micrologix 1500

[dougpdf 0]

I have a micrologix 1500 with combo analog in and out with an lpvt on a cylinder for position and a proportional valve for "pressure" Is it possible to control the speed using feedback from input and send volts out that constantly is correcting itself to maintain a certain speed"Closed loop?" I know you can get velocity mods for other controllers, but are not available on micros. Could I use PID? Thanks Doug

[Guest Guest_PhilipW]

This kind of application always fascinates me...the correct answer is going to be very dependent on how fast and how accurate you need the servo loop to update. Most standalone motion type velocity loops run the motion coarse planner at around 1-2mS intervals, with the actual loop closed faster than that. This is much faster than a Micro1500 can manage....but all is not lost if your application can survive loop closure in the order of 10mS or so. It isn't easy to give you a clear cut yes or no without know a lot more about the actual equipment the hydraulics is controlling and the acel/decel rates you need. However the 1500 is quite capable of running a fast PID loop inside an STI file at 5 or 10mS. A lot will depend on how much other logic is also running. If the STI file runs too often or takes too long then the main scan will extend out dramatically and ultimately you get a scan watchdog timer error. To fix this you have to either reduce the STI file excecution time, or run it less often Another easily overlooked aspect is the need for the I/O to have response risetimes comparable. You will need to ensure that any filtering on the input is set to as fast as possible. ie 500Hz which is around the 2mS mark. Alternately I would seriously consider using encoder or pulse feedback from the motion axis and the 1769-HSCE module to provide a velocity/position value. Outputs can be done either with analog or better still why not use a High Speed pulse output and simply low pass filter the pulses with a simple RC combination? In the absence of more details I can only give some ideas (some of which may not be applicable)...but overall the answer to your question is a conditional yes. If the actual motion you are trying to control is not too demanding it is certainly worth trying it out a Micro1500 as you suggest. It won't be as simple nor will the result be quite as good as a true motion control system, but it may well be quite good enough and a lot cheaper. If your