Author Topic: Controls Braking vs Dynamic Braking for Motors  (Read 2156 times)

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Offline Tranq97Topic starter

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Controls Braking vs Dynamic Braking for Motors
« on: October 15, 2014, 03:55:48 PM »
Okay, so for a very large brushless DC motor, I need to be able to stop and slow the motor at chosen rates. I've looked up a lot of different options:
Mechanical Braking wear would cause too much hassle.
'Crossing the Poles' wouldn't be powerful enough and I wouldn't be able to chose the deceleration rates.
Controls braking, where the H-Bridge is toggled to run back on the motor, giving an opposing force against the momentum for the motor.
And dynamic braking, mostly used for locomotives and gives the 'generator effect' a hard time by giving the motor a load of resistance.
These motors would go on the arms of the robot, not wheels.
I simply need to explanation as to which braking method I should use and why.
I found a lot of the information here: http://www.societyofrobots.com/schematics_dcmotorbraking.shtml.
Thank you in advance.

Offline Kohanbash

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #1 on: October 19, 2014, 04:33:09 PM »

In many digital motor drives you can specify acceleration and deceleration rates.  After that a lot will depend on the type of gearing that you use and how much resistance that puts on your drive motor.

How much braking do you need? What do you consider a "large" motor?
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Offline Tranq97Topic starter

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #2 on: October 20, 2014, 03:35:51 PM »
Large motor, large motor... I'm not talking about one I possess, and I'm not sure how to judge it in kW. Let's just say the nominal dimensions are 150mm/150mm/250mm. That's probably as much as I can give you, sorry.

Now, this thing is going to go at high RPM and low torque, attached to a couple of gears to convert that to low RPM and high torque for the robot's arm. Before, I was trying to design a type of motor which could just run at high torque, low RPM. But, I found that that would hinder it's precision, accuracy and braking mechanisms. So, then I decided to use the gears with a high RPM motor. So, the braking on the arm will have to be virtually instant on the arm (big gear), so I don't know how quickly the motor (small gear) will have to stop to make that happen.

Basically, I'm building the motor and controller myself. The controller I have sorted. But electronic motors have always made me a little confused. With all this back EMF and whatnot. The things I need help with are as follows:
- Calculating motor requirements
- How to build a motor and account for those requirements
- What system can I use to change the acceleration rate?
- What system can I use to change the deceleration rate?
- What system can I use to hold a stationary motor in place?
- How to calculate gear sizes
- Are there any systems which can make the large gear more compact?
I'm 17, I've been interested in robotics for 4 years and it's all been textbooks, internet and college. I haven't had much practical experience at all, and about half of my education hasn't been learning, but rather developing ideas. As for the last point I made, basically I am worried that the large gear will be too big to fit in the robot's arm. So, are there any special gears which have the same effect as a big gear but a more compact? You seem like an expert on robotics, David, so I was wondering if you could give me a hand with these hurdles?

Offline Kohanbash

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #3 on: October 21, 2014, 09:50:30 AM »
Hi
Using gearing to get the high torque low RPM you need is the standard way of doing this. Most people do not make a custom motor for this (even though I know of several exceptions to this).

If you get a professional quality motor driver such as an Elmo or a Copley (I have a post on Copley drives that I am releasing in the next few days) you ca configure the acceleration and deceleration rates (it is just a parameter).

The nice thing wit using a bunch of gearing is that it will give you some resistance so there is a good chance you will not need any active braking to slow down. This should not be confused with a motor brake which you might need if you need to hold a position with the motor unpowered (or when stationary to conserve power). Note that motor brakes are typically only to be used when the motor is not in motion and not to slow down.

If you look on the main SOR site (or google it) you can find calculators for determining motor specs required.

Often harmonic drive gears are used for the compact size with good torque and low (or no) backdriving of the motor (which helps hold position).

A downside to professional hardware (motors, gears, controllers, etc..) is that they are often much more expensive than hobby level hardware. If you can find a hobby level digital motor drive it might be able to do the acceleration/deceleration stuff also.
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Offline Tranq97Topic starter

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #4 on: October 21, 2014, 10:16:58 AM »
Hi, thanks. This isn't for a hobby, I plan to start a business using a theory I have with computer architecture. But more to the point:
1. What types of motor brakes are there available?
2. I will have to build the motor driver myself (long story), the digital side of it isn't a problem, but I need to know what it's physically going to do to the motor (which I will also be building myself) in order to accelerate and decelerate at different rates.
3. I've seen a lot of designs where they increase torque and decrease RPM with many gears, rather than just one small one and one large one. Why is this?

Thanks. It's unbelievably hard to find someone on the internet willing to help.

Offline Tranq97Topic starter

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #5 on: October 21, 2014, 12:52:18 PM »
I've just used the motor sizing calculator on SoR, one of the motors was 16.2 kW but that's just from estimated data.

Offline Kohanbash

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #6 on: October 21, 2014, 02:59:55 PM »
1. Just google "motor brake". Often when you order a motor you can get encoders, brakes, gears, etc.. from the same vendor.

2. For the most part you know the position of your motor from hall sensors or an encoder. And the rate you spin it can change when you start or stop the motor. When you issue a stop command instead of instantly stopping you send the intermediate commands from current speed to 0 based on how fast you want to decelerate. In an extreme case you can imagine commanding the motor to go backwards to reach a desired deceleration rate.

Based on how you are braking or if you have gravity forces on your motor you might also need a shunt regulator in your design.

3. There are a few reasons. The simplest can be to achieve a desired form factor. Another common reason is that certain gear boxes will have a max input speed, so you need another gear box to slow the shaft enough for the second gear box.

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Offline Tranq97Topic starter

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #7 on: October 22, 2014, 04:26:22 AM »
Thanks!

Offline SeekingVision

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Re: Controls Braking vs Dynamic Braking for Motors
« Reply #8 on: November 19, 2014, 01:08:17 PM »
I have dealt with this issue with DC motors above 2 kW and have settled on H-bridge with active breaking through 1ohm resistors and PWM control that with shaft encoder will set speed or no speed if required. Diodes bypassing the mosfets solve any inductance issues.
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