Victor braking feature

[MattB703]

Has anybody out there experimented with the brake feature on the victors. It does not seem to be documented very well. Here are some of the questions I have been trying to answer.

1. I think that the back torque on a motor that is connected to a victor set on brake is linearly proportional to the speed that it is backdriven at. Is this true in theory? Is it true in practice?

2. If a motor is hooked to a Victor that is set on brake and the motor is spun at its free speed, will it develop a back torque that is theoretically the same as the stall torque?

3. What happens if you spin it faster than its free speed?

4. If you keep spinning the motor faster what will be the failure mode? Melting the victor? "Grenading" the motor?

Thanks
Matt Berube
Teams 902 and 703

[ateene]

The victor dynamic breaking essentially shorts the motor leads when the inputs are neutral. This makes the motor work like a genertor causing incresing the resistance thus stoping it faster. It doesn't effect normal operation. You can use it to stop your motors faster, but you would need to consider the impact on your drive system componets.

We typically don't use dynamic breaking for our drive, but have used it for arms.

Andres
Teams 159

[The Lucas]

It may be alright to use the brake function on your drive system if you have two drive motors, but definitely not if you have four drive motors. The two different types of motors will brake at different speeds, fighting each other along the way. This could destroy carpet, gears, motors, etc.

[ahecht]

A motor with the victor's "brake" on is much like a motor that is connected to a spike. The spike or victor shorts out the motor preventing it from moving. You can try this yourself. Try spinning one of your motors, then connect the two leads together and try again. It should be much harder to turn.

[Al Skierkiewicz]

To make this more accurate,
DC, brush type, permanent magnet, motors will act as generators when the shaft is turned. The output voltage is proportional to the speed of the shaft rotation. If a load is connected to the motor, then current will flow. In the case of the speed controller "brake" function when there is no speed command the output transistors are turned on in such a way that two of the four output banks are turned on across the motor. This produces a very low resistance, (less than an ohm, close to a short) and all the motor current is dumped into the speed controller. The current flowing produces an effect known as "back EMF" which acts against the rotation of the motor shaft and thus the braking. Trains, subway cars, and a variety of other appliances use this to slow down, a technigue known as "dynamic braking". In large motors, a bank of resistors are switched in groups to control the amount of braking. The lower the resistance the more braking. They are very easy to see on the under side of subway cars and trains.
Most teams use this feature with care, the braking can be so severe that robots with a high center of gravity can easily tip when braking.
Hope this helps.

[MattB703]

I really appreciate all of the replies to my post. However, it is not anything I didn't all ready know.

I don't want to go into the details of my idea just yet, but I am not planning on using the brake feature on primary drive motors.

What i really need to be able to do is predict the toque value I will get from a motor being backdriven at a certain speed. If the motor is backdriven against a torque then work is being done. Where does the energy go? I think it comes out of the victor as heat. Is this correct? How much of this can the victor take? And what happens if the motor is backdriven at a very high rate? Will the voltage and current continue to climb until something melts?

Thanks again
Matt

[Matt Attallah]

A word of warning...

We used the breaking option last year, with tank treads and 2 motors, and we blew breakers like nothing. Watch out for that!

[Al Skierkiewicz]

Matt,
You are correct, the power dumped by the backdriven motor into a speed controller is lost as heat. Part in the speed controller and the rest in the wire (both motor internal and external) The speed controller is capable of 600 amps so I don't think you will ever get to the point where the speed contoller is at failure. The current generated by the motor is a function of how fast and how much wire is passing through the magnetic field inside the motor. The current is higher at high RPM. While the current is climbing, the back EMF and the braking effect are also climbing. There are so many variables involved, you may not be able to accurately predict the results. If you wanted to build power resistors into a custom circuit board box with some relays, you may be able to get variable control of braking. I am just thinking off the top of my head here, but it might be possible.
Using braking on some motors in a multimotor drive is a tricky thing to do, if something goes awry you are producing your own load and the current skyrockets in the driven motors, resulting in tripped breakers.
Good Luck

[Digo]

I'll try to explain how this brake works for the people that don't know much of electrical things (like me). Correct me if I'm wrong, please.

