February 18, 2012
Marty Kanner
THE PID REVISITED
Chapter 2
ELECTRO-MECHANICAL POSITIONAL CLOSED LOOPS
A velocity closed loop was described to introduce the concept of closed loops and a comparison was made to an open loop velocity control configuration. We saw that the closed loop would be used whenever accuracy and overall performance is required. Although a comparison was made between open loop and closed loop in the previous chapter, it will not be done here. The improved performance possibilities of the closed loop over the open are essentially the same. Note that the positional closed loop is a type 1 control loop.
The purpose of a positional closed loop is just that; to command a desired position and have a desired load move to the commanded position. The problems of design are generally in the details but I don’t think it worthwhile at this point to completely specify a positional loop. This introductory discussion just describes the typical functional elements you would find in a positional loop, figure 1. In this case we take a command voltage that represents the position of a large block that we want to precisely position over a one foot range. In this case our feedback transducer could be a precision potentiometer instead of the tachometer used in chapter 1. The precision potentiometer is coupled to the block such that when the block is moved through the desired one foot range, the arm of the potentiometer moves through its complete range. By providing a regulated voltage across the ends of the potentiometer, the voltage at the output of the potentiometer arm will linearly represent the position of the block.
As in chapter 1, the command voltage is compared to the voltage feedback. This time the feedback is from the potentiometer. The voltage comparison is made in what is called a summing junction. This function can be implemented in many ways but in the end, the difference between the voltages is the error voltage that would then be amplified. The amplified voltage then powers a motor that is selected to be large enough to move the load at a desired speed. Motors are generally designed to run at high speed. They are then matched to the load using some form of speed reduction (i.e. gear train speed reducers). The motor automatically moves in the direction to drive the error voltage toward zero within the limits of the design. Here again, the higher the gain, the smaller the error. The limit in gain is again the concern for loop stability. For reference purposes only, note that the electro-mechanical positional loop described is a type 1 control loop. These loops can be designed to position to 0.10% accuracy. We could position a load to within .001 feet over the one foot throw.
When the design requirement dictates small error voltages under dynamic conditions, another element is added to the design. This device is called an integrator which literally integrates the error voltage. This minimizes static and dynamic errors and is described in chapter 4.
Please reply with any questions
