I need to control the output torque of an Electromagnetic Brake by means of the 0-5V output of my DAQ system.

Is there a comercially available transconductance amp which comes reletively plug and play which will let me do this.

Rating is 24V 1A max at 5V reference.

Cheers

 

You could likely use an op amp with a transistor follower buffer at the output. What is your supply voltage?

 

supply to the EM brake will be 24V but could get away with lower if needed. Its the 1A that im worried about, that seems a lot for a standard op amp emitter follower.

 

You can go to a current-limiting switching-mode control. There are quite a few available that can handle this. To minimize dissipation, you would want a synchronous switching regulator with at least 30V supply range and >1.5A max current.

 

would this work? can the op amp supply be +ve and Gnd, or does it have to be -V12V?

 

You can go to a current-limiting switching-mode control. There are quite a few available that can handle this. To minimize dissipation, you would want a synchronous switching regulator with at least 30V supply range and >1.5A max current.

Ok now im lost... any links you could throw my way?

Cheers for the input guys

 

would this work? can the op amp supply be +ve and Gnd, or does it have to be -V12V?

With the right op-amp, that could work. I think if you search a bit for power op-amps you can find one.

But it does require a large power dissipation by the op-amp, potentially just as much as your brake. It's a "linear" solution. The switch mode approach eliminates that power loss.

 

You can go with an opamp and an LM317 regulator or darlington power transistor on a moderately large heatsink. Assuming the brake has 24 Ohm DC resistance, and you run it from a 24V supply, you would have to dissipate around 6 Watts with an output of 12V (approx worst case). There may be linear regulator kits you can find.

For switching-mode, I looked at this on Jameco - switching-mode regulator for motor control - up to 35V and up to 6.5A -- cost is $24:
http://www.jameco.com/webapp/wcs/st...01&productId=120539&catalogId=10001&CID=MERCH
Another, more elegant design is a development kit from Maxim for their MAX15020EVKIT+ - available at:
http://www.digikey.com/
for $56. - allows up to 30V output at up to 2A.
Switching-mode circuitry is not for the novice -- if you want to use it, use a pre-built design like one of these.

 

I've never used one, but this op-amp sounds like it would be more than adequate.

 

That opamp (OPA548) looks fine -- just remember you will need to heatsink it. It appears that it should work fine from a single supply. Be sure to use good bypassing. If you use your earlier circuit, you will dissipate 5W in the current sense resistor and lose 5V from the supply voltage, in addition to the voltage drop within the IC (probably about 3V @ 1A), so you would need a power supply of at least 32V to get 24V across the brake solenoid. Another possibility would be to use a 1 Ohm resistor to sense the current and a 5:1 resistor divider to the input of the opamp to give you the proper scale factor. This will reduce the dissipation in the sense resistor to 1W max (use a part rated to at least 2W), and the required supply voltage to 28V. You can use a higher supply voltage power supply and just dissipate the rest in the IC.
Good luck!

 

Depending on the type of brake and if you need variable torque control, you might consider a commercially available purpose-built brake controller from the likes of magpowr, Stearns, et. al. Because sometimes it isn't as simple as delivering a specific current. There are things like hysteresis and stiction to consider, which these products are designed to account for.

 

Depending on the type of brake and if you need variable torque control, you might consider a commercially available purpose-built brake controller from the likes of magpowr, Stearns, et. al. Because sometimes it isn't as simple as delivering a specific current. There are things like hysteresis and stiction to consider, which these products are designed to account for.

Its funny you mention stiction, im usign the EM brake in a rig to measure spool sticktion using return times with varying oposing forces (hense the variable torque)

I will have a look at these ideas, thanks all of you for the help, you are pure class.

 

DC to PWM modulator looks like the way to go and should be acurate enough for my purpose. Cant believe i forgot about using PWM instead of steady DC...

Thankyou so much for the help guys.

 

I have use the LM759 to drive a servo valve, it should work with a brake of that current.
Max.

 

It seems to me as if this would be very hard to control, if it's the most common kind of brake that uses a disk that's pulled forward to contact a high-friction surface. You'd put a certain amount of current through the coil, and at some point (unknowable in advance) the disk would jump forward and the brake would be on, and then the more current you apply, the more torque would be developed. Then when you reduce the current, the torque would lessen until at some point (again unknowable, but less than the pull-in current) the spring would snap the disk back and there'd be no more torque. But really, electromagnetic brakes are designed for on-off operation. If you want something that's got a linear current/torque relationship, you'd need a magnetic particle brake, where the elements (a disk and a drum of particles) are in contact all the time, and the drag depends on the magnetic field. Like this:

http://www.warnerelectric.com/magnetic-particle-clutches-brakes.asp

Or you can set up an eddy current brake, which has no contacting parts at all, but it depends on relative movement to create the braking force.