LEM Current Transducer

I've used 3 of the LEM LA50-P current sensors for just that, testing a motor controller.

What exactly are your concerns?

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: "skypeanalog"  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
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Well, I'm gonna send a PWM output command from a dsPIC to a MOSFET. Then I'll connect the Motor to the MOSFET (I think you know better then me those steps) and I'll send back the current absorbed from the motor to the dsPIC. Hence having the current value as a feedback I can develop a controller which will change the PWM duty cycle in the dsPIC. For this purpose I need to build a PCB. I'm not very skilled in that but I'm improving myself everyday (at least I try). I want to know if I have to adapt the signal output of the current transducer or I can use the Vout directly as input of the dsPIC A/D.

Thank you in advance, Pasquale.

My motor control had an ANALOG brain ;-)

EC (commutation) feedback from the winding ends.

I only used the LEM modules to display 'scope information.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: "skypeanalog"  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave

What kind of motor? There may be a lot of noise riding on the current transducer output. Might need some filtering or, in the case of stepper motors, sampling.

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Regards, Joerg

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It's not a stepper. It's DC motor and I think should be a low pass filter in itself.

Thanks, Pasquale.

See if your dsPIC can do "symmetric" PWM - instead of a sawtooth going into the PWM comparator, it uses a triangle.

This ensures that the peaks and troughs of the triangle *always* occur at the centre of the output pulses, regardless of duty cycle.

these peaks and troughs can be used to trigger interrupts (well they can in a sensible micro, who knows what the dsPIC might do)

Then sample the current inputs at the peaks and troughs. This puts the current samples as far away as possible from the noise-generating edges.

I havent used this particular LEM transducer, but I have used (read as: designed into commercial AC motor controllers) LOTS of other LEM transducers

you will almost certainly need to alter the LEM CT output to suit your dsPIC. The LEM transducer outputs a 0-5V signal, centered at 2.5V. the CT will measure up to 3x rated current (which is 6A for 1T, 3A for 2T or

2A for 3T), giving 2.5V +/- 1.875V

I am way too lazy to look up the dsPIC analogue input specs, but I wont be surprised if the analogue inputs are, say, 0-3.3V. So you may need to run the LEM CTs thru a voltage divider first. you must NEVER overdrive ADC inputs, if you want believable results.

Until you know what a P-I controller is, you wont have much success. I suggest you go buy a copy of Tim Wescotts book "Applied Control Theory for Embedded Systems". I havent read it but Tim is a regular at SED and knows stupidly large amounts about building embedded control loops (Tim, I will buy your book, Im just waiting for it to turn up 2nd hand ;)

Cheers Terry

If it's a regular brushed motor it will generate a lot of spikes. Best is to check the LEM output with a fast oscilloscope.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

"Joerg"

** ROTFL !!!!!!!!

So now you need a " fast oscilloscope " to check the output signal of an analog device with only 250 kHz bandwidth ??

Hate to see one of the slow scopes the * Jerk * imagines exist.

..... Phil

Hi, thank you very much for you explanation. I've found it really interesting. I think I need a voltage divider because the dsPIC input in

0-3.3V : any suggestion ? Which component I should use ? I've downloaded the LEM transducer datasheet but I cannot find a lot of information. Do you have some more stuff to let me see ? Luckly I know quite good P-I controller so at least in this stuff I shouldn't have problem (at least I hope).

Thank you, Pasquale.