Problem #1
I am replacing the BLDC driver and motor on my mill with a Durapulse VFD.  The Durapulse VFD uses a 0-10v analog signal input for spindle RPM.  The proprietary hardware electronics of the mill outputs a PWM signal so I have a C41 board (rev.1) from CNC4PC to convert the PWM signal to an analog voltage signal for the VFD.  Sound simple right?  Well the problem is that the PWM signal from the machine is inverted, meaning 0% duty cycle is max RPM and 100% duty cycle is min RPM or an active low configuration.  This is the way the supplied BLDC driver needs the signal.  The Mach3 interface has the same configuration to make it compatible.  I can change the Mach3 step setting to active high but I can't change the hardware of the machine so it will always output an active low signal.  The C41 board however only reads active high.

I think I need to put a circuit in between the Mill hardware that is outputting the backwards PWM signal and the C41 board to make it work.  The question is how do I invert the PWM signal, I think it should be easy to do but really don't know where to begin.  I'm assuming I have to make some type of op amp circuit.  Please help or offer suggestions.

Problem #2
Even when supplying the correct active high PWM signal directly from Mach3 to the C41 card I get no dynamic response on the output analog voltage.  There is just a constant ~1.3v which I can adjust slightly with the pot.  No matter what the input signal is to the c41 I can't get the output voltage to change or even get close to 10v.  Perhaps it's wired wrong?  I am supplying the external power with +12VDC and -12VDC for ground (I'm not sure if that is the correct way to do it).  I don't have the relays wired yet, I'm just trying to get an analog output voltage first.  Please tell me what I'm doing wrong.

Here is how it's wired:


Also here are some relevant images of the VFD terminals:


I'm banging my head against a wall trying to figure this out.

 

Problem #1) Assuming your PWM signal is 5 Volt then you could use just one gate of a simple TTL logic chip to invert the signal.

For example if using a 74LS00 then…
Input – pins 1 and 2 connected together.
Output – pin 3
+5 volt (regulated) – pin 14
GND – pin 7
The other unused inputs (pins 4,5,9,10,12,13) should also be connected to GND

Problem #2) When you say you are supplying -12 volts to the GND terminal I assume you really mean 0 volts ??

Tweakie.

Thanks tweaki!  I actually ended up using a 74ls04 not gate hex inverter thinking that would just be simpler.  It works!  The chip eats a little voltage though and when I add some indicator LEDs I really loose voltage, by about half.  But it does work and the pwm is going on the right direction.

I still can't get the c41 to do anything though.  There is just a constant output of 1.4v no matter what I do with the pwm input, even if I disconnect it completely.

The power is coming off the machine's breakout board and is supplied by two terminals from an on board 12v power supply that reads +V and -V and the voltage between the two measures 12.15v.  I have the +V connected to pwr and the -V connected to grd.  This is as I understand it the correct way to power it.  The -V/grd is not earth ground.

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Duh I think I see what the problem is.   The grd under the pwm input is for reference and not connected to the power grd like I thought.

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I everything seems to be basically working now.  But there is a new problem.  The original inverted pwm signal will cause the c41 output to reach 10v (when set to min rpm) but when I run the signal through the 74ls04 inverter chip the c41 will only reach 6v at max rpm.  The pot is maxed out.  I assume this is because the inverter eats too much voltage.  How can I fix this?

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If the circuit which you are connecting the TTL output to applies too much loading and the TTL voltage is falling then perhaps consider including a simple transistor driver.

This page may be of interest  http://www.bristolwatch.com/ele/transistor_drivers.htm

Tweakie.

The chip eats a little voltage though and when I add some indicator LEDs I really loose voltage, by about half.

You have a series resistor of the appropriate value on each LED yes?

I had a conversation with someone about this who knows a little about logic circuits.  He suggested that it may be the output current of the 74LS04 chip that is the problem.  The L stands for low power and outputs half of the current of a regular 7404 chip.  Does this make sense?  He Ali suggested trying a different ground between the c41 and the inverter chip.

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Basically, things are working in a general sense.  The unaltered pwm signal will get me 10v on the c41 but only at minimum rpm.  Inverting the pwm signal with a not gate hex inverter chip gets me the correct signal orientation on the c41 (duty cycle goes up as rpm goes up), but the max analog output voltage is only around 6v.  I did get it up to 8v once and I'm not sure how but now it's back to 6v.

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