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Electronics Tips                    

 

Combat robots require at least three major electronics systems. The power electronics, the control electronics, and the battery charging electronics.

Power Electronics:

Controlling the electric motors on a combat robot is usually done with a device called an electronic speed controller. The speed controller takes the raw power from the battery and modulates it, or reverses its polarity, so that your motors can be run at any speed from full forward to full reverse, or anywhere in between.

 

Control Electronics:

Combat robots are controlled by the same radio transmitters and receivers that are used for model cars and airplanes. These radios come in three basic flavors: AM, FM, and PCM. AM radios are cheap and unreliable. BattleBots does not allow the use of AM radios. FM radios are good, but the best type are the PCM radios.

It is a challenge to set up your system to get interference-free reception in the radio-frequency polluted environment of the BattleBox. Also making it difficult is the fact that your robot is in a big metal box, and you (and your transmitter), are standing outside of that box. There is nothing you can do about the metal box and all the RF noise, but there are a few things you can do to improve reception:

  • Use a PCM radio.
  • Mount the receiver antenna at least an inch away from any metal surface.
  • Your transmitter and receiver batteries should be fully charged.
  • Use resistor spark plugs on gas engines.
  • Install capacitors on all electric motors.
  • Separate your radio and power electronics as much as possible.
  • Your receiver antenna should be as exposed as possible.

I wish all of BioHazard's opponents would follow the last tip. I would then gladly come along and sever their antennas! This is one of the biggest trade-offs with radios. You're antenna should be out in the open for good reception, but it should also be hidden and well protected. In BioHazard, I use a thick plastic window through which the radio signal can (usually) enter, and which (hopefully) protects it.

BioHazard ElectronicsHere is a picture of the layout of the electronics in BioHazard. The radio is in the lower left corner. It is buried in a big block of balsa wood for cushioning. Notice that the controllers are separated from the radio by two magnesium bulkheads.

The following is an excerpt from the Engineering Design Reference Manual (Prof. M. Malone, University of Toronto): "Capacitors leading from each motor terminal to the metal case of the motor help quench spikes right at the motor. Putting these capacitors even inches away turns the long capacitors leads into antenni  that broadcast EM interference. The leads of the motor should then be twisted so that the magnetic fields generated around the wires flip sign with every twist. Lastly, in cases where the noise persists, the leads can be wrapped with a grounded conductive shield such as tinfoil." [Ground the shield at at one end only. Grounding at both ends can cause problems with current loops.]

Another thing you can do is to make sure your antenna is exactly the correct length. Your antenna should be 1/4 of the wavelength of the frequency of your radio. You can determine the wavelength by dividing the speed of light in meters per second (300,000,000), by the frequency. For example, the wavelength for 72MHz is 300,000,000/72,000,000 = 4.167 meters. One quarter of that is 1.042 meters or 41 inches. An antenna for 72Mhz works best at 41". Radio receivers come with antennas that are supposed to be trimmed to the right length, but it wouldn't hurt to check them.

 

Batteries:

There are several performance figures associated with a battery. To compare one battery to another, you have to consider each one of the following parameters: Capacity (measured in Amp-hours), Voltage, maximum rate of discharge (power), weight, size, speed of recharge, and cost.

Don't confuse Amp-hours with power. Amp-hours tells us the total ENERGY in the battery, POWER tells us how quickly that energy can be withdrawn from the battery.

BattleBots contestants use lead acid gel cells or NiCd or NiMH or lithium batteries.

BioHazard uses two 12 Volt, 16Ah, lead-acid batteries. These batteries have very low internal resistance. This is important because batteries with low internal resistance let you get the most possible performance from your motors. They will put out a solid 90 Amps for five minutes, and 680 Amps for five seconds. They weigh 14 pounds each.

Another advantage of the lead acid gel cell is the quick recharge time. If you use a constant-voltage charger, you do not need to limit the the charging current. The limit is set only by your budget and the shear weight of good high-power chargers. BioHazard usually finishes a match with at least 50% of its battery capacity still in reserve. BattleBots allows a minimum of 20 minutes between rounds to change or recharge your battery, and with a 20 Amp charger, it can get very close to fully recharged in 20 minutes.

 

 

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