Best way to brake a DC motor

cmartinez

Joined Jan 17, 2007
6,886
How far does the Motor travel, after power off, with a high amp Schottky Reverse Bias Diode attached to the motor?
If too much, then add a low ohm resistor in series with diode?
Is that enough Dynamic Braking?
And Mvas is right. Did you connect an inverse parallel diode to the motor? A schottky type would be best.
 

Thread Starter

gerstley

Joined Nov 20, 2015
58
I am using a monostable 555. The sensor causes the 555 output to go low and stop the motor. The footswitch makes the 555 go high and starts the motor regardless of whether the light sensor output is on or off.
And Mvas is right. Did you connect an inverse parallel diode to the motor? A schottky type would be best
I was going to ask about that. I did put an N4004 diode in parallel as a flyback diode. Would that be the same idea? It had no effect on the motor stopping. I also tried putting a 2 ohm resistor in parallel. It also had no effect. Maybe I was doing something wrong.
 

cmartinez

Joined Jan 17, 2007
6,886
I am using a monostable 555. The sensor causes the 555 output to go low and stop the motor. The footswitch makes the 555 go high and starts the motor regardless of whether the light sensor output is on or off.

I was going to ask about that. I did put an N4004 diode in parallel as a flyback diode. Would that be the same idea? It had no effect on the motor stopping. I also tried putting a 2 ohm resistor in parallel. It also had no effect. Maybe I was doing something wrong.
You do not need to use the 555, at all. In fact, it might be contributing to your problem. All you need is the proper interface circuitry to the Fet you're using.

Can you post a sketch of how things are exactly arranged at this moment?
 

djsfantasi

Joined Apr 11, 2010
6,338
Why that particular motor? I would think that a less powerful one could be used, which wouldn’t run out as much and be easier to stop.
 

Thread Starter

gerstley

Joined Nov 20, 2015
58
The light sensor detects the narrow gap between coupons. By the time the motor stops, the gap will have passed by the sensor. The 555 stops the motor as soon as light is detected and keeps the motor off even when the sensor sees darkness because it has drifted beyond the gap. The footswitch turns on the motor whether the sensor sees light or darkness. This is current circuit I am working with.
motor control5.jpg
 

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mvas

Joined Jun 19, 2017
537
Another possibility with this motor is ...
Use a Full-H Bridge = Fwd & Rev
a) Press the foot pedal to eject a coupon
b) Stop the motor, as best as possible, when the Photo Sensor says STOP = some over-shoot
c) Then reverse the motor at some slower speed ( 25% ? ) and retract the overshoot
d) Stop the motor ( now more accurately ) when the Photo Sensor says STOP again.
e) Tear off the coupon
repeat ...

Precisely stopping a high speed motor that is geared down is difficult to do in an Open Loop configuration.
Especially when Dynamic Braking seems to have little impact.
A Closed-Loop Servo or a Stepper Motor makes more sense here.
 
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MaxHeadRoom

Joined Jul 18, 2013
20,425
I can see a 8krpm motor stopping and starting every few seconds getting pretty hot, even without the braking.
A closed loop would imply a deceleration of some time but within that small a window it doesn't seem practical to me.
It may not be possible but I still would have thought that a continuous running motor and a mechanical disconnect of the drive would be a better solution.
Max.
.
 

cmartinez

Joined Jan 17, 2007
6,886
I agree with Mvas. And also think that using a stepper would be a better choice. But maybe you've already invested too many resources in this project to change paths.

Gotta get busy, I'll be back tonight and take a closer look.
 

MaxHeadRoom

Joined Jul 18, 2013
20,425
I would agree that a stepper would be more practical than closed loop, but a problem of getting a stepper to run at 8+krpm, unless different gearing.
Max.
 

cmartinez

Joined Jan 17, 2007
6,886
I would agree that a stepper would be more practical than closed loop, but a problem of getting a stepper to run at 8+krpm, unless different gearing.
Max.
Yeap, a stepper with a different gearbox ratio would be ideal. And then you'd need to create the proper accel/decel profile circuitry.
 

