Wednesday, February 1, 2017

Create a Customized On/Off Process Control Unit

In control theory, an on/off controller is a feedback controller that switches abruptly between two states. It is often used as a control method for a process which can tolerate ongoing change in the process value within a band, referred to as the hysteresis. A common example of an on/off temperature control operation is a residential thermostat. They control the temperature of your home, turning off the heating or cooling system at your comfort setting, waiting for some significant change to occur, then turning on again to eliminate that difference. The process cycles continually. Many process operations can utilize simple on/off control action to maintain suitable conditions.

sanitary temperature transmitter RTD
Temperature Transmitter
Courtesy Smart Sensors
A PLC (programmable logic controller) can be a good candidate for creating your own on/off temperature controller with specialized additional functionality that suits your process. Implementing the temperature control operation is not difficult, and the use of a PLC allows the designer to integrate other useful functions into a single piece of hardware, functions that might not be available in a commercially available process controller.

The primary input device will be a transmitter with analog output compatible with the analog input on the PLC. For this discussion, let's assume this is a temperature control application that requires heating of the process. So, a temperature transmitter will be our primary input device. The primary output device will be a heater contactor or other power control device, the input of which must be compatible with the output of the PLC. Any other switches, pilot lights, alarms, or other devices will need to also be associated with a compatible PLC I/O point. 

The logic portion of the temperature control activity is not complex. The input signal from the temperature transmitter is converted to a working value. Depending upon the numeric muscle of your PLC, this value may be a floating point number, but integer values work suitably. Here are the logic steps needed.

  • Read temperature input value
  • Is temperature greater than or equal to the setpoint? If yes, turn off output for heater and proceed to the next step. If no, go to next step.
  • Is temperature less than or equal to the value of setpoint minus a deadband value (more on deadband below)? If yes, turn on output for heater. If no, continue to other commands that provide your additional desired functions.
Here are some points to consider.
  • Use a greater than or equal to, or less than or equal to comparison to assure that all possible numeric scenarios for process temperature are handled.
  • Deadband is a value that you employ to keep your control output from chattering rapidly between the on and off state when the process value is very close to the setpoint. It can also be used, in this case, to slow down the on/off switching of the heater and reduce wear on a mechanical contactor. Keep in mind that a 16 or 32 bit number, which is what the PLC will use for internal processing of your temperature reading, may actually pass across the setpoint value rapidly, even though a digital display of temperature will appear to be relatively stable. The constantly changing values would cause rapid changes in the output if the comparison logic did not include a deadband value. The use of a deadband creates a range of process temperature where no change in the output occurs. 
  • For this particular application, with its heating action, a separate limit control is advised. The device should derive its input signal from a source other than that of the PLC and the output of the limit control should provide a positive means of de-energizing the heater.
  • Other functions easily programmed into the PLC include alarms, pilot lights to reflect heating activity, an on/off switch for the process, and other items limited only by your ingenuity.
Not every process needs PID control. This illustration focused on temperature, but the principles are the same for almost any process. A modestly powerful PLC can provide the processing power, and input devices for temperature, humidity, moisture, pH, liquid level, flow, pressure, and more are available. Share your challenges with a process measurement specialist and develop an effective customized solution for control of your process.