EXPERIMENT 5 (a) PRESSURE, PROXIMITY AND MAGNETIC FIELD SENSORS

Transcription

1 EXPERIMENT 5 (a) PRESSURE, PROXIMITY AND MAGNETIC FIELD SENSORS 1. OBJECTIVES: 1.1 To study the characteristics of a semiconductor type pressure sensor and an electronic pressure switch circuit 1.2 To experiment a proximity switch operation 1.3 To understand the working principle and application of a Hall sensor 2. COMPONENTS & EQUIPMENTS: 1 DC power supply 1 Sensor Unit SU Multimeter 1 Handy Compressor 3. INTRODUCTION: This experiment shall concentrate on the applications of three types of existing sensors used widely in the industry pressure, proximity and magnetic field sensors. 3.1 Pressure Sensor There are two types of pressure sensors that are available at present load cell, which is commonly used in electronic scales, and solid state technology based sensor, which is used in measuring gas and liquid pressure. In this experiment, the solid state semiconductor type will be used. Semiconductor based pressures make use of the piezoresistance effect of a semiconductor material which exhibits varying resistance to the applied pressure. Figure 5.1 shows one type of the semiconductor pressure sensors in which a stainless steel diaphragm transmits the pressure to the semiconductor chip at the end. Circuit shown in Figure 5.1 is used with the sensor to develop a detected signal. This sensor is mechanically compact, and finds a wide range of applications. Particularly, the sensor works well in detecting pressure developed from gas or liquid. This sensor has fast response time, typically in 1 to 10 millisecond range. Its usage involves mainly with fluid control applications, fluid leakage test, and pressure and level check of a storage tank.

2 Figure 5.1: Solid state semiconductor type pressure sensor construction diagram Figure 5.2: Pressure sensor output detection circuit 3.2 Proximity Sensor A proximity switch detects presence of an object and generates a flag without having a direct contact with the object. It can be classified either as an oscillation type, or an induced type. Another way to classify a proximity switch is as a capacitance type or an inductance type. Listed below are some of the advantages and disadvantages of using proximity switch: Advantages: Since there is no mechanical stress, the life of a proximity switch is very long When the object is well within the detection range, the sensor can detect not only the presence, but also the motion of the object Because there is no moving part, it is very easy to seal the sensor, protecting it from the environment

3 Certain types of proximity switches respond to only specific materials. Such a property can be used in detecting specific types of materials. Disadvantages: Ambient temperature deviation can cause false detection Susceptible to external noise To control large current or high voltage, a buffer, such as relay, is needed Requires power to energize the sensor circuit. Reliability is reduced due to the increased component counts in the sensor unit Figure 5.3: Capacitor type proximity switch Figure 5.4: Oscillator type proximity switch 3.3 Hall / Magnetic Field Sensor If a specimen (metal or semiconductor) carrying a current I is placed in a transverse magnetic field B, an electric field, called Hall voltage, is induced across a and b in Figure 5.5 (b). This electric field is in the direction perpendicular to both I and B. This phenomenon is known as the Hall effect.

4 Figure 5.5: Hall Sensors and Hall Effect A magnetic resistance element is a current carrying semiconductor placed in a magnetic field. A force will be exerted on the current carries regardless whether the current carrier is free electrons or holes. This force will modify the traveling path of the carriers in such a way that the length of the path is extended. Because the current carriers have to travel longer distance, the specimen will exhibit increased resistance. Hall sensors are temperature sensitive; therefore a bridge detector or a compensation network is necessary. Besides that, it is easy to miniaturize and ideal to be used as a peripheral component for computers. Its output shows good linearity to the magnetic field. Hall sensors are being used in flux meters, FDD and HDD, motor rotation detection, VTR and current measurements. 4. PROCEDURE: 4.1 Pressure Detection using a Pressure Sensor Cautions: Do not apply pressure that exceeds the specification. The ON/OFF point output is set to 1kg/cm 2, which is the maximum pressure it can take. Therefore, when the Det Indicator LED turns on, do not apply any more pressure. Also, do not alter the maximum pressure setting. The sensor output is an analog signal. The ON/OFF point output is used to drive the Det LED. Make sure no other objects other than air enter at the pressure input. Use only the Handy compressor supplied with the system to generate pressure. No other compressors are allowed with ED-6800B system. Close the sensor opening when not in use.

5 Procedures: 1. Prepare a power supply (± 15 V) and keep the power off. Set up equipment as shown in Figure 5.6. Remove the cover at the pressure input of the sensor and connect the Handy compressor. Figure Keep the three sensor switches as well as the motor switch off, and set the multimeter to DC 15V. Turn the power supply on. 3. Turn the air inlet of the pressure sensor so that there is no pressure build-up in the Handy compressor. Turn the pressure sensor power on. 4. Increase the pressure until the Det LED on the Sensor Unit is turned on. Define the pressure at this point as 1 kg/cm 2. Record the value in Table Proximity Detection using a Proximity Sensor Cautions: The detection range decreases as the target object gets smaller. In case the object is too small, it is not possible to detect. When other metal objects are around the target, the likelihood of an erroneous detection increases. Also, the detection sensitivity becomes poor. When installing a proximity sensor, avoid a place with electrical noise or interference signals. In case the sensor output is DC, shield the output from noise. For an area with high moisture or severe temperature change, consider the reduction of the detection range.

6 Procedures: 1. With the power off, wire the sensor unit to the DC power supply as in Figure 5.7. Keep the motor switch as well as the three sensor switches off. Figure Set the Motor SPEED to 0, and turn the Proximity Switch on. 3. Turn on the power supply. Turn the SPEED knob slowly clockwise, and see if the Det ED turns on when the objects pass by the proximity sensor. For what type of object the LED turns on? 4. Stop the motor when the iron object aligns with the proximity sensor. Increase the distance between the sensor and object, and find the maximum detection distance and record it in Table Magnetic Field Detection using a Hall Sensor 1. With the DC power supply off, connect it with the sensor unit as shown in Figure 5.8. Set the digital multimeter to DC 20 V range. Keep all the switches, the motor switch and the three sensor switches, in OFF position.

7 Figure Turn the power supply on. 3. Set the motor speed to minimum (0), and turn only the power switch on at the Hall sensor. 4. Turn the motor on. Turn the speed knob slowly clockwise, and observe the moment the LED turns on. Explain what type of material has caused LED to light. 5. Stop the disk when LED is on, and measure the output voltage. 6. Turn off the power supply. Change the distance between the magnet and sensor, and measure the output voltage at each distance as specified in the Table 5.3. Record the value in the same table.

8 Name: Matrix No.: Date: 5. RESULTS Table 5.1 Pressure (kg/cm 2 ) 1 Output (V) Table 5.2 Maximum detection distance (mm) Table 5.3 Distance (mm) Output (V) Instructor Approval:. Date:

9 Name: Matrix No.: Date: 6. DISCUSSION 7. CONCLUSION Instructor Approval:. Date:

10 Name: Matrix No.: Date: PROBLEMS 1. Define stress. How does stress would affect strain? 2. Two strain gauges are bonded onto a steel bar so that one measures the longitudinal strain and one the transverse strain. Given the longitudinal strain as - 5 x 10-4 and the Poisson s ratio of 0.4, calculate its transverse strain. Instructor Approval:. Date: