Do 4QD controllers have closed loop control?

Closed loop control

The Pro, DNO, and Porter controllers do not incorporate internal armature voltage sensing. For most applications this is an unnecessary complication since the operator compensates for speed changes automatically. The 4QD series does incorporate armature voltage sensing. However this alone gives a limited improvement since the motor’s speed still changes because of the voltage loss in its internal resistance (IR loss). It is possible to compensate for motor IR loss, but the controller then must be set up for each different motor: this causes much confusion as setting up requires much skill, or a tachometer and a method of loading the motor whilst measuring the speed. The other problem is that the existing internal current sensing (which could be used for IR compensation, is sensitive to the MOSFETs internal temperature, so the IR compensation would drift.

The best method of closed loop control is via a tacho-generator. It is relatively simple to add on to the 4QD series as the internal circuitry used for armature voltage sensing can accept a tacho generator circuitry with minimal change.

It is also possible to add a tacho generator to the Pro 120 or the DNO with our optional tacho feedback board. Other controllers can also use this same board or, if you wish to make a circuit yourself, see Tacho generator motor speed feedback for the circuit of a suitable error amplifier to compare tacho feedback voltage with the demand speed.

It is also possible to use a pulse generator to measure motor speeds. If using analogue control this needs to be processed via a frequency to voltage converter to be used as feedback. The slowest motor speed will determine the maximum time between pulses and this will determine the minimum response speed of the F to V converter: if it responds too quickly the motor will accelerate between pulses! But if you chose a long time you will need slow response which can cause problems with motor speed stability. Pulse circuitry is not direction sensitive (unlike a tacho generator) so the error amplifier need only be uni-directional.

Because of these timing constraints, pulse generators are usually used with computer control systems: in this case the computer can give the controller a demand speed via a digital to analogue converter: the software can control response times and difference amplification (if the demand speed is input digitally), or the D-A can output to a conventional analogue error amplifier, working direct from the demand speed control.

The 4QD, Pro and DNO series have, at their inputs, sophisticated analogue ramps: ideally the error amplifier should be inserted after the ramping circuit and before the modulator: contact 4QD for more assistance.
See also Speed Stability


Category: Features

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