values received from load cell and gives them to user for better calibration in the programming. 5.7: Gear Calculations: There are 10 orifices present on the piston within 160º. Hence, angle between two orifices is given as, Angle between two orifice = 160 10 = 16 Hence, gear design is done in such a way that, for one rotation of gear meshing teeth, 8º rotation will be available i.e. one orifice will be open. Hence, number of teeth on driver and driven gear must be equal. No. of teeth on driver and driven gear = 360 8 = 45 50 Gear material is selected as nylon for weight reduction of the gears. Teeth on pinion and gear are calculated as 50. Module for gear is selected depending upon space considerations as 2. Hence, gear dimensions are given by Equation 6.1, where, T = T = 50 D = D = T m (5.1) D = D = 50 2 = 100 mm T = T m D = D : No. of teeth on pinion and gear : Module of gear : Pitch circle diameter of pinion and gear Face width of gear is selected higher than pinion in order to compensate movement of orifice opening and closing mechanism which is given as, b = 10 mm, b = 20 mm Respectively.
132 Circular pitch diameter of gear is given by Equation 5.2, P = = 6.28 mm (5.2) Outer gear diameter Inner diameter for pinion Inner diameter for gear = P. C. D. + 2 Addendum = 100 + 2 2 = 104 mm = 15.8 mm = 28.2 mm Other Dimensions of Gear as Per Standard: Addendum = 1 m = 2 mm, Deddendum = 1.25 m = 2.5 mm Working depth = 2 m = 4 mm, Tooth thickness = 1.5708 m = 3.1416 mm Minimum clearance = 0.25 m = 0.5 mm Fillet radius at root = 0.4 m =0.8 mm Figure 5.7 (a): Block Diagram of the Assembly of Components
133 Figure 5.7 (a) shows the block diagram of the assembly of components and Figure 5.7 (b) shows the actual connections of the entire automatic mechanism for the variable stiffness suspension system. Figure 5.7(b): Actual Connections for Control Mechanism Figure 5.8: Block Diagram of Stepper Motor Mechanism
134 Block Diagram of the Stepper Motor mechanism for driving orifice opening and closing mechanism is shown in Figure 5.8. The gearing mechanism for driving orifice opening and closing mechanism is shown in Figure 5.9. Figure 5.9: Gearing Mechanism for Control Actuation 5.8: Investigation to Study Control Effects: The following parameters have been investigated to study their control effects. i. Response time ii. Control effect iii Flow condition at the piston holes iv. Equilibrium position of the prototype after each reading taken during the experimentation
135 i. Response Time: The presented system has been proposed to vary the shock absorber configuration in response to the mass supported by the vehicle. For practical implementation of the prototype system, rotary actuator and a weight/mass sensor (i.e. load cell) have been measured. Block diagram for implementation of the system is shown below in Figure 5.10. Figure 5.10: Block Diagram of Testing Variable Stiffness Suspension System Load cell monitors the vehicle sprung mass (i.e. weight) and the controller operates the recommended settings for the adjustable dampers. Actuators and mechanical arrangement has been used to vary the damper setting. Response time of the control circuit includes response time of load cell, actuator and the mechanical arrangement which is close to 200 mili seconds. The proposed system has to monitor only static load on the vehicle and it does not respond to the dynamic load fluctuations, which are encountered during the vehicle running. Response time of 200 ms will not affect the system dynamics, since the system has been designed to monitor only laden mass of the vehicle. ii. Control Effect: Prototype system with a rotary actuator has been discussed earlier. The proposed system can also be operated with a linear actuator and mechanical gears as illustrated in Figure 5.11. A linear actuator with multiple positions may be used along with rack and pinion gear. The linear actuator has been designed to operate the rack teeth, which in turn will rotate the pinion gear and cover piece. Linear actuator (operating with 12 V DC supply) travel positions and gear configuration will rotate the cover piece through desired angle. This will facilitate to uncover required number of holes. Further, the arrangement of actuator may be provided in a protective housing to ensure reliable and robust operation.
