Electronic Control

It is very common to design electronic control circuit and I have students from doing the senior project to working in industries not quite aware of the similarity in various way to control.

Not starting from a feedback control system but starting from electronic circuitry logic, it starts with sensor sensing either temperature, pressure, light, moisture, strength, electric or magnetic field etc. Any control system will need a sensor capturing something and provides a voltage at the output. If the voltage is small, an instrumentation or difference amplifier will be used to buffer and amplify this tiny input voltage signal.

Then the amplified voltage corresponding to the sensed quantity will need to be rectified and filtered. The purpose is to get the energy of the detected and pass this down to the comparator to elliminate the probability of noise addtion.

The result of this detection will provide the logic 1 or 0, high or low digital levels which can be 5V or 0 V or even 3V or 0V. Here is the problem. Students is faced with different circuitry to take this logic output to turn another device on or off. This device is usally called actuator in control terminology.

1. Basic Control using relay.

2. Control using transistor.

3. Control using SCR.

4. Control using TRIAC.

All of the above are based on something called ON/OFF control. For a more precise control, something called Proportional control will be used. In addition, there are Integral Control and Derivative Control and for the general control circuitry, PID controller is what in the industry is used.

The following diagram illustrates the similiarity of control with AC power using relay, transistor, SCR and TRIAC. Note that in transistor control alone, a bridge rectifier is used

Control AC
AC control