Run Amok Combat Robotics
Transmitter Tweeks for Better Driving Control
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Builders inexperienced with the full range of control options available on a modern computerized twin-stick transmitter too often believe that radio set-up ends as soon as they slap on an Elevon Mix and set the correct Failsafe response. In the heat of a fight with some adrenaline pumping they suddenly find their 'bot quite difficult to control: darting off in odd directions, unwilling to hold a straight line, impossible to hold in a smooth turn, and pointing anywhere but the direction needed. They start to think they aren't good drivers, when the problem may actually lay in their radio equipment and setup. Most transmitter-related tracking problems stem from four sources:
It's not uncommon for one of the motor controllers to 'kick in' and start motor motion on one side of the robot just a bit sooner than the other side. One motor starting a bit early will throw the 'bot off at an angle and require delicate correction to re-aim under acceleration. If your ESCs can be calibrated to your radio you can reduce this problem, but to eliminate the problem completely you may need to access the 'subtrim' function to set forward motion start points equal.
The mechanical trim settings on the transmitter adjust the center position of a stick axis. If multiple output channels are ‘mixed’ on that stick they will all be affected.
Once you have the motor controllers synchronized for forward motion the signals are aligned, but they may not centered in the 'deadband' of the motor controllers. Having the signals off-center will cause one motor to start early when turning. This is a much smaller annoyance than forward motion de-synch, but if you like you can correct it:
Subtrim adjusts the center positions of individual receiver outputs. Two motor controllers mixed to a single transmitter stick can be individually adjusted. Move up the subtrim for the channel controling the motor that is 'slow to respond' until it starts up in synch with the other motor.
Move both ESC subtrims one step 'forward' and try a slow turn again:
Throttle/Steering Interaction
If the above process does not reduce turning imbalance: return the subtrims to their starting positions and try again by moving CH1 subtrim forward and CH2 subtrim reverse. Continue movement if improving, or reverse (CH2 forward, CH1 reverse) if synch worstens. Running an 'aileron' channel mix with the US normal 'Mode 2' channel stick assignments puts both throttle and steering control on the right-hand stick. In the heat of combat it's easy to nudge the stick a little to the left or right while intending to push straight up. This throws the 'bot off to one side and requires a delicate correction to recover.
Switching the transmitter from 'Mode 2' to 'Mode 1' moves the elevon mix throttle control from the right stick over to the left stick but leaves steering control on the right stick, eliminating any accidental interaction. You'll need to add 'spring centering' to the left sick vertical axis, but it's well worth the effort.
It can be difficult to maintain the delicate touch needed to correct path deviations in combat, and having the robot respond to a small stick input with a large turning motion makes it much more difficult. An intended small correction becomes way too much and you zigzag ineffectively across the arena. Two transmitter adjustments are available to bring this problem under control: 'Dual Rate' and 'Exponential Response'.
Dual Rate assigns a transmitter switch that will reduce the response of the robot to a specific transmitter stick axis by a specified amount. Setting a dual rate on the steering channel will reduce your robot's turn rate to keep your ‘bot composed under normal conditions, but allow a full-rate 'righting spin' or 'victory dance' at the flip of a switch.
Set