Thread Starter

gerstley

Joined Nov 20, 2015
58
Precisely stopping a high speed motor that is geared down is difficult to do in an Open Loop configuration
Overshoot by the motor is actually not a problem as long as the overshoot is consistently the same from coupon to coupon. The overshoot is compensated for when setting up for a run of coupons. The problem comes when the overshoot varies from coupon to coupon. When the overshoot is 1/10th of a revolution, a +/-10% overshoot is not significant. When the overshoot is 1 revolution, a +/-10% overshoot becomes a problem. That is why I want to brake the motor. I was hoping there would be a simple way to use an inexpensive brushed motor but I am sure a stepper motor would be a better solution. It all comes down to money I suppose.
 

MaxHeadRoom

Joined Jul 18, 2013
20,425
The up side to a stepper it may not need accel/decel, IOW if a stepper motor is stopped, i.e. no input command with motor power still applied, its stationary holding torque kicks in providing automatic braking.
Max.
 

cmartinez

Joined Jan 17, 2007
6,886
The up side to a stepper it may not need accel/decel, IOW if a stepper motor is stopped, i.e. no input command with motor power still applied, its stationary holding torque kicks in providing automatic braking.
Max.
With a little rattling during start/stop possibly... but yes, it could work in a consistent way
 

BR-549

Joined Sep 22, 2013
4,938
I'm with Max. It's hard to determine the actual problem without more details. A high ratio gearbox can store momentum. There might be mechanical tension adjustments.

Now a days.....discreet drive is the way to go.
 

Tonyr1084

Joined Sep 24, 2015
4,375
For me the thing a stepper means with each signal to dispense you can have the stepper step N number of steps and stop. If the coupon requires 200 steps to dispense a coupon then that is easy to program in. Have the motor run so far and no further. A sensor can verify the correct position, and if needed - programming can move the coupon a little further or retract a slight bit till the coupon is perfectly dispensed. As for speed; what's the difference if it takes a coupon 1.1 seconds to dispense or 1.2 seconds? Or even 2.0 seconds. If I'm getting a coupon that is going to save me some change waiting the extra second isn't going to kill me. I'll just wait.
 

mvas

Joined Jun 19, 2017
537
I can see a 8krpm motor stopping and starting every few seconds getting pretty hot, even without the braking.
A closed loop would imply a deceleration of some time but within that small a window it doesn't seem practical to me.
It may not be possible but I still would have thought that a continuous running motor and a mechanical disconnect of the drive would be a better solution.
Max.
.
Closed Loop control can decelerate and stop the motor with +/- 1 Encoder Count after the Photo Sensor says "STOP" = repeatability.
Repeatability means, you can now adjust the position of the Photo Sensor so that the tear-off on the coupon is perfect every time.

There are inexpensive Closed Loop PWM H-Bridge Motor Controllers for PM DC Motors with encoders that have ...
a) Precise Acceleration Ramps
b) Precise Velocity
c) Precise Deceleration Ramps
d) And even Holding Torque when stopped !

Holding Torque prevents the spur gears from turning when you pull on the coupon.
 
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MaxHeadRoom

Joined Jul 18, 2013
20,425
Closed Loop control can decelerate and stop the motor with +/- 1 Encoder Count after the Photo Sensor says "STOP" = repeatability.
Repeatability means, you can now adjust the position of the Photo Sensor so that the tear-off on the coupon is perfect every time.

There are inexpensive Closed Loop PWM H-Bridge Motor Controllers for PM DC Motors with encoders that have ...
a) Precise Acceleration Ramps
b) Precise Velocity
c) Precise Deceleration Ramps
d) And even Holding Torque when stopped !
I am aware what closed loop is having worked with and designed systems since the 80's, but it appears to me that your itemized list shows velocity control, not positioning which for a DC or BLDC motor generally requires a PID loop.
The stepper in this application offers positioning on pulse count and braking at stop using the steppers natural holding torque.
If the demands on the motor is fairly low rpm, it could be within the torque drop off point, allowing a very simple drive and power supply to be used.
The stepper would certainly be the route I would consider if designing the mechanism as described.
A Conclusive decision could only be made by witnessing the application in person.
Max.
 
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