Motor Control - how to "stop dead"

Hi there,

My question is about how much "opposite" current is required to stop a motor dead.

e.g. 60 RPM geared DC regular motor is turning, we want it to stop a X angle/point. If the power is simply "removed/stopped" there will be a minor amount of ....not sure the word...... lag? where the motor keeps rotating for a few milliseconds from its momentum.

At the point of "removing/stopping" current to the motor, how much [opposite] power should be applied to stop it dead? (i.e. turning anticlockwise, power removed/turned off, then how much clockwise power should be applied to stop it dead?).

The motor:

6 v
60RPM
controlling 0.15 KGCM

Not expecting definate answer as not enough detail - would just like some pointers.

thanks in advance / john

If you stop the motor that fast then you will probably strip the gears.

There are a lot of variables to stopping a motor by applying a counter pulse, and I don't think it can be done without some sort of feedback loop. I did a similar thing a few years ago, but incorporated a magnet and two Hall detectors (to determine direction), then applied the counter pulse as a PWM until the feedback loop indicated the motor had stopped. It stopped quickly, but there was a little bit of overshoot, about 1/2 coasting revolution in the opposite direction.

Now, 60 RPM is pretty slow, and you may be able to stop it with more accuracy. What type of accuracy do you need? Is the motor free wheeling or is it driving something. In the latter case, it would probably be easier to avoid overshoot. John

John

If you short a spinning DC motor then it stops very quickly. Model RC electric cars do that.

The motor I referenced was series wound and needed the impulse. I suspect the OP's motor is PM and a controlled short, like with a mosfet or SCR, might work. John

Edit: That's how the brake on our model aircraft motors work. My mind must have been off-line when I wrote my first response.

The motor stops rapidly if you short it after switching off.

If that is too slow use a coil activated brake to apply simutaneously when switching off.

Braking action directly at the motor shaft won't do any harm to the reduction gear. Also, the masses are less.

I am thinking about the heavy propellor on my electric model airplane. It is a very serious strain on the gears when the motor speeds up or slows down quickly.
A model car would also have momentum which might strip the grears if the motor is quickly rev'd or stopped.

audioguru said:

I am thinking about the heavy propellor on my electric model airplane. It is a very serious strain on the gears when the motor speeds up or slows down quickly.
A model car would also have momentum which might strip the grears if the motor is quickly rev'd or stopped.

Click to expand...

Are you using a reduction gear to propell a plane?

Boncuk said:

Braking action directly at the motor shaft won't do any harm to the reduction gear. Also, the masses are less.

Click to expand...

Dynamic braking in gearhead motors must be analyzed carefully if they are being used to drive inertial loads. The gearhead sits between the rotor and the inertial load. Because the inertial load wants to keep rotating and backdrive a gearhead after power is removed, both the inertia of the motor's rotor and the external load can subject the gearhead components to dynamic stresses far beyond their design points.

It is indeed something to keep in mind...

hi odm,
If the system requires that you stop the existing motor 'dead' ie: instantaneously, you are using the wrong type of motor.

What's the type of motor being used and for what application/system.?

Boncuk said:

Are you using a reduction gear to propell a plane?

Click to expand...

Yes and the propeller is large and creates a high thrust. But the propeller frequently breaks during a landing.
I have an identical motor without reduction gears and it has a small folding propeller that has low thrust but it has never broken.

Hi oem_odm,

The word is overshoot.

The 6v DC motor is geared down to run a shaft for some purpose which involves the angular positioning of the shaft.
You wish to stop the rotation of the motor and shaft when something or the other lines up relative to the shaft.
Have you tried short circuiting the motor in order to see how much angular overshoot occurs ?

Have you tried repositioning the angular position of the sensor,
so that when it operates and shorts the motor,
the shaft will then stop in the appropriate place?

I am guessing that this motor is on a worm.
I think you will need a mechanical brake assembly attached to the motor,
to operate in conjunction with cutting the feed to the motor.

And possibly a small speed-relative small spring loaded part to adjust the photo sensing arrangement a few degrees to take into account overshoot at low speeds,
before the motor is up to running speed.

Unless your arrangement is such that the shaft is never required to stop without the motor reaching running speed. (thats why i put possibly).

Such a small spring loaded part need only be a little piece that has a hole for the light to go through, pivots a little against a spring, and alters with motor speed.

If such a part is required, don't forget that when the motor is stopped, it will return to rest position.
This may cause the light to be interrupted, so have the circuit such that this won't make the motor start again.

Please post more details on the assembly. Pictures if possible.

Cheers, John

feedback and thanks

Thank you everyone, I didn't expect such support.

Some more details:

- motor drives a 50 mm dia shaft
- simple optical sensor used to count position, then stop (5 possible positions)
- i expect the angular position of optical sensing to be played about with, so, overshoot achieves desired position
- motor is not freewheeling, drives a small (cam like) load of 125 grams worth.
- motor is 4:1 geared
like this : https://en.wikipedia.org/wiki/Image:Annular_(PSF).svg
- motor has 0.1 kgcm torque, amounting to .4 kgcm at the shaft through the gearing
- need a regular dc motor as final application is for volume production - costs need to hit budget
- maybe stop dead is a little harsh. I want low cost methods of reducing overshoot.
- worried short circuiting motor will dramatically reduce motor lifetime
- can't use spring meachanism due to several positions required of motor
- motor needs to function in both directions anyway

thanks / john