PWM output from CVS stops working

[KevD]

I’m using the NI CVS 1456 to generate a PWM to control some motors for a robot. I’m able to successfully generate a PWM from the TTL outputs but for some reason it stops working and stays high/low after a short amount of time. I’ve tried experimenting with different solutions and it appears to shutoff much quicker when i change the width of the PWM by a lot and do it fast. For example, if the width changes from 500 microseconds to 200 microseconds and this value changes every second, the PWM stops working after around 10 seconds. If i decrease the max step size the width can change and increase the amount of time it changes then it appears to last longer, around 30 seconds. If i leave it constant without changing anything then it works constantly. When it stops working, nothing else appears to change, the other outputs still work and the CVS still has power and runs the labview program, any help would be appreciated, thanks.

[Wee-Bey]

How are you setting your triggers up within LabVIEW? This could be an issue with the loops and how you're configuring your CVS to produce triggers.

[KevD]

Hi Ravi,

Thanks for the reply. The trigger is set to immediate. i want it to continuosly output pwm signals as soon as i start so immediate seemed like the right choice. Also, the mode is set to continuous. Im not sure if that matters but i tried all the other modes and nothing really changed. 

[Wee-Bey]

It sounds like your code might be somehow setting the pulses to stop after a certain amount of time while you are changing the pulse width. What happens if you just change the pulse width once?

[KevD]

It depends on how big the jump in width is. If it goes from say 800 to 50 microseconds it stops outputting immediately but smaller jumps seem more successful. I have the same problem with the example program Pulse Generation Output.vi. Ive attached the block diagram to help explain, but this is essentially what im using in my program. When I make large changes in the width the pwm stop working, perhaps you can explain why its doing that in the example program, thanks

[Wee-Bey]

Hmm, this could end up being an issue with the hardware if it's behaving like this. Have you tried this with other ports on the device?

[KevD]

I have tried it on all the TTL outputs and the 2 trig outputs that can do pwm and I get the same behavior from all of them. 

[Wee-Bey]

Also, how are you confirming that this is what the behavior is? What is the device you are using to measure this pulse that seems to measure it dying after a while?

[KevD]

through an oscilloscope, but thats not the problem because it still fails even if i dont attach anything to it and check it after a few minutes. If you could just give me the reasons the read/write for some outputs stop working while others continue to work that would be very helpful since i cant find anything on that subject

[Wee-Bey]

You mentioned that this is being used to drive motors. Does the behavior exhibit itself even when you're not connected to these motors? What are the motor's specifications? If you have a model number or spec sheet, verify that they are being driven correctly. What you're experiencing isn't expected behavior, so it could be an issue with the circuit that you've implemented (aka, the motors and their characteristics).