The PID controller has its origins within the process control industry going back over 50 years. Today it remains an industry standard in this and many other applications, including hydraulic servodrives.
In the early days, the 3 terms of the controller P, I & D, ((P)ropotional, (I)ntegral, (D)erivative) were implemented in analog electronics. Hence the commonly used name “3 Term Controller.” More recently, digital (microprocessor based) closed-loop electronics have become commonplace. However, the PID controller concept has remained intact as the means of achieving closed-loop control in hydraulic servo systems.
What is PID Control and what role does each term play?
The abbreviation PID is descriptive of the action taken by the controller in achieving high accuracy and/or fast and stable dynamics in a closed-loop system. In the standard configuration (taking hydraulic drives as an example), the P, I, and D, terms act upon the system control error to produce a valve drive output signal which is a summation of the 3 components. These are produced in the following manner and have a distinctive action in the controller:
- Sets the proportional algorithm (P) output to a level depending on multiplying the error by a gain Kp
- Increases system static accuracy and dynamic response
- Direct function of the error and gain setting Kp such that P=ƒ[K? · error]
- Increases the integral algorithm (I) output at a rate depending on the error multiplied by the integral gain Ki
- Gives further increase in static accuracy, but often at the expense of dynamic response
- Function of the accumulation of error with time and gain setting Ki such that I=ƒ[?Ki· (error/dt)]
- Sets the derivative algorithm (D) output to a level depending on the rate of change of error multiplied by the gain term Kd
- Increases or improves the dynamic response
- Function of the time rate of change of error and gain setting, Kd such that D=ƒ[?KD· d(error/dt)]
The optimum value for the 3 gains (Kp, Ki, and Kd) is normally found during the commissioning process and is dependant on the characteristics of the controlled system.
What if PID is not enough?
The wide acceptance of PID control as a standard makes it a good starting point for most servo control applications. However, a little extra in terms of control capability is often required to achieve a good result; for example when dealing with high inertia drives or particular types of load or pressure control. Moog has developed the M3000 and MSC (Moog Servo Controller) to deal with these issues.