PWM Settings

[Neale]

I have sometimes wondered how these things work at all. The Gecko output is a PWM signal. That is, a series of pulses that are on for some percentage of the time and off for the rest. The idea is that the average is equivalent to the desired speed. 10% on-time give 1V average; 50% on-time gives 5V average, and so on. The Gecko also has an opto-isolated "analogue" (actually, pulsed digital) output, which is a good thing. I get the impression that a lot of North American users use DC motors and associated controllers. The Geckos are probably going to work fine with a DC motor controller as the motor will average out the pulses. However, the area of uncertainty is with a VFD. This has an analogue input that is going to turn the analogue input voltage into the equivalent frequency and hence motor speed. But the VFD isn't seeing a constant input voltage, slowly changing as speed required changes. What it does see is sometimes a "high" voltage pulse, sometime a "low" voltage, changing between the two at maybe 1KHz, but never a real analogue input. I don't know if there is any smoothing being done anywhere to effectively convert the PWM signal into an analogue voltage. I know that this does work for other combinations of hardware (my IP/M works fine with a typical Chinese VFD and plenty of people use this kind of setup) but given that the target market for Gecko is possibly not primarily focused on the VFD user but the DC or similar motor controller user, I wonder if there is some kind of incompatibility here. Could be easily fixed, I would suspect, with a suitable capacitor across the analogue output to smooth the output pulses. Unfortunately, measuring the "analogue" voltage with a digital meter is likely to be equally misleading as the meter's sampling circuit is also going to get confused (but might well see something different to the VFD). A traditional analogue meter would show a better approximation.

Personally, and if you don't have an oscilloscope to actually see what's happening, then I would stick a 10uF capacitor across the analogue output to smooth the pulse stream and see what happens. Can't believe that it would do any harm but might help.

Or maybe it's something else completely. A Google search suggests that this can be a problematic area with the Gecko, but they aren't widely used in the UK/Europe based on typical machine descriptions on this forum. I'm assuming that you are taking +10V from the VFD to the Gecko?