PDM (Pulse Density Modulation) Question

[PJRitz]

Newbie here,

Trying to create a servo system with the loop closed in EMC, I would like to know if PDM in EMC has the capability to give me my -10v to +10v signal to drive the servo drives using only one output pin.

In other PDM systems, the PDM signal can be as such: 50% duty cycle would represent 0v, 100% duty cycle would represent full scale positive voltage and 0% duty cycle would represent full scale negative voltage. Where all I would have to do is filter, amplify, and level shift the 5v signal from the parallel port.

Is EMC able to produce this type of output? or would I have to use a seperate PDM signal for each direction, inverting one and summing the two together?

Thanks
PJ Ritz

[PCW_MESA]

Newbie here,

Trying to create a servo system with the loop closed in EMC, I would like to know if PDM in EMC has the capability to give me my -10v to +10v signal to drive the servo drives using only one output pin.

In other PDM systems, the PDM signal can be as such: 50% duty cycle would represent 0v, 100% duty cycle would represent full scale positive voltage and 0% duty cycle would represent full scale negative voltage. Where all I would have to do is filter, amplify, and level shift the 5v signal from the parallel port.

Is EMC able to produce this type of output? or would I have to use a seperate PDM signal for each direction, inverting one and summing the two together?

Thanks
PJ Ritz

Sure, EMCs PDM component has Scale and Offset parameters so if you offset the output 50% and set the scale right it should be possible. Of course with 50% = 0 you need something else to enable the drives or you will be at full + or - motor drive at startup before the PDM is running.

[PJRitz]

Yes, I do have an enable input on the drive so as long as I have the PDM signal on and at 50% for a 0v output before the drive enable is turned on that should work fine. I am not really trying to save output pins, but rather simplify the PDM to -10v to +10v circuit

Thanks
PJ Ritz

[jmelson]

Newbie here,

Trying to create a servo system with the loop closed in EMC, I would like to know if PDM in EMC has the capability to give me my -10v to +10v signal to drive the servo drives using only one output pin.

In other PDM systems, the PDM signal can be as such: 50% duty cycle would represent 0v, 100% duty cycle would represent full scale positive voltage and 0% duty cycle would represent full scale negative voltage. Where all I would have to do is filter, amplify, and level shift the 5v signal from the parallel port.

What you describe above sounds like Pulse WIDTH modulation, where the density (inverse frequency) of the pulses is constant, but the width of the pulses changes.

Is EMC able to produce this type of output? or would I have to use a seperate PDM signal for each direction, inverting one and summing the two together?
PJ Ritz

A stepper pulse system is an example of PDM, so sure, EMC can do that.
The problem with software generation of either PWM or PDM is that the timing granularity is coarse. With the fastest PCs, you can get timing granularity down to about 20 us. So, if you
need a 1 KHz bandwidth at the servo drive, you need a PWM carrier of about 2 KHz, and so
with a 20 us interval, you get about 25 "clock ticks" per PWM cycle. So, you have a granularity
of 4%. This might actually work for a servo drive, but it won't work very well. The analog servo drive interfaces I make for EMC have a 16-bit DAC, so you get a resolution of .003%, which is probably overkill. (It is only .003% because the DAC has to provide voltages in both polarities, so you only get 32768 discrete voltages in each direction.)

I also make stepper and PWM controller boards that can produce PDM and PWM signals at much higher rates and with much finer resolution that software. But, really, if you want ANALOG output, you really can't beat a good digital to ANALOG converter!

Jon