A PID controller can take the form of a PI or PD controller, or just I or P. Not all applications require all three parameters. Derivative control is often eliminated due to system noise. A PID controller calculates three measurable parameters to minimize errors and compensate for changes in the system. The simplest controls are fine for basic thermostatic systems, but industrial uses require constant monitoring. Factors to consider include sensor input, output needs, and number of exits required.
Choosing the best proportional-integral-derivative (PID) controller will depend on your specific needs. A PID controller can take the form of a PI or PD controller, or just I or P. Not all applications require the use of all three parameters. Derivative control is the one most likely to be eliminated as it makes measurements based on system noise. Elimination can be done by setting unwanted parameters to zero.
Widely used as industrial controls, a PID controller calculates three separate measurable parameters, characteristics, or factors. Error value calculations are made by taking the difference between a measured amount and the desired amount. Errors are minimized by adjusting inputs to the control system.
In a PID controller, any proportional changes that are too large can cause system instability. If the changes themselves are too small, the system will become unresponsive. Integral control measures the amount of error and tries to minimize it. Derivative controls reduce the slew rate, but can slow response times and introduce more noise into the system.
To understand the control process, a good example is manually adjusting the water temperature on a two-tap faucet. Both hot and cold taps are opened and then adjusted by the user to the desired combined temperature. Adjustments must be made precisely or the user will go back and forth between water that is too hot or too cold. Fully proportional controls eliminate on and off cycles in the system. A PID controller will automatically compensate when changes are detected in the system.
The simplest control systems can be used for basic thermostatic systems. A PID controller in a furnace can only work best with proportional and integral controls. The derivative function may cause erratic changes due to electrical noise or interference. The control, functioning properly, allows the oven to heat up to the desired temperature and then switches on and off to maintain it. Heating is slowed down as the oven reaches the desired temperature to avoid exceeding the set point.
The basic on and off controls are fine in systems that don’t require constant exact temperatures. Home heating and cooling units can use it, but you will get better efficiency with a proportional or PID controller. Industrial uses normally require constant monitoring for laboratory type uses. Motion, temperature and flow control requirements can all be met with PID functions. When a steady state error (SSE) is critical, all three controls working together will provide the desired result.
Factors that need to be considered are the type of sensor input to the system and the range of results allowed. Next, the output needs must be met. The outputs can be an electromechanical relay, an analog receiver, or a solid state relay (SSR). Finally, take into account the number of exits required. PID controllers commonly ship with a list of all the input and output types they work best with.
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