Connecting a Transmitter/Receiver.

 The following schematic outlines the basics of how to take a transmitter/receiver and use them to power a robot.

 

Definitions:

 

The transmitter “encodes” the electrical signal from the controller, sends the signal over the FM bandwidth (note only 75  MHZ transmitter/receivers are allowed for ground  control) and is “decoded” into channels by the receiver.  FM transmitter come in two flavors: plain Jane FM and PCM.  PCM uses the same wavelengths as FM but “encodes” the signal such that interference is minimized; if you plan to go to Battle Bots go ahead and get the PCM, its required, otherwise use your judgement.  See http://www.futaba-rc.com/ for additional info on transmitters/receivers and PCM.

Connecting the receiver to the Speed Controller:  Two routes are basically available (see schematic).

1)     connecting the receiver to a servo (usually included in the transmitter/receiver pack) and using the servo to turn a potentiometer POT).  The POT is then hooked up to the speed-controller.

2)     connecting the receiver directly to the speed-controller.  The advantage is it’s direct, the disadvantage is there can be electrical feedback interference from the motor.  The electrical-mechanical-electrical system in number one bypasses this effect, but is more complicated to build.  It often comes down to the type of speed controller you buy.

Speed Controllers:  There are lots of GREAT places you can get speed controllers.  The web sites have TONS of info on how they work and what they do.  Also check out teams like Biohazard who give overviews of how they work and what they do.  The basic function is they take a low voltage signal from a POT or Receiver, amplify the signal, and run the motors.  You will need to buy a speed controller which can support the AMPs your motor will need, or else the motor will not be at its peak performance.  The AMPs the motor “will” need will be based on (1) the motor, (2) the torque you plan to generate, and (3) the gearing you use.  Make these decisions based on torque needed, speed, the gearing, and the size of wheels, etc.  Anyone competing in this competition is smart, use what you know from statics and dynamics to figure this out; also, we have included a helpful drive train section to unlock some of the mysteries.

Ultimately, the manner in which your speed controllers connect to your radio receiver depends on what type of speed controller you use.  Some, such as the IFI Victor and Vantec 33 series speed controllers just require you to plug a cable into the output of your receiver and the other end into the speed controllers.  Others, such as 4QD, require the servo-controlled potentiometer setup described above.  Still others, such as the OSMC, require an interfacing board.  You plug the cable from the receiver into the interfacing board, and another cable goes from the interfacing board to the “speed controller”.

A speed controller can be thought of as a current “gate.”  When you move the stick of your transmitter a little, it receives the signal and opens the gate a little, allowing a little current through, which makes the motor to be driven slowly.  If you move your transmitter stick a lot, it will open up the gate a lot, allowing a lot of current so the motor will drive very fast.  For a detailed description of “how” speed controllers actually work, look at the web sites for the controllers.  Speed controllers are actually governed by PCM, pulse controlled modulation.  The speed controllers switch voltage from +/- very quickly in short pulses.  The longer the + pulse compared to the – pulse controls how much “the gate” is opened.  For full speed forward, the majority of the pulse will be +, with a brief – pulse.  The duration of the pulse is much shorter than a physical device could operate, so to the motor, it looks like a smoothly varying curve.  The web sites have additional information on the exact mechanics if you would like to know more.

Be very careful when wiring up your speed controller!!  The input of a speed controller comes from your batteries.  Positive goes to positive, and negative goes to negative!!  Most speed controllers will fry! if you hook them up backwards!  Opps, you just lost a huge investment.  Check the connection every time, and create a system where the + battery input is always connected to the + side of the battery. 

The output side of your speed controller goes to your motor or motors.  You will have to test to determine which way the motors spin when you move your transmitter stick forward and which way when you move backwards.

Batteries and Motors. There are a lot of good links out there about Batteries and Motors. In this competition, ALL BATTERIES HAVE TO BE SEALED, so look at sealed lead-acid, NiCad or similar. Batteries and motors can be available from a host of sources. Drill motors, appliance motors, automotive motors, etc. can be good sources for motors and often batteries (e.g. cordless drills). There are lots of places out there; also a search of battlebots sites can provide links to many sources of motors. And don’t forget to look at our tutorial of sizing a motor and drive train system.

The batteries you pick must be capable of the amp draw of your motors or. NiCad battery packs can usually output 40 amps each for small packs and 80 amps each for large packs. NiMH batteries can usually output 40 amps each or less. SLA batteries can output large amounts of current, but the total current draw affects they’re capacity. They are rated by the number of hours they can output one amp of current for (amp*hours). However, a good rule of thumb is that for every time you multiply the current draw by five, you lose about 20% of capacity. (25 amps is 60%, 125 amps is 40%, more than 125 and it drops very fast) A number often indicated is that 100 amps gives you 44%, which seems to fit well with this curve.