This "brake" is something you can set in the robot's programming. What it does is the same as connecting both the leads of the motor, when you press a button.

When you spin a shaft attached to the motor with your hand, you can notice a voltage on its leads.
For example: if you turn it to the right, you will produce 5 Volts. But if you put 5 Volts on its leads, it will turn to the left. If you put -5 Volts between its leads it will turn to the right.

So when you turn the shaft with your hands you are producing the opposite voltage it needs to spin that way. If the leads are connected, there will be a "contradiction", resulting in no movement, because it always produce for itself the voltage to spin in the way it's NOT spinning.

Let's say that your robot has this electrical brake on and another robot tries to push it. They are pushing you to go backwards, making your wheels spin backwards (as if you were spinning the shaft with your hands). The motor will work as a "generator", but the voltage it produces will try to make it go forward. So theoretically your robot won't move! And it's easier than using file cards!! Maybe not so efficient...

This can be easiliy tested with the fisher-price motor with its reduction box!! Try to spin the white round thing with your hand, and then connect the motor's leads and try again!
And it's incredibly fun (at the first time you do it) to connect the Fisher-Price motor's leads to another motor's leads and then spin the white plastic thing on the FP motor's transmission box. The other motor will start working, and if you don't know anything about electricals it will look like magic

I hope it was easy to understand (but probably it wasn't!).
There are so many words I don't know in English, so it takes me too long to explain something easy.

[Kristophe85]

Actually, the "brake" isn't something set in software. It is a Jumper on the victor itself. You use a tiny jumper to connect either pins A and B or B and C to create either "Brake" or "Coast" mode. Brake mode simply slows the motors much more, and makes it harder to backdrive. A common use could be on a mechanical arm, to prevent it from lowering too fast while power is not being applied. Coast mode is usually used in two or more motor drive systems, to prevent braking at different rates, and ultimately making magic smoke. Learn from experience and make sure you set all ESC's in the same mode, if you set one side to brake and one side to coast, you will have major drift. It's kind of upsetting trying to figure out what is causing this dift at 2:00am in the mech room.. *sigh*


-Kris Erickson

[Jnadke]

Originally posted by Digo

I hope it was easy to understand (but probably it wasn't!).
There are so many words I don't know in English, so it takes me too long to explain something easy.

Almost right, but not quite...

In break mode, the speed controller connects both motor wires together to form a closed loop. When the magnets inside the motor turn, they induce a magnetic field in the wire... the field created is an opposite magnetic field that opposes the motion. The harder you turn, the harder it opposes it. Any person who has taken physics will know that this is called Lenz's law...

Of course... as said above... since electricity and magnetism go hand-in-hand... there is current created, which is dumped back into the speed controller...

Heh... in collge I plan on double majoring... Physics and Electrical Engineering... so, I'll be quite familiar with this stuff soon...

[Lloyd Burns]

We used braking on the FP motor / gearbox that drove a leadscrew to raise and lower a basket. That year, the idea was to catch a horizontal bar, and have the robot chin itself, hanging on the bar after the power went off. It would lose the scoring if it touched the ramp beneath within ten seconds of the end of the match.

Before we shipped, it worked acceptably, with about 80 pounds of robot suspended on the screw. After the screw wore in, however, the friction was low enough the the load could backdrive the motor, causing the body of our robot to descend slowly, for the longest ten seconds after the match. Had we allowed the basket to rise more slowly with a shorter lead, it would have had more friction as it unwound in descent, and it would have been harder to backdrive.

It is possible to backdrive a "braked" motor, more or less fast, depending on its load, and resistance in the shorting circuit.

[Al Skierkiewicz]

Lloyd,
See my earlier post. The answer is of course yes, trains do it all the time. With the Victor braking, there is no provision. It puts a dead short across the motor. Until the new rule about custom circuit boards was introduced, there was no option. Under the new rules I think there is some possibility but I would be wary of judges decisions during inspection. Under the current rules, (and if I was a judge) I don't think it would be allowed.