Ask about Generate and Measure PWM with PXI6624

[Thang Nguyen 2]

I posted this queston on NI forum but I still not get any asnwer for it, so I hope I can get help here. I need to finish this project soon.

I am working on a project which require both measure and generate PWM signals.

Currently I have some issues with both of these.

I use PXI NI 6624 card.

1. Issue with measure PWM.

I connect the signal to Gate + and GND to Gate -. I expect I can read the width of the high pulse, casue the gate will start the counter.

I use the code as in example of measure the duty cycle (in picture 1). The problem I have is the values in the array also include the low pulse, and these two values will be shifted countinously in the array. So if I index from the array I cannot get a stable number (in picture 2).

Ex:

1. 2-5-2-5 <-----

2. 5-2-5-2 <-----

3. 2-5-2-5 <-----

I want to know the reason and how to fix it.

2. Issue with generate PWM.

I use another counter to generate the PWM.

I connect Vdd and Vss with a 13.8V DC Power Supply. I use a 10 KOhm resister connect between Vss and Out. I got a right signal with 13.8 V.

a> My first problem is the frequency is not stable. I set it to 122 Hz, and I saw it jump from 119Hz to 125Hz.

b> My second problem is when I connect this to the measure PWM I did above the voltage will down to 2.5 V. I tried with 1 KOhm resister and it downs to 6V. When I connect a real controller with a 12V PWM, this signal is stable after I connect with measure PWM channel above.

I would like to know if I did anything wrong. Cause this PWM will be use to simulate a sensor value and it must be stable.

I highly appreciate reading and answering my questions.

Best regards,

Thang Nguyen

Edited August 7, 2009 by Thang Nguyen 2

[Thang Nguyen]

Hi,

Currently, I can measure the PWM quite good but when the duty cycle is more than 70%, the low pulse time start fluctuate between the low pulse time and high pulse time. This lead to the duty cycle will fluctuate between the right value and 50%.

Some one with experience please help me on this.

I attach here my test program which has this issue.

TestPWM.zip