Especially in precision motion control applications, the question often comes up, “What are control loops and what do they do?” To answer that, let’s look at one of your activities. When you drive a car, you are part of at least one control loop. If you control the speed without using the speedometer, you are using open-loop control (assuming you neglect the inputs from the seat-of-the-pants). By contrast, if you observe the speedometer, you have then closed the control loop. That is, you specify an action by pressing on the accelerator and you receive a precise feedback on the final results. Then you mentally note any error between your intended speed and the actual speed, and take the necessary corrective action, Figure 1.

In reality, everything you do is — in some manner — a closed loop system. Any action taken produces a response, a feedback, and an attempt to correct the action by the controlling devices. Some loops take a long time to close, or complete the loop. Others are completed in a few thousandths of a second.

When controlling motion, control loops are usually associated with the physical control of a motor shaft that positions something in relation to something else measured in distance or time. Controlling an electric motor, requires controlling three basic functions of the motor shaft:

Position — the location of one part referenced to some other physical point within the application. For example, a point on an indexing table relative to a drill head, or a point on a linear slide in relation to an inserter.

Speed — usually defined in revolutions per minute (rpm), radians/sec (rad/sec), or some other measure of velocity with respect to time.

Torque — generally defined by the electrical current flowing through the motor, which is indicative of the torque delivered by the motor.

Position control loop

A position control loop is used when you want to control the location of one device relative to the location of another device. During normal driving on a crowded highway, you use a driver’s version of a closed position loop. You establish a gap between the front of your vehicle and the back bumper of a vehicle directly in front of you, Figure 2. You attempt to keep the gap distance constant by adjusting your speed.

If the vehicle in front slows down, the gap gets smaller. To maintain the desired gap, you slow down at the same rate plus a “bit” more. The bit more makes up for the gap change that occurs before you respond to the change.

The motion control term used to describe the change in the gap is following error. It represents a difference between the actual gap and the desired gap. This distance related to time includes the time it takes to notice the change plus the time needed to take action.

Response time is quantified with “bandwidth,” the reciprocal of response time (1/response time). It establishes the limit of performance for a control loop or for a drive and system being controlled. A larger bandwidth value means a device can execute more commands per unit of time.

Speed control loop

To control position, you must control speed, as we discussed previously with closed-loop control and Figure 1. Continuing the traveling example, assume one car after another passes you, so you decide to check the accuracy of your speedometer. You use the mile markers and the car clock as a speed feedback loop. Traveling at an indicated 60 mph, you know that each mile should take one minute. If the time between each mile marker is 66 instead of 60 seconds, you have a 10% error signal, so you increase the speed shown on the speedometer by 10% to reduce the time between mile markers.

With one marker per mile, the speedloop correction time is several seconds. If there were markers at ½-mile intervals, the correction time is less. With 60 markers per mile, correction time is less yet, and you can maintain good speed regulation.

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