This anticipation tends to add damping to the system, thereby decreasing overshoot. With derivative control, the control signal can become large if the error begins sloping upward, even while the magnitude of the error is still relatively small. With simple proportional control, if Kp is fixed, the only way that the control will increase is if the error increases.
![rslogix 500 pid example rslogix 500 pid example](https://instrumentationtools.com/wp-content/uploads/2020/06/PID-Block-in-RSLogix-500.png)
The addition of a derivative term to the controller, Kd, adds the ability of the controller to anticipate an error. This has the adverse effect of making a system sluggish and oscillatory since when the error signal changes sign, it may take a while for the integrator to unwind, leading to an overshoot in the opposite direction. If there is a persistent, steady error, the integrator builds over time, increasing the control signal and driving the error down. The addition of an integral term to the controller, Ki, helps to reduce the steady-state error. This is another way of saying that the response has reached a steady state. The steady-state error is the difference between the input and the output of a system as time goes to infinity. Increasing Kp also tends to reduce, but not eliminate, the steady-state error. This makes the controller react quicker for a given error in the closed-loop system, but it also means that it will likely overshoot more as well. Increasing the proportional gain, Kp, has the effect of proportionally increasing the control signal for the same level of error. u(t) is the controller output, and e(t) is the error value. The analog output module converts the digital value in the range of 6242 to 30 1208 to an analog signal of 4 to 20 milliamps.The important parts of this equation are Kp, Ki, and Kd. The SCP output parameter is the file address of the analog output module. Scale maximum value is programmed as 31208 representing the high end of the analog output module range. The input value is then scaled between the ranges programmed in scaled minimum and scaled maximum parameters scale minimum value is programmed at 6242 representing the low end of the analog output module range. The input minimum and input maximum are programmed as the smallest and largest values of the input range. the input parameter of the SCP contains the memory address of the PID output values that will be scaled. Next, the control variable range of 0 to 16383 is scaled to an analog output module using the second scale with parameter instruction. The control variable parameter is programmed as an integer which is the memory address for the output value. In the PID instruction the process variable parameter is programmed as an integer file which is the scale value from the SCP instruction. The SCP output parameter is the integer file address where the scaled output is stored and then used by the PID instruction as the process variable.įor example, when the minimum pressure of 0 psi is measured a scale value of zero is transferred to the SCP output, and when 100 PSI has measured a scale value of 16383 is transferred to the SCP output.
![rslogix 500 pid example rslogix 500 pid example](https://i.ytimg.com/vi/CrX7BMakDs0/hqdefault.jpg)
Scale maximum value is programmed as 16383 representing the high end of the output range. The input values are then scaled between the ranges programmed in scaled minimum and scaled maximum parameters, scale minimum value is programmed is 0 representing the low end of the output range.