How to sense motor current to actuate brake?

Here is the "Current Measurement Applications Handbook" from Zetex.

Heres Zetex's site."sensing for high voltage motor control"

Current Limiter for the Motor Control ICs from nxp

If you search Digikey or other distrubutors for current sense you will find a couple of hundred integrated high side current sensors of varying prices and accuaracies.

The current increases with the load on the motor. If you use that to apply a brake, you have *positive* feedback, which will quickly lock the shaft. What about measuring the actual force on the load?

Unless he's using the motor in regen state to slow down a load? We do that our self's at work if the motor exhibits to much current in regen state for over a set time.. The brake has an DC amplifier/Drive with slow accel set.

Is this for user feedback or some sort ot automatic control loop?

Bye. Jasen

yeah, that's what I was thinking; shouldn't it be the less current, the more the brake? which also gives you the opportunity of applying the brake more or less proportional to the voltage across the motor.

If the brake exerts force on a shaft, it's just another servo-motor, isn't it? That suggests you use two motors, two amplifiers, and some proportional inputs to the amplifiers.

Or, do you mean 'drag' in the sense of viscous drag, a force that is proportional to the square of velocity?

You mention 'force feedback', is that because there is some unknown third quantity (variation of the driven mass, or friction, or inelastic deformation) in the system, and you want to compensate in some way for that quantity, in real time, rather than prepare/design for a fixed value?

Thanks for the info Hammy. I'll check out the links. Eric

I wasn't clear in my first post. Re-reading it I see that I imply that the brake goes on the motor shaft. The brake actually is for another shaft that is turned by hand. This shaft has an encoder and a dial on it. I have already made the setup and turning the shaft with the dial causes the servo motor to follow. The next step is to load the servo and for me to sense the load by a drag put on the shaft I'm turning. Thanks for all the replies so far. Eric

--- Nicely stated.

You want force feedback so that as the load on the servo motor varies, you want that load to be reflected back to the crank you're turning by making it harder to turn when the servo's heavily loaded and easier to turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a permanent-magnet DC motor to the shaft you turn by hand, and then use a MOSFET's drain and source terminals connected across the input terminals of the motor in order to change its [the MOSFET's] resistance as a function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the resistance of the MOSFET and the more difficult it would be turn your shaft, according to Lenz's law. JF

Greetings John, If I understand your post the voltage across a shunt placed in one of the servo motor power leads could be used, by connecting to the gate of a MOSFET, as a variable resistor shorting the power leads of a PMDC motor in order to make it hard to turn. Is that correct? I have shorted the leads on several PMDC motors that I have and none of them have enough drag. A couple servo motors I have come close but they are way too big, and too valuable, to use. My idea is to use a magnetic brake. I already have one and a mock up shows that it will work fine. This is good because it's a stock item and I might need lots of them. Thanks, Eric

Greetings John, Thanks for your replies so far. I don't have a complete solution yet because I don't really know how to do the elecronics part. I should have been even more clear and stated that I had the brake already. It is a 24 volt device and it draws only about 3 amps. And even though I kind of understand in theory about how a MOSFET works I don't know enough yet how to decide which one to use. On top of that, I had a hard time believing it would be so simple, just a shunt and a MOSFET. Since the MOSFET will be acting as a variable reisitor will it need to dissipate as many watts as a variable resistor would? And in order to make the brake adjustable electrically wouldn't it be best to vary the voltage supply to the brake instead of using a variable resistor in series with the power supply to the brake? Thanks, Eric

And the goal is to make the brake respond proportional to the (very large) current in the servo motor, assuming current is proportional to the motor force.

If you rewound the brake so it was (what was it? 80 amps?) instead of 3 amps, you could just wire it in series with the servo motor. What you want is proportional current in the brake and the motor.

If the servo amplifier is a switching type, you can probably program a switchmode power supply with a copy of the servo internal switch signal, to make a slave current source to drive the brake.

Thanks for the ideas. But what I need is something adjustable and as simple and as inexpensive as possible. The amount of force feedback will be different for different setups and people. So the ability to use stock components and easy adjustability are important. Thanks again, Eric

Greetings John, The schematics help some. I thought I might need an op-amp. Now I'll look in the books to figure out how to choose which op-amp and what R1 and R2 do. I'll let you know how things work out. Thanks, Eric