Learn something new every day. I’ve known for some time that it’s better to have your motor controller plugged into the cortex directly, and then use extensions to get the rest of the way to the motor, but I didn’t realize there was an official/scientific reason for doing so. Well, I just came across something on the VEX Forum that had me actually *read* the product page for motor controllers, and lo! and behold:
Note: We recommend using no more more than one (1) 3-Wire extension cable in between the Motor Controller 29 and a VEX Microcontroller. For longer extensions, use 2-Wire extension cables in between the Motor Controller 29 and a VEX 2-Wire motor
We have a lab full of 3-wire extensions; I think we don’t even own 2-wire extensions. If you’re like me with a bin full of 3-wires, there’s good news! You can use your 3-wire extension in place of a 2-wire extension (but not the other way around).
Side note: You’ll still need 3-wire extensions for connecting sensors to the cortex, and short 3-wire extensions to go from the cortex to the power expander.
What Those Wires Do
Some background for the uninitiated. Most cortex ports have 3-pin plugs; one end of a motor controller also has 3-pins. Pretty obvious what happens there. The other end of the motor controller has a 2-wire extension, which matches up to the 2-wire lead coming out of the motor.
Why not 3 wires all the way to the motor? What’s the difference?
- The red (orange) wire, as usual, carries current.
- The black wire, as usual, is the ground.
- The white wire is for what’s called “signal”.
As described in my post on motor controllers, the MCs are what control the electricity flow from the cortex to the motor itself, using what’s called PWM (pulse-width modulation). MCs turn on & off rapidly to allow power to get to the motor. For 127 (full) power, the motor controller is sending power through constantly, like an open spigot. For 64 (half) power, the MC is sending 100% power through, 50% of the time. It’s turning the spigot on & off very rapidly so that half the time it’s allowing 0 and the other half of the time it’s sending 100%. There is no actual “half power” for VEX motors; it’s either 0 or 100%, and it’s the amount of time, essentially, that electricity is getting through to the motors that produces different speeds/outputs on the robot.
So how does the motor controller know what pulse rate to use? How does it know how much to turn on & off? Well, that’s where the white wire comes in. The white wire’s purpose (in regards to motors) is to allow the cortex to send instructions—”signal”—to the MC.
And that’s why the white wire only travels from the cortex as far as the motor controller. It’s those 2 things that need to talk to each other.
Back to the product page advice: If the signal from the cortex to the MC has to travel over a series of really long extensions, the signal will degrade, so that the MC will not operate as well as if it were closer. The awesome jpearman from this VEX Forum post explains the technical details:
The instance when a motor starts it is by definition stalled and will draw high current (4.8A per the spec for a 393). The high current will cause a voltage drop at the MC29 due to the resistance of the wire if it’s on a long extension. If the voltage at the input to the MC29 drops below 5V then the MC29 can reset and disable the motor, current now drops to zero and the voltage rises again. This sequence can repeat until the motor starts to turn.
But All I Have Is 3-Wire Extensions!
Never fear. The world works in your favor this time. You can indeed use what you have, and you don’t *have* to buy more. The first image here shows where in the cortex–MC–motor system each extension wire is used.
And with the somewhat-hokey-but-perfectly-acceptable method of using a 3-wire in place of a 2-wire, you’d have it like this:
So never fear if all you have is 3-wire extensions. You’re still good to go.
Mentors: if the “only 1 extension between the cortex and motor controller” rule is news to you, it’s probably news to your students too; pass the word.