In the last post there was some discussion of how positioning is accomplished from a general perspective. As with all things in the world of mechatronics there are at least three perspective on the subject; mechanical, electrical and control. In order to understand the performance requirement, the underlying variable that ties everything together is the time. When it comes to positioning, time provides some important insights that require careful consideration.
Positioning from the perspective of time has two important elements that need to be considered before beginning a new project; speed and error. Each has unique attributes.
Speed of positioning with respect to the move profile is the most obvious concern. The faster you need to do the move, the more acceleration is require. Acceleration is manifested as the rate of current inrush over time. Coincidentally this is also the limit of performance for the solid state power transistors that are the core of all drives and servo amplifiers. When the application is very demanding, squeezing the last 10 milliseconds from the move profile can push the limits of performance of the power transistors.
Secondarily, somewhat less common in today’s technology, the feedback circuit in the control system has frequency limits that cannot be exceeded. Many of today’s servo motors are sporting super high resolution feedback, 20 bit encoders being fairly common. That’s a 1 million pulse per revolution accuracy. Cool for the spec sheet. The problem is that for a 5000 RPM motor, the feedback frequency rate could hit 83 megahertz, well beyond the common input frequency of many controllers. So an important decision to make is; How much feedback is enough? Generally speaking 5-10 times the actual desired error measurement is more than enough.
Error detection is a control system behavior that depends on the feedback, but requires additional definition. Position error can be an absolute value, based on the target position at the end of the move profile, or it can be “following error”, a measurement of permissible error that is continuously measured throughout the execution of the move. These are actual control system features that are part of the controller programming.
Position accuracy is expressed as a +/- measurement, a “bandwidth” of accuracy. If the bandwidth of error is applied continuously, which is common in machine tools and robots, then the behavior is ‘following error’. If the position accuracy is measured at the end of the motion profile, typically in a PLC controller, then a simple correction may take place at the end of travel. It is the ‘following error’ type of control that makes possible complex motion such as drawing a circle with two linear actuators.
Knowing the difference between the two makes a huge difference in the control system architecture.