PWM controller interfacing help needed

[Black6spdZ]

I was wondering if anyone had knowledge in PWM control and could help me interface an off the shelf controller with a 0-5v pressure sensor to create a constant pressure pump controller. I was looking at this controller, a 15A controller with POT or 0-5v input
http://www.criticalvelocity.com/products/datasheets/cv-spd-2115e.pdf

and this 1 bar 0-5v linear pressure sensor
http://www.aemelectronics.com/1-bar-15-psig-brass-sensor-kit-1179

I'm not quite sure how to connect the two while allowing an adjustment to set the desired output pressure of my pump. For instance.. if I wanted to maintain 170kPa ~10 psi this would be about 3.1v output from the sensor.. what else would be needed? some sort of amplifier I'm guessing to keep a narrow pressure range. Any input would be greatly appreciated. Thanks

 

[Harald Kapp]

It depends on teh characteristic of the controller:

If at 3.1 V input the generated pressure is above your goal (170kPa), then you obviously need to lower the input signal. A simple potentiometer can do the job and allows you to adjust the pressure level.

If at 3.1 V input the generated pressure is below your goal (170kPa), then you obviously need an amplifier.

Assuming the input/output transfer characteristics of both sensor and controller/pump are linear. If the characteristics don't match, things become more compülicated. You may then need to insert a servo amplifier between sensor and controller.

In any case stability can become a problem. By connecting the sensor to the controller and the controller (via pump and the medium) back to the sensor) you create feedback. Depending on the parameters of the feedback loop (gain, phase, delay), the system may start to oscillate (as in: sensor registers low pressure -> controller increases pressure -> pressure rises above setpoint -> sensor registers high pressure -> controler lower pressure ...)

I have only limited knowledge of control theory so I can't help with this issue in detail. You may want to just try it out. If the system starts oscillating, one way of stabilizing it is to reduce the gain of the feedback loop. Since you probably have no influence on the transfer characteristics of either sensor or controller, you will then have to put a stabilizing circuit in the signal path from sensor to controller. the setup in principle could look like this:

The potentiometer allows you to reduce the gain while the adjustable voltage source allows you to set the operating point (in practice you wouldn't use a battery but some electronic circuit to achieve this).

Harald

 

[BobK]

Harald,

Isn't your circuit providing a control voltage in the wrong direction? Your circuit may shift the level and the gain, but it is still providing a higher control voltage for a higher pressure, and (I presume) the higher control voltage raises the pressure. Isn't an inverting amplifiter needed, such the a higher pressure lowers the control voltage?

Bob

bob

 

[Black6spdZ]

BobK, I think you are correct the sensor raises voltage with pressure and the controller raises PWM % with voltage so it wouldn't work without an inverted input. If I had to build a complicated secondary circuit to achieve the desired results I might as well build a programmable controller from a microcontroller. What if a could find a suitable resistive pressure sensor? could we then create a simple voltage divider with adjusting POTs to come up with the same goal?

 

[Harald Kapp]

I admit I didn't look at the datasheets in detail. I guess you're right, an inverting amplifier will be required.
I beg your pardon for my lazyness.

You could achieve the required direction of the control voltage by placing the sensor inverted (sensor "+" to battery "+") in series with a constant voltage source (adjustable for setting the operating point), Thus a higher sensor voltage will effectively lower the voltage of the series circuit. This works as long as the sensor has a separate power supply from the battery. One would still have to thin k of a circuit (poti or similar) to set the gain.

All in all I think a simple µcontroller could be the easier solution here.



Harald

 

[Black6spdZ]

I think I have found the perfect controller for my project.. almost perfect anyway

http://www.pololu.com/catalog/category/94

Small, compact programmable controller with 0-3.3v analog input "would need a simple voltage divider to take my pressure sensor's 5v to 3.3v max on the controller's input. According to the manual the input can be scaled to any input range and inverted.. perfect! I wonder what would be needed to keep the pump from oscillating? maybe a very narrow input range?