136 Figure 5.11: Actuating Mechanism of Piston and Cover Piece with Linear Actuator As has been explained earlier the system will monitor the vehicle mass and not dynamic load variation. Therefore, response time and inertia of the mechanical components will not affect system performance. iii. Flow Conditions at the Piston Holes: The adjustable damper in the prototype has been modeled in MATLAB simscape. Piston holes diameter in the piston are kept lower to ensure laminar flow. Turbulent fluid flow will result in quadratic damping force. However, for the shock absorber damper laminar flow is preferred (Cimbala et al., 2006) Finite Element simulation has been performed to evaluate oil flow in the adjustable damper. Simulations have been performed with ANSYS Fluent Release 16.0; which uses Bernoulli s equations with K-epsilon (k-ε) turbulence model for the analysis. Figure 5.12 (a) shows CFD mesh with piston, cylinder and fluid. During the simulation cylinder was fixed and velocity of 2 m/s was given to the piston. The piston velocity has been kept higher than the normal suspension velocities, which are upto 1.2 m/s. It has been observed from Figure 5.12 (b) that Reynolds Number for the flow in the piston orifice is less than 2000, which shows that the orifice oil flow is laminar.
137 Figure 5.12: (a) Meshing of CFD model Figure 5.12: (b) CFD simulation results 5.9: Testing of Control Mechanism: In order to demonstrate control of the adjustable damper, an exercise has been carried out with a rotary actuator, load cell and arrangement of mechanical gears. Arrangement in the prototype has been shown in Figure 5.14. The prototype system has been fabricated to change the damper setting in response to variation in the sprung mass. The system has been designed to modulate damping coefficient of the adjustable
138 damper in response to the sprung mass. The controller (LPC 2138 ARM 7 Microcontroller) measures the sprung mass with the load cell and operates the rotary actuator to achieve the desired control strategy. Figure 5.13: Piston and Cover Piece As illustrated in Figure 5.13, the piston has multiple orifice holes that can be blocked by rotating the cover piece. Desired damping coefficient has been controlled by opening number of piston orifice holes by the cover piece. The cover piece has been connected to the rotary actuator (24 V Stepper motor) through a spur gear pair. Rotary actuator gives rotation to one of the gear, which rotates the other gear to open/close desired number of piston holes. The controller has been designed to monitor only the static load (mass) and it does not operate the actuator in case of dynamic load variations at the sprung mass. The actuator (stepper motor) rotates through 1.8 for each pulse. Furthermore, the gears are designed to rotate through 7.5 for each pulse given to the motor. The control strategy ensures that two orifice hole is opened/ closed with each pulse given to the actuator. 5.10: Experimentation Results: To find out whether control mechanism is working as shown in Figure 5.14 some experiments have been performed for finding effect of change of sprung mass on
139 different orifice holes opening at different frequencies. The sample cases have been explained here. Figure 5.14: Test Rig Assembly with Rotary Actuator and Gears Case I Sprung Mass=50kg and 10 orifices open: In this case, 10 orifices are open and sprung mass value is 50kg. Motor speed variation is controlled by regulator; motor speed range of 60 to 300 rpm measured by contact type tachometer. Accelerometer placed above the sprung mass plate takes values of sprung mass acceleration and these values are recorded with the help of PULSE 14.1.1 analysis software. Values obtained for the corresponding speed ranges are represented in Table 5.3. Table 5.3: Sprung Mass Acceleration Values for 10 Orifices Open Sr. No. Motor speed (rpm) Sprung mass acceleration (m/s 2 ) 1. 60 0.370845 2. 120 1.03 3. 180 1.152 4. 240 1.53 5. 300 1.809
140 software. Sprung mass acceleration plots achieved with the help of PULSE 14.1.1 analysis Case II Sprung Mass=100 kg and 8 Orifices Open: In this case, 8 orifices are open and sprung mass value is 100kg. Motor speed variation is controlled by regulator; motor speed range of 60 to 300 rpm measured by contact type tachometer. Values obtained for the corresponding speed ranges are represented in Table 5.4. Table 5.4: Sprung Mass Acceleration Values for 8 Orifices Open Sr. No. Motor speed (rpm) Sprung mass acceleration (m/s 2 ) 1. 60 0.304245 2. 120 0.58986 3. 180 0.71559 4. 240 0.9891 5. 300 1.2141 Case III Sprung Mass =150 kg and 6 Orifices Open: In this case, 6 orifices are open and sprung mass value is 150kg. Motor speed variation is controlled by regulator; motor speed range of 60 to 300 rpm measured by contact type tachometer. Values obtained for the corresponding speed ranges are represented in Table 5.5. Table 5.5: Sprung Mass Acceleration Values for 6 Orifices Open Sr. No. Motor speed (rpm) Sprung mass acceleration (m/s 2 ) 1. 60 0.248355 2. 120 0.54882 3. 180 0.76464 4. 240 1.0215 5. 300 1.1871
