Experiment 5: Measurement of rotational speed using Optical pick-up and Magnetic pick-up.

Experiment 5(a) : Measurement of Rotational Speed using Electromagnetic pick-up.

Setup:

The setup has two units

• Signal Conditioning and Processing unit.
• Sensor Module

Fig.1 signal conditioning and processing unit

• The signal conditioning board has a variable DC power supply source to change the speed of Motor.
• It has a 4 digit LED display to show the RPM of motor.
• There are three test points, one after the wave shaper block, another after the multiplier block and other is for testing 1Hz signal. All the test signals are measured w.r.t ground point.
• There are five sockets for sensor module interface. Two for motor (red, black) and remaining three for sensor (yellow, blue and green).
• The sensor module consists of a 12V DC motor, a 15 tooth metallic circular disc connected to the shaft of DC motor and an electromagnetic switch fitted on a clamp near to the slots of the disc.

Working Principle: 

• When a magnetic material is brought near to the electromagnetic switch, it gives a logic high (5V) at the output. And gives logic low (0V) when there is no magnetic material near to it.
• When the motor gets supply it rotates along with slotted circular disc.
• The electromagnetic switch has been fitted close to the slot of the disc.
• During rotation when a teeth passes the electromagnetic switch it gives a logic high signal and a logic low signal which immediately follows due to the air gap between two slots.
• So in a complete rotation of the disc the sensor gives 15 pulses.
• These pulses are feed to the signal conditioning for further processing

Fig.2 Sensor Module

• In the signal conditioning board, these pulses are immediately passes through a wave shaping circuit to give them a perfect square wave shape. Then they are multiplied by a multiplier circuit by a factor four to show the speed directly in RPM.
• After multiplication these signals are feed to a frequency counter that counts the pulse for one second and it is then passed to the display unit.
• There is a toggle switch for the display to show the speed in RPM and the no. of counts. Two push button switches named as reset and push to count are associated for counting purpose. By counting the pulses for particular second we can calculate the speed manually.

Appratus Required:

• DSO
• Multimeter
• Trace paper/Pen drive of 4GB or less

Procedure to Experiment:

• Rotate the power supply knob anticlockwise so that it should reach at the extreme left position.
• Connect the sensor and motor to their respective socket on the board.
• Switch ON the power to the board.
• Make sure that the toggle switch below the display is pointing towards the RPM.
• Connect a multimeter in 20V DC range at the motor socket on the board.
• Take a dual trace CRO, connect one channel at the output of wave shaper test point (TP-1) and other channel at the output of multiplier test point (TP-2) w.r.t ground.
• Rotate the power supply knob slowly in clockwise direction to start the motor to rotate.
• Note down the supply voltage reading on the multimeter, RPM reading on the display and frequency of both the channel on the CRO screen.
• Rotate the knob to change the supply voltage and note down all the above parameters again.(continue for 10 readings

Manually measurement of RPM:

• Toggle the toggle switch to SPEED.
• Push the Reset button 2 to 3 times until getting 0000 reading on the display.
• Start the stopwatch then push and hold the Push to count button.
• Stop the stop watch after some second say (T), release the push button switch and note down the count on the display say (C).
• Then RPM = 4C/T

Observation:

Sl no	Supply voltage	Frequency at Wave shaper Output	Frequency at Multiplier output

Experiment 5(b): Measurement of Rotational Speed using Opto-coupler:

Setup:

• The setup is almost similar to electromagnetic pick-up. The only difference exist in sensor module is, here a slotted optocoupler is used as sensor instead of electromagnetic pick-up.
•  A slotted optocoupler consists of an IR transmitting (Source) and receiving (Detector) LED pair.
• They are confined inside a plastic package separated from each other by a small air gap (slot).
•  There is a small hole in the package on both side in a line so that the transmitted IR ray from source can directly fall on the receiving LED.
• The optocoupler has been placed in such a way that the slotted disc connected with the motor shaft will pass through the air gap between the source and detecto

Fig.1 Signal Conditioning and Processing Unit

Working Principle:

• When the teeth of the disc comes exactly in between the source and detector of the sensor, the IR ray is obstructed so we get a logic low signal (0V) at the output.
•  And we get a logic high (5V) signal at the output when the IR ray is not obstructed by the disc.
• Hence we get 15 pulses at the output per one rotation of the motor.

Fig.2 Sensor Module

The signal conditioning and processing unit does the same thing what it does in electromagnetic pick-up. Procedure is also same as that of electromagnetic pick-up.

Observation:

Sl no	Supply voltage	Frequency at Wave shaper Output	Frequency at Multiplier output