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Flexible PWM measurement method?
David Moerman
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08-30-2004 10:00 PM
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I have a need for measuring up to 12 PWM inputs simultaneously with the following parameter ranges:
Voltage: 24V
Current: 250mA to 1500mA
Frequency: 200Hz to 2000Hz
Data acquisition update rate: 2 Hz
As you can see, a fairly slow update of a fairly broad range of signals. What is the best* way to do this? Since my signal amplitudes vary, the problem is a little tricky. A couple of ideas:
1) Since I have an SCXI chassis, I could use an SCXI switch to multiplex these 12 onto an NI-4060 DMM (after sampling with some low value resistors). I've seen electricians simply use their handheld multimeter's RMS-current function, and I believe the NI-4060 DMM could do the same (albeit a voltage RMS).
Drawbacks: lots of switching (
I would need to use solid state -- can I?), potentially slow DMM update. Would it work?
2) Use a counter card to time the duty cycle, and somehow use each rising edge to hardware-trigger an analog input measurement (preferably with some small built-in delay). Is this feasible? Sounds like a job for FPGA?
Thanks for your suggestions!
David Moerman
TruVIEW Technology Integration Ltd.
*least cost
Voltage: 24V
Current: 250mA to 1500mA
Frequency: 200Hz to 2000Hz
Data acquisition update rate: 2 Hz
As you can see, a fairly slow update of a fairly broad range of signals. What is the best* way to do this? Since my signal amplitudes vary, the problem is a little tricky. A couple of ideas:
1) Since I have an SCXI chassis, I could use an SCXI switch to multiplex these 12 onto an NI-4060 DMM (after sampling with some low value resistors). I've seen electricians simply use their handheld multimeter's RMS-current function, and I believe the NI-4060 DMM could do the same (albeit a voltage RMS).
Drawbacks: lots of switching (
I would need to use solid state -- can I?), potentially slow DMM update. Would it work?
2) Use a counter card to time the duty cycle, and somehow use each rising edge to hardware-trigger an analog input measurement (preferably with some small built-in delay). Is this feasible? Sounds like a job for FPGA?
Thanks for your suggestions!
David Moerman
TruVIEW Technology Integration Ltd.
*least cost
Stuart_G
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08-31-2004 07:01 AM
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Are you trying to get the PWM inputs back in terms of Voltage, Current or Frequency? Is all you want to do is read the value from the PWM? If so I would think you could do this with just a DAQ card.
Stuart G
Stuart G
08-31-2004 10:23 AM
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I am measuring the control current to hydraulic proportional valves, therefore I need to know the average current. This will also allow me to detect an open circuit. However, since I think I need to sample the current with a small-value high-power resistor, from the measurement hardware's perspective it will be in terms of voltage.
Also, I was having second thoughts about my 2 Hz sample requirement: I think I need to go faster than this if I want to plot the PWM signal.
So, even with a dedicated DAQ card sampling @ 20kS/s per channel, I don't get much accuracy if my PWM is operating at 2000Hz. Although this may have possibly worked if I just go with a slow DC average, then I would just need to average out a whole lot of samples. One problem
I have with this is an unnecessary processing tax on my system, since I am monitoring about 40 other channels simultaneously at 500Hz.
-Dave
Also, I was having second thoughts about my 2 Hz sample requirement: I think I need to go faster than this if I want to plot the PWM signal.
So, even with a dedicated DAQ card sampling @ 20kS/s per channel, I don't get much accuracy if my PWM is operating at 2000Hz. Although this may have possibly worked if I just go with a slow DC average, then I would just need to average out a whole lot of samples. One problem
I have with this is an unnecessary processing tax on my system, since I am monitoring about 40 other channels simultaneously at 500Hz.
-Dave
