Torque vs. Speed

[=Martin=Taylor=]

Okay I'm sure this has been discussed somewhere. I have searched but couldn't find exactly what I wanted...

I recently had a discussion with our team's programming mentor. I told her I had designed our prototype drive train to run at the desired speed when the motors were turning at 50% of their max speed. I did this because I figured at this point speed and torque would be highest (proportional to each other).

She said that in reality 75% of max speed is a better number to aim for, and that 50% was just theoretical. I then asked my dad and he said it didn't matter, and that 100% would be just fine.

So who's right? I don't want my drivetrain running at max power constantly just to maintain the desired speed. That would drain the battery. I also don't want to have poor acceleration because there isn't enough power.

Any help?

[MrForbes]

If you have the "2007 Guidelines_Tips_Good Practices_RevA.pdf" file, look on page 8 at the little chart of the CIM motor, you'll see that max efficiency is somewhere around 80% of max speed.

I'd shoot for somewhere in that neighborhood.

Also the current that the motor draws is proportional to the mechanical load on it. So, if you're cruising along at 80% of max speed, you won't be using much current...unless you're also pushing 3 other robots across the field at the same time!

[eshteyn]

theoretically speaking 50% is the ideal number for just the motor itself, however depending on your drive train configuration, your gearing ratio, and your power delivery system the number varies. Also remember that this world isn't perfect and friction as well as inefficiency of design do exist and have to be taken into consideration. 75% sounds like the better percentage to achieve what you want, but theres nothing wrong with delivering a full 100% to the drive system, your batteries wont drain too much in the 2 and a half minutes your running them.

[MrForbes]

The problem with trying to run the motors at 100% speed is that you can't get any work out of them at that speed! they will slow down to less than that when you apply a mechanical load to them.

[eshteyn]

The driver can always lean back on the throttle a little bit, you don't want to be stuck in a situation where you need the speed but don't have it. Also a button may be installed to switch the drive from 100 to 80 percent in pushing scenarios.

[MrForbes]

You don't need a button to slow down the robot, the robot you're pushing will do that for you

See if you can figure out what this chart is telling you. Notice that torque is zero when speed is 100%.

This is important stuff, it takes a while to understand it, so please take some time to think about it and discuss it more. It's likely a real expert will jump in and point us to a relevant thread, or give a better explanation than I can (I'm a mechanical engineer....)

[eshteyn]

I believe there is a misunderstanding, are we talking about motor speed or are we talking about the PWM power setting? Because if were talking about motor speed then you are absolutely correct, there is no torque, but if we are talkign about the pwm then its a whole different story.

as for your ability to explain things, i feel your pain, i'm just as bad, i can't explain anything to anyone

[MrForbes]

Oh yeah, you're talking about the PWM command signal...I think Hachiban is talking about motor speed...and they are not the same thing.

Hachiban is worried about the motor running at near full power, just because it has been set up to run at near full speed (determined of drivetrain gearing). This is not how it works, the current draw of the motor depends on load as well as well as the speed.

I think he doesn't understand the relationship between torque, speed, current draw, and power. Again, it takes a while to figure out how it works.

[=Martin=Taylor=]

You see our mentor's primary concern was not with the mechanics at all. She was worried that it would be very difficult to run a Victor at 50%.

(apparently they don't like to run at this level or something??.. doesn't really make any sense to me, but then again I'm easily confused by electrical stuff).

[MrForbes]

In real life, you will hardly ever have the victor running at 50%....especially if the motors are set up to run at 50% of their no load speed at "normal" driving speed.....

It would help if you did understand the torque/speed/load/power stuff.

You can think of it like driving a car (this might not help if you don't drive a car)...when you drive on the highway, you have the gas pedal pushed just a little bit, but the engine is running at perhaps 50% of it's maximum rpm. You come to an up hill, you have to push harder on the gas pedal just to keep the car going the same speed. You get to a down hill, you have to let off the gas pedal to keep from speeding up more.

When you start from a stop, you have to push the pedal pretty far to get going, and the engine speed changes from slow to faster then slows down again after the transmission shifts, even though all this time the gas pedal is about the same position.

