Designing a System to Measure & Display a Shaft Rotation Speed

We would like to design a system to measure and display the rotational speed of a shaft. To accomplish this, we shall use a number of integrated circuits (ICs) to create the desired functionality of this system. A simple technique to measure the speed is to count the number of shaft rotations during a given period of time. The resulting count will be directly proportional to the shaft speed.

We have several different types of sensors that can be used to detect shaft rotation; a good example is the proximity sensor that employs magnetic, optical, or mechanical principles to detect some feature on the shaft. In our case, we can use a LED-phototransistor pair as an optical sensor and place a small piece of reflective tape on the shaft. Each time the tape passes by the photo-optic pair, the 7414 Schmitt trigger inverter provides a single pulse to a counting circuit. This is demonstrated in the figure below:

We also need a circuit to count and display the pulses over a given interval of time T. The figure below shows the components that we need:

The 7490 decade counter counts the pulses and is reset by a negative edge on signal R_t, after the time T. The period T is set by a resistor-capacitor combination using a 555 oscillator circuit. If the count can exceed nine during the period T, additional 7490s must be cascaded to provide the full count. Just prior to counter reset, the output is stored by 7475 data latches that are enabled by a brief pulse on signal L_t. The latches are required to hold the previous count for display while the counter begins a new count cycle. One of the two 74123 one-shots is positive edge-triggered by the clock signal CLK to generate a latch pulse L_t of length Δt. Take note of the fact that, the latch and reset pulse widths must be small (Δt << T) to maintain count accuracy. The trailing edge of the latch pulse triggers the second one-shot, which is negative edge-triggered, to produce a delayed reset pulse R_t for the counter. The 7447 LED decoder and driver converts the latched BCD count into the seven signals needed to drive the LED display. The display reports the number of pulses that have occurred during the counting period T.

The shaft speed in revolutions per minute is related to the displayed pulse count by:

Where ppr is the number of pulses per revolution generated by the sensor, for example 1 for a single piece of reflective tape.

Also Read: Mechatronics System