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Speed Controllers

Speed Controllers, the mystifying electronic brick that sits deep within the recesses of your average bot and makes the thing work. Normally this is done by way of a solid-state (no moving parts) H-bridge. An H-bridge is an electronic goodie that causes a circuit (usually a motor) to either go foward, stop, or reverse. The speed controllers commonly used in Battlebots, Robot Wars and the like are variable H-bridges. What they do is take a signal from the radio receiver and turn that into thousands of on/off commands per second. When the transmitter control stick is at center, no on commands are sent. At half throttle, 50% of the commands are on and 50% are off. At full throttle, all are on commands. This is an over-simplification, but it gives you an idea. The electronics a speed controller uses are quite finnicky. Unlike a motor, it is not reccomended to run a speed controller over its listed voltage. It WILL blow. A speed controller comes from the factory with generally a minimum voltage, maximum voltage, maximum current, and continuous current. The voltages are pretty much clear, but the maxiumum and continuous currents can be a bit confusing. The maximum current is how many amps a speed controller can handle for about 2 seconds before failure. The continuous current means how many amps the controller can handle all day long without failure. Listed below are the maximum current, continuous current, voltage, and details of some of the more popular speed controllers used in battlebots. The retail price reflects all of the parts necesary to hook up two motors and interface to the radio system as this often requres more than one piece. Here they are, listed in decending order of continuous current.
Name Retail Price Voltage range Continous Current Maximum Current Details Where to buy
OSMC $390 with uMOB, $420 with standard MOB, $475 with Deluxe MOB 13v-50v 160 amps 400+ amps configurable, on/off outputs for relays/solenoids, built-in mixing. Robot-power
4qd 150 24/36 $566 18v-45v 140 amps 400 amps Single control of each motor, requires seperate radio interface, bulky as all heck. 4qd
Thor 883 $720 12v-30v 120 amps 250 amps Single contained unit per motor, very compact, easy to mount. IFI robotics
Vantec RDFR38E $753 9v-32v 80 amps 220 amps Single contained unit for both motors, built-in mixing. Vantec
Vantec RDFR47E $785 9v-43v 75 amps 220 amps Single contained unit for both motors, built-in mixing. Vantec
Victor 883 $320 12v-30v 60 amps 180 amps Single contained unit per motor, very compact, easy to mount. IFI Robotics
Vantec RDFR33 $465 9v-36v 35 amps 100 amps Single contained unit for both motors, built-in mixing. Vantec
Vantec RDFR23 $340 4.5v-30v 30 amps 60 amps Single contained unit for both motors, built-in mixing. Vantec

Now generally when choosing a speed controller you want a higher current capacity for more powerful motors and more powerful motors are typically used in the higher weight classes. From this one can conclude that the higher the weight class the more powerful the speed controller, right? Maybe. Even very powerful motors if geared down enough can draw small amounts of current. Complete Control uses 4 1.5 horsepower motors that can pull nearly 200 amps each if stalled. His drivetrain, however, has been adapted for this. The motors are geared down 25:1 and he has a top speed of just 8mph, with a maximum linear force in excess of 1500 pounds. Because the maximum linear force is so high compared to the actual weight of the bot, the motors will draw very little current from the batteries during normal opperation, allowing him to save weight on batteries that could be used to create a beefy weapon system. Complete Control, despite using these very powerful motors, only uses 4 victor883s for drive.

But let's say you're entirely out of your mind and want to use The X950 for drive. What DO you do? Well in short, get the biggest damn controller you can find. If stalled, the X950 can draw in excess of 750 amps at 48 volts. By comparisson, an average welder uses approximately 200 amps. Scary stuff. For ultra-powerful motors such as the X950 and such, I would reccomend the 4qd-300 which can handle 240 amps for a full minute straight. This does require an external radio interface and it is bulky, but my god that's a lot of amps to be working with. A prudent bot builder would have a lot of batteries onboard.

Ok, you've got your motors all set up but how do you figure out how much current they will realistically draw during a match? Well my friend, you have come to the right place. Every motor has a constant ammount of current that it will draw regardless of voltage and entirely dependant on the load that's put on it, which is usually expressed in oz-in/amp. That is how many oz-inches of torque the motor will exert per amp drawn and since the motor's amp draw is completely dependant on how much torque it's putting out and since it only puts out enough torque as the load that is put on it, figure out the load on the motor and you can figure out how much current it will draw and thus what speed controller to buy! Yay!

Now here comes the math, try to stay with me. First figure out the weight of your robot, which shouldn't be too dificult. Let's say it's a 120 pound middleweight. Divide that by however many motors you have in your drivetrain. Let's say it uses 2 EV Warrior motors. Multiply by the radius of the wheel. Assume 6 inch wheels, so multiply by 3. Now divide by the gear ratio. Assume a standard 6:1 reduction so divide by 6. Now you have the load, in inchpounds, on the motor. Now multiply by 16 to turn inchpounds into oz-in and then divide by the torque constant to get the average number of amps drawn at any given time. For the EV warrior that is 6.6 in-oz per amp. So let's put it all together: 120/2x3/6x16/6.6 and we get an average load of about 73 amps. This is a bit on the high side because that's how much current each motor would draw if you put it against a curb and gunned the throttle, so It's pretty safe to say that a victor883 could easily handle this configuration. Below is a handy dandy chart of various popular motors and their torque constants. I love tables!
Motor name Torque Constant
EV Warrior 6.6 oz-in/amp
4 inch magmotor 8.5 oz-in/amp
3 inch magmotor 6.57 oz-in/amp
Bosch 750 8.7 oz-in/amp
NPC 1200 9.5 oz-in/amp
NPC 64038 121.6 oz-in/amp (geared)
DeWalt Hammerdrill 24v 73.3 oz-in/amp (geared)
NPC X950 (E-tek) 18.2 oz-in/amp
NPC 74038 112.8 oz-in/amp (geared)
NPC 80522 136.5 oz-in/amp (geared)
NPC 84088 152 oz-in/amp (geared)