141 Case IV Sprung Mass = 200 kg and 4 Orifices Open: In this case, 4 orifices are open and sprung mass value is 200kg. Motor speed variation is controlled by regulator; motor speed range of 60 to 300 rpm measured by contact type tachometer. Values obtained for the corresponding speed ranges are represented in Table 5.6. Table 5.6: Sprung Mass Acceleration Values for 4 Orifices Open Sr. No. Motor speed (rpm) Sprung mass acceleration (m/s 2 ) 1. 60 0.23157 2. 120 0.6147 3. 180 0.89964 4. 240 1.071 5. 300 1.13895 Case V Sprung Mass=250 kg and 2 orifices open: In this case, 2 orifices are open and sprung mass value is 250 kg. Motor speed variation is controlled by regulator; motor speed range of 60 to 300 rpm measured by contact type tachometer. Values obtained for the corresponding speed ranges are represented in Table 5.7. Table 5.7: Sprung Mass Acceleration Values for 2 Orifices Open Sr. No. Motor speed (rpm) Sprung mass acceleration (m/s 2 ) 1. 60 0.2259 2. 120 0.65439 3. 180 0.86283 4. 240 0.94095 5. 300 0.9666
142 This is the relation between the instrumentation with test setup. Experimental result shows that, as number of orifice opening decreases, it results in decrease in sprung mass acceleration. This is resulted due to increase in damping forces as well as increase in equivalent stiffness. 5.11: Damper Characteristics: From simulation results obtained, damping force and the corresponding values of damper rod velocities are given as shown in Figure 5.15, damper characteristics i.e. F(V) plot for the sinusoidal excitation can be plotted. For damper rod velocities the values are shown depending upon the sinusoidal excitation i.e. cyclic excitation. Hence, there are both positive as well as negative velocities are shown. Similarly for damping force, the values are shown positive as well as negative depending upon the rebound chamber damping force and compression chamber damping force. Figure 5.15: F(V) Plot for the Sinusoidal Excitation Above superimposed graph shows results obtained by simulation for damper characteristics i.e. F(V) plot for the sinusoidal excitation. Figure 5.17 shows graph between damping force versus damper rod velocity. Graph shows natures of curves are non-linear. Positive values of damping force indicates damping force produced in
143 rebound chamber and similarly negative values of damping force indicates damping force produced in compression chamber. This is the plot for variable stiffness suspension system having asymmetrical fixed orifices. Depending upon the number of orifice opening i.e. due to change in the damping coefficient, corresponding values of damping forces are plotted. From simulation, maximum value of the damping force produced is 456.1 N which is close to 475.4184 N which is the value of designed for damping force produced by damper. 5.12: Closure: Experimental & simulation results are validated with each other in this chapter. For ride comfort, sprung mass acceleration produced due to input excitation is considered as main parameter. Apart from ride comfort, damper characteristics curve i.e. F(V) plot for variable stiffness suspension system have been plotted. However, from the comparison of results it has been found that for higher input excitation frequency, the passenger ride comfort will be increased but this value of sprung mass acceleration is higher. Hence as per design, natural frequency of variable stiffness suspension system should be kept in the range of 1-2 Hz. For lower input excitation frequency of 1-2 Hz, sprung mass acceleration is 45% less as compared to that at 3-5 Hz. At higher input frequency 3-5 Hz, although sprung mass acceleration values are decreasing but these values are still higher. It will result in harsh ride for passengers. Hence as per design, natural frequency of variable stiffness suspension system should be kept in the range of 1-2 Hz. Hence it has been observed that as orifice opening decreases from 10 to 0 at sprung mass natural frequencies, it will result in increase in stiffness of variable stiffness suspension system up to 57%. And for increased input excitation frequencies from 1-5 Hz, 38-78.4% increase in the equivalent stiffness is achieved.