I have noticed over the past few weeks that several people are confused by the 2 wire motors and why they only have 2 wires vs 3. Let me try to explain the fundamental differences between the 3 wire motors and the 2 wire motors.
Basic Construction:
[INDENT]1. The 3 wire motor was originally based on a Futaba servo. The only difference is that we removed the potentiometer from the servo to make it a continuous rotation motor.
The 3 wire motors had plastic gears that would strip if over torqued. The new 2 wire motors have steel gears that are much, much stronger.
[/INDENT] Wiring and Control
[INDENT]1. All DC motors that require dynamic operation and variable speeds need what is called a speed controller. The main item in a speed controller is called an H-bridge and is made up of Field Effect Transistors (FETs). The three wire motor had its H-bridge inside the motor. The 2 wire motors do not have an integrated H-bridge so they are literally just dumb motors that take a DC voltage up to 7.2 nominal volts (~8.4 max).
Basically, we separated the speed controller from the motor. I have seen this device called a “dongle” on this forum. It is much more than a dongle. It is an H-bridge speed controller that takes a standard hobby PWM 3 wire input and “converts” it to a steady state voltage between -7.2 and + 7.2 volts.
In addition, there are two H-bridges integrated into the Cortex controller. One is on port 1 and the other is on port 10.
The main advantage of this new design that decouples the speed controller from the motor is replacement cost for each item. If a motor fails or breaks, then you only need $13 to replace the 269 motor. If the speed controller fails, then you only need $10 to replace it.
In addition, with the new system, you get a much longer cable for each motor that uses a speed controller which enables more design flexibility.
Lastly, this enables VEX to continually develop new speed controller technology without changing the motor as transistor technology is always getting better.
[/INDENT]
I hope this answers some questions regarding our new 2 wire motors and speed controllers.
My understanding is that the servo PWM signal was a 1-2ms high pulse in a 20ms period, where 1ms = -127 and 2ms = +127, or something like that.
My understanding of an Hbridge speed controller, is that the output is also PWM with a different standard: Something like 0-100% duty cycle at positive rail or negative rail, with a period at some frequency like 1kHz. This PWM duty cycle variation provides better motor control at low speeds than an analog steady state voltage near zero. In old electric train controller language, this method was sometimes called a “DC chopper”.
The slower of these two duty cycles limits the useful frequency of motor update, as mentioned on one of the “drive straight” threads about using two optical encoders to make the left and right side of robot wheels go the same speed. ie The motor driver timer has already sent the high pulse, and has to wait 18ms before starting the next one, so there is no point in telling it to change the motor speed 18 more times before it can do anything about it.
Do the DC motors use brushes, or are they brushless (as in diskdrives)?
It will likely be a while until “switched reluctance” DC motor technology makes it down to the hobby motor scale.
Using an Oscope to view the inputs and output of the #29 motor controller would be an excellent demonstration for Industry mentors to show their teams. It might make a nice educational video to enter.
This is a good thread and a nice explanation of some of the basic concepts behind DC motors.
I used two 2-wire motors on my last robot and they are absolutely solid. Seriously, anyone who has any doubts with these motors (at least the 269s I used) should rest assured that they are ROBUST, solid, and useful. Moving the speed controller off-board is another great move - if you figure out how to bust these motors you save a couple bucks.
My favorite feature though is a very small one. They switched the side the wire comes out on. Since 90% of teams mount the output end “up”, having the wire come down instead of around saves a nice inch or two that I always seem to end up needing anyway.
Another nice feature of the 2-wire motors is that their direction can be reversed by plugging it in the other-way-around. Of course you can also reverse a motor in software, but this provides a quick workaround if you didn’t think to do that.
More importantly, it lets you drive a pair of counter-rotating motors from a single motor port: Motor Port -> 3-wire-Y -> pair of #29’s -> pair of #269’s/#393’s with one reverse-attached. There is no way to do that in software unless you use two motor ports.
I like the new 2-wire motors in that they allow you to add more power to your competition robot. Four 393 motors plus six 269 motors is roughly equivalent in power to 12.4 original motors. Also, having two different sizes of motors does help with distributing power.
However, in my experience 2-wire motors do not seem to be much more robust than the original motors. I have had ten 269 motor modules burn out and/or break the internal gears in the last two months. You wouldn’t think that metal gears could break. Before that I had six 393 motors spontaneously burn out within the space of about a month. Obviously, this being a competition robot, motors will fail, I was just hoping that it wouldn’t happen so often.
looks like you have a VERY aggressive robot o.0
and where did you get the number for “12.4 original motors”?
after a few burned out 3 wire motors from last year, we learned our lessons and used the motors what they were made for
(you should post some pics of the shredded METAL internal gears ;))
Yes, like I said some of the mechanisms on my robot are fairly demanding. High Hanging for one is obviously not going to be easy on the motors. At the same time, I have my gear ratios backed off enough so that overheating is never a problem. Thus, yes, it may be somewhat aggressive, but I really don not think that I am abusing the motors in any way.
As for what I said about “12.4 original vex motors”. I ran some tests with the 393 motors. What I found is that they do seem to offer roughly 60% more torque than the original vex motors. The 269 motors have roughly the same performance as the original vex motors. On a competition robot you are allowed to use four 393 motors which means you can also implement six 269 motors for a total of ten.