Think of the gas pedal position as the PWM signal, and the engine RPM as the motor speed, and the car's speed as the robot speed.

(This is a bad analogy, but might help you to understand the relationships)

[ChrisH]

Originally Posted by Hachiban VIII

Okay I'm sure this has been discussed somewhere. I have searched but couldn't find exactly what I wanted...

I recently had a discussion with our team's programming mentor. I told her I had designed our prototype drive train to run at the desired speed when the motors were turning at 50% of their max speed. I did this because I figured at this point speed and torque would be highest (proportional to each other).

She said that in reality 75% of max speed is a better number to aim for, and that 50% was just theoretical. I then asked my dad and he said it didn't matter, and that 100% would be just fine.

So who's right? I don't want my drivetrain running at max power constantly just to maintain the desired speed. That would drain the battery. I also don't want to have poor acceleration because there isn't enough power.

Any help?

I ALWAYS set up motors and gear trains so the desired motor speed is 75% of freespeed. There are some basic reasons for this.

First, no power transmssion system is going to run at freespeed. There is always drag on the system that slows things down even if it is just a little bit. Going with 75% gives you a little margin. Margin is almost always a good thing.

Second, as you stated earlier, you don't want to run things flat out, this is wisdom. See comments about margin above.

Third, running at 75% gives you margin in terms of POWER output. A motor's power output is maximum at 50% of freespeed. At 75% and 25% of freespeed you are getting about 50% power output. If you run at 75% and run into an obstacle that slows you down, you actually begin to get more power from the motor. This extra power might help you blow hrough the obstruction. (hopefully it is not a hard stop on your arm) On the other hand if you run at 50 %, when you slow down you will be getting even less power than you were before.

This holds true for DC electric motors in general. Drive, arm, it doesn't matter.

i hope this helps

[kramarczyk]

Don't forget to keep an eye on current. Your motor can vist 50 Amps, but it can't live there.

This presentation by Copoili & Patton covered most of the things in discussed here. http://www.chiefdelphi.com/media/papers/1682

[Qbranch]

Personally, if i'm trying to make a nearly perfect gear reduction, I start out by making a rough mathematical model of the frictions that must be overcome by the robot, in other words, how much braking torque the robot has when its just moving along flat ground. Then, from this torque, I can figure what the top RPM of the motor will be about 12VDC.

If you don't want to make a mathematical model (it doesn't take long to make a rough one so i suggest you do) then you can just work your reductions out to hit normal load.

Now, this is all welll and good if your robot is going ONLY for top speed. If the pushing power of your robot at 40A (the power available through a victor for a limited but decent duty cycle) isn't enough for you, then you'll have to reach a compromise, change your requirements of your drivetrain, or use a multi-gear or continuously variable transmission.

I've attached the spec sheet for the 2.5" CIM motor. A few parting notes: Remember that when designing a transmission you can have more than one motor, so divide or multiply your torques accordingly. Also, a good figure to remember: A 2.5" CIM motor provides about 101.1oz-in torque @ 1564RPM when battery voltage is 12VDC and current is 40A (480Wi).

Questions? Comments? Post!

-q

[Dick Linn]

Just because you might be running your motors at half speed at the desired travel speed of the robot, that doesn't mean you are getting maximum power out. Unless you have a real load, it shouldn't take a full 12 volts to get there (the gas pedal analogy).

[=Martin=Taylor=]

Thanks for all the help. I'm pretty sure I understand now.

Originally Posted by Hachiban VIII

(apparently they don't like to run at this level or something??.. doesn't really make any sense to me, but then again I'm easily confused by electrical stuff).

What I mean't to say was...

Originally Posted by Laura Rhodes

When the operator is running the robot at a mid-joystick position, the Victor
speed controllers pulse the voltage applied to the
motor on and off rapidly (this is called pulse width modulation). This
effectively lowers the average voltage applied to the motor.
The torque and speed curves of the motor look similar at the lower voltages but
maximum no-load speed is slower and the stall torque
(when motor speed is zero) is lower. The Victor speed controllers have
limitations which make trying to control the speed of motors
at low speeds difficult.

I forgot this was how Victors worked. Silly me.