Convert PWM to Analog out

[Kyle Grathwol]

Kyle!
Happy Christmas and good luck with your project.

Merry Christmas to you Bits & Bytes. Thanks for all your help.

Hi Norm, The Moeller controllers you suggested look a lot like AB's PICO line. I am using PICO's in some horrible environments and they have proven to be very reliable. Good suggestion though. I think I will have to go chip level for this project.

OT:By the way Norm I purchased this http://www.factorycomm.com/unigateclco.html for the CANOpen communications that you and I discussed in the past. I will be working on it over Christmas break.

Merry Christmas to all and thank you all very much for your help!

 

[ndzied1]

Hi Norm, The Moeller controllers you suggested look a lot like AB's PICO line. I am using PICO's in some horrible environments and they have proven to be very reliable. Good suggestion though.

There is a very good reason for that. AB buys them from Moeller and puts their name and the Pico brand name on them

 

[chois]

Bringing this one back from the dead, but as it relates closely to an application I am trying to solve, I figured that would be better than a fresh post.

First of all, I am way out of my knowledge base here, and I know it. I am an ME trying to put together a control system for an oddball application. I'll lay out what I am doing, why and why I think I may be able to use the RC filter concept to accomplish it. Any feed back would be greatly appreciated.

The application is controling the fuel injection system on one race car - not an industrial or commercial thread to it. Just trying to build a better mouse trap and win a national championship.

The rule set requires that we retain the original type of fuel injection, which in this case is a continuous flow Bosch CIS type system. It differes from common current technology in that today you have digitally operated solenoid valves acting as fuel injectors, while the CIS stuff has mechanical valves that spay fuel continuously. Modern systems vary the duty cycle of the injector via PWM, and fire the injectors based on engine speed, so the frequency does vary. The CIS system controls the fuel flow to the injector lines with mechanical or electro mechanical devices.

There are several off the shelf user programmable fuel injection control systems available - all of them designed for modern electronic injectors.

I would like to utilize one of these systems, as they already accomodate all of the inputs and outputs, as well as appropriate algorithms to control the engine as desired (spark, fueling, appropriate reaction to temperature, load, and other factors). The only fly in the ointment is the duty cycle signal coming out of the system.

I would like to convert this to an analog signal - voltage or current - to drive a flow control valve, thus metering fuel through the old hardware with programable control.

Here is some more detailed information on a typical engine management system with fuel injection control.
http://www.bgsoflex.com/megasquirt.html

So I have a signal that switches to ground in order to fire solenoid style injectors, operating on a frequency proportional to engine speed (4 cycle engine, 4 cylinders, 1000-9500rpm = approx 33 -> 158 Hz), varying the pulse width based on the fuel required at the load/rpm state of the engine, and need to turn that into an analog signal (integrate the 'area under the curve' of the PWM injector signal?).

Can anyone help?

Thank you - Chris

 

[Peter Nachtwey]

I think a RC filter would work poorly because of the lag. You need to use a micro controller, DSP or FPGA to measure the pulse with using a high speed counter. This pulse width needs to be immediate scaled to output to a digital to analog converter. Now there is no delay. This is a hardware problem. Just about any 16 bit DSP would do the trick. You could buy a DSP starter or development kit for a few hundred bucks and get a prototype going. When you know what you want you lay out a four board ( at least ) using a board lay service like PCBexpress. A good hardware person could get this done within two weeks.

 

[rdrast]

Or, if you don't want to play around with a DSP, you might consider giving the reliable old standby a shot.. A CMOS 4046 PLL Chip. At those frequencies, the lock up and tracking time should be no problem.

For some in-depth guidance along those lines, get a copy of Don Lancaster's "CMOS Cookbook". He devotes at least a chapter to that chip, and goes into decent detail about configuring the loop filter.

 

[chois]

Thank you both for nudging me off in a sensible direction. I did a little looking last night and see that I have a fair amount of self education to complete in order to achieve the goal with those options.

thanks,
Chris