A guide to radio system functions useful for control of combat robots."

Run Amok Combat Robotics
Combat Robot Radio Systems - what functions do you actually need?
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As a robot builder you will learn very quickly that hobby R/C systems are NOT made to control combat robots. Most radio gear is sold for model helicopters, drones, and fixed-wing aircraft; the user manuals are written in aircraft jargon. Fortunately the features found on advanced radio systems can meet the needs of any combat robot – if you can decipher the terminology to figure out which of the dozens of available features are needed or useful. Deciphering the aircraft language isn’t easy:

  • Does a robot need ‘5-point Revo Curve Mixing’? (No)
  • What type of ‘swashplate’ does a combat robot have? (No swashplate)
  • How do I use ‘Flaperon’ on my robot? (You don’t)

This guide lists the R/C system functions and features that typical combat robot designs either require or will find useful. Do not pay extra for features not listed here unless you have a very specific need.


  • Check the rules for the competitions you plan to enter to find specific technical requirements for radio gear. An event may require the use of specific radio frequencies and may mandate failsafe functions depending on the weight class of your robot and the type of weapon you equip.

  • Most combat robots use no more than three R/C channels: two for the drive train, and one for an active weapon. Some will use an additional channel to control a piezo gyro or to activate an invert function on a speed controller.

  • You will probably want a radio with five or six channels – not for the extra channels, but for the other useful features that come with these more advanced radios. Radios with seven or more channels will have still more features, but nothing that will be of any practical use in robot combat.

Listed below are features that you can put to good use, along with brief descriptions of how each is used. I have placed the features into three categories depicted by a number of 'stars':

*** Absolutely must have
**  Realistically need
*   Probably want


Combat robot rules generally require that larger robots and any robot with an active weapon must ‘failsafe’ on loss of signal from your transmitter: it must quickly stop moving and shut down active weapons. Unless you plan to spend your whole robot career building insect class wedges and rammers, you must have a radio system with proper failsafe response.

Radio systems that claim ‘failsafe’ capability may actually do one of several things on loss of the receiver signal:

  • Continue last command received - If the weapon was active it stays active, and if the robot was moving it continues to move. This is a completely unacceptable failsafe for combat robot drivetrains and spinner weapons. It is useful only for hammer/crusher/flipper weapons that you don’t want to unexpectedly change position on signal loss.

  • Stop all receiver output - This is not really a radio ‘failsafe’ as the receiver does not directly control the actions needed to shut down the robot properly. A specific motor controller or other device plugged into the receiver may or may not respond correctly on loss of the receiver signal. This is a marginally acceptable failsafe option.

  • Revert to pre-set position - This is the preferred response for drive trains and spinner weapons. On loss of transmitter signal, the receiver will send a command signal to the connected devices to perform a specific action – as in stop all motion. Make sure the pre-set position can be user set to accommodate your needs.

Be aware that failsafe is a function performed by the receiver, so radio systems that are compatible with multiple receivers may failsafe differently based on your receiver choice.

Some receivers offer a pre-set position failsafe active only for the throttle channel (channel 3) while the other channels switch off their output signal. This is awkward. You will most likely want this failsafe on your weapon, and this forces placement of weapon control on a gimbaled stick axis.

Some radio systems will give you a choice of failsafe type on each of the channels. This is highly desirable as it offers flexibility in incorporating various weapon designs.

Some very inexpensive radio systems have a large selection of features, but have user manuals that were written in Chinese and very poorly translated. Here’s an actual example:

The time-recorder is used calculating comparable bo stipulated time unexpectedly, or the possible time of flight under the state that the fuel fill it up with, it is very convenient. The pattern of the timer-recorder is the count-down. Pour time-recorder from set for time is it is it count to change, show surplus time at interface to begin.

If you can’t read the manual, you’re going to get extremely frustrated trying to setup your radio for best performance. Download the manual before you buy the radio. If you can’t make sense of it, consider buying a different radio.

Combat robots commonly have two motor controllers: one each for the left and right sides of the robot. This allows the robot to turn by running the two sides of the robot drive at different speeds and/or directions. Most drivers have a strong preference to have both motor controllers respond together to move the robot forward and backward under the control of a single control stick axis, and to have both controllers respond in opposite directions to turn the robot under the control of a second stick axis. This is done by 'Channel Mixing'.

There are many available types of channel mixing. Look for a radio system with at least ‘Elevon’ mixing (AKA ‘Delta Wing’). The Elevon mix will allow you to steer with the horizontal axis of the right-hand stick and control forward/reverse throttle with the vertical axis of either the right stick (Mode 2) or left stick (Mode 1). A radio that also has 'V-Tail' mixing will provide additional choices in choosing which stick axis controls which function.

Mixing setup help is in 'Appendix A' to this guide (below).

If you have more than one robot you need this. Model memory allows you to save the full set-up for each robot on the transmitter and switch quickly between them. This saves time, effort, and errors when switching between robots.
During the radio set-up process, you may discover that one or both of the drive train motor controllers respond in the wrong direction: the motor backs up when you command ‘forward’ – or you may discover that the toggle switch controlling your flipper weapon has the ‘down’ and ‘up’ positions reversed. You could physically reverse the motor polarity or re-plumb your pneumatics – but with this feature, you can simply instruct the transmitter to reverse the signal it sends to the receiver. Very handy, and almost every hobby grade radio has this feature.
Another set of features that will save you lots of time in set-up. 'Trim' sets the center point for the transmitter controls, and 'Sub Trim' sets the center point for each of the receiver outputs. You need both because with ‘Channel Mixing’ (see below) one transmitter input can affect multiple receiver outputs. Again, these are very common features.
A transmitter with the features you want for combat robotics will greatly benefit from an display screen to assist in setting up and adjusting those features. Transmitters without display screens either have very limited adjustability options or require that you to plug the transmitter into a computer to change feature settings. Pay a little more and upgrade to a flexible system with a display screen.
This function allows you to choose between two differing amounts of receiver response to maximum transmitter stick deflection with the flip of a toggle switch. Commonly used to limit steering response to prevent wild gyrations in adrenaline-fueled combat, but still allow an instant change to a rapid steering spin rate for special situations.

With Elevon mixing, try setting channel 1 (aileron) dual rates to 50% and 100%. Adjust as needed to provide proper steering response. Note that some cheap radios have a single ‘fixed’ dual rate setting that does not allow adjustment -- not a good choice for combat.

Different manufacturers may call this ‘ATV’, ‘End Point Adjust’, or ‘Travel Adjust’. If you don’t have ‘Dual Rates’, then this becomes a ‘need’ rather than a ‘want’ feature. If you have adjustable dual rates, leave this alone and set a dual rate instead.

Too much steering sensitivity will make your robot very difficult to control in combat. You’ve probably seen video of robots that keep overshooting their steering corrections and either spin out or just never get pointed in the right direction. That may not be poor driving – just poor radio setup. Start at about 50% response for the steering channel and experiment.

Another adjustment to make steering control a little smoother. Also called ‘Exponential Rate’, this function is commonly used to decrease steering sensitivity near the center point to allow finer control while still allowing full range of motion. IMHO, this function is not as effective as setting an ATV or Dual Rate, but it can improve the ‘feel’ of the robot control. Consult the radio manual for setting instructions.
There are two common layouts for the control stick assignments:

  • Mode 1: the vertical axis of the right stick controls throttle (channel 3), and the vertical axis of the left stick controls elevator (channel 4). This layout is common in Europe.

  • Mode 2: the vertical axis of the right stick controls channel 4 (elevator), and the vertical axis of the left stick controls channel 3 (throttle). This layout is common in the USA.

Some radio systems require that you purchase the transmitter in one or the other mode. Converting these systems from one mode to another requires a physical rewire of the transmitter. Other radios allow you to switch modes via a menu selection. This allows for greater flexibility in stick assignment when using channel mixing. If you don’t yet have a strong preference for stick function assignment, this feature adds flexibility that you may find useful.

Most manufacturers mention this option in the User Manual. Oddly, the Spektrum DX6i transmitter can switch modes, but the menu screen is both well hidden and undocumented. The secret is revealed in 'Appendix B' to this guide (below).

Note: the throttle (channel 3) stick is usually NOT spring centered like the other stick axis. This makes sense for an airplane or helicopter, but is annoying for a robot. You may be able to purchase the pieces needed to convert the throttle axis to spring centered from the manufacturer, or you can just transfer the spring mechanism from one stick axis to another if you need a particular axis centered

Transmitters from different manufacturers use differing 'protocols' to establish a connection and send information to the receiver. The transmitter and receiver must be able to communicate using the same protocol. Common protocols include:
  • DSM (Spektrum)
  • ACCST (FrSky)
  • FASST (Futaba)
  • AFHDS (FlySky)
Protocols evolve over time and develop new 'flavors' within a protocol (DSM2, DSMX...) that may not be compatable with earlier versions. If you require a very small receiver to fit into your very small robot you should check that such receivers are available in the protocol of the transmitter you are considering.

Some transmitters offer 'Multi Protocol' capability, either built-in or via a plug-in module. This can be handy in the long term if your favorite receiver gos out of production (common) and the best new choice is a differing protocol.

Those are pretty much the only functions you’ll need or use for a combat robot. Advanced radio systems offer dozens of highly specialized functions that I have not listed here. If you are sufficiently advanced to design a robot that needs any of those other R/C functions, you probably already know what they do and why you need them.


Combat Robot Elevon Mixing

Setting up channel mixing can be frustrating. You enable mixing and plug everything in only to discover that the robot is not responding correctly: it backs up when you command a turn and spins to the left when you try to go forward. Where do you go from there?

Quickest Option Try the new Team Run Amok: Combat Robot Mixer Fixer. Answer three questions about your robot and our interactive 'Mixer Fixer' will give you a complete and accurate solution to your mixing problems.

If you're more of a 'hands on' builder and want to better understand channel mixing and the types of things that can go wrong, the process below provides a step-by-step process to get your differential steer robot responding correctly to a transmitter Elevon (AKA 'Delta Wing') mix. This mix places steering and throttle control on the right stick of a Mode 2 transmitter. For other stick assignments, consult our FlySky FS-i6 Programming Guide.

  1. Consult your radio manual and enable Elevon mixing on your transmitter. If your motor controller (ESC) has its own on-board mixing, disable it.
  2. If applicable for your radio, set both Aileron and Elevator response to 100%.
  3. Plug the left side motor controller into the channel 1 (Aileron) receiver port.
  4. Plug the right side motor controller into the channel 2 (Elevator) receiver port.

Throttle Test Power up the robot and push the right stick forward a little (forward throttle command).

  • If the robot moves forward, good! Skip down to the Turning Test.
  • If the robot moves backward, select Servo Reversing from the transmitter setup and reverse both the Elevator and Aileron channels.
  • If the robot spins to the right, reverse only the Elevator channel.
  • If the robot spins to the left, reverse only the Aileron channel.

    Note: if your 'bot has multiple motors on each side of the 'bot and wheels on the same side of the robot are turning in opposite directions, reverse the motor leads to the ESC for any motor(s) spinning the wrong direction.

Turning Test Move the right stick a little to the right (spin right command):

  • If the robot spins right, your basic setup is complete.
  • If the robot spins left, swap your motor controller plugs (left into Elevator, right into Aileron) and repeat the Throttle Test with ‘left’ and ‘right’ corrections reversed.

Turning response will likely be too sensitive for combat. Scroll up to the above sections on Dual Rates, Throttle Volume, and Exponential Response in the main section of this guide to adjust control sensitivity.

Problem: When I push the stick up only the right side motor spins, and when I push the stick to the side only the left motor spins. What's wrong and how do I fix it?

Solution: You have either all mixing turned off, or BOTH transmitter mixing and speed controller mixing turned on. Adjust so one and only one mixer is active.

See Also: Transmitter Tweeks for Better Driving Control

APPENDIX B: Hidden Mode Switch Menu for the Spektrum DX6I Transmitter

Many computerized transmitters have a menu option to swap stick assignment modes. The DX6i has one, but it's undocumented and really well hidden. Here's how you find it:

  1. Press the selection roller and hold down while turning on the transmitter.

  2. Release the roller when 'SETUP LIST' appears.

  3. Rotate the roller to highlight 'COPY/RESET' and press the roller.

  4. Rotate the roller to highlight 'RESET' and press the roller.

  5. When prompted with 'SURE NO/YES', rotate the roller to highlight 'List'.

  6. Toggle the 'AIL D/R' switch (top right of the transmitter) from position 1 to position 0 a few times.

  7. Secret hidden menu! Select your desired mode with the roller and press to accept.

Please send notes for improvement, correction, or clarification to joerger@toast.net.

Copyright 2012 Mark Joerger, Team Run Amok

Run Amok Combat Robotics homepage

More robot help: Peizo Gyros for Combat Robots -- Transmitter Programming Guide -- FlySky i6 Transmitter Combat Set-up -- Ask Aaron - Combat Robot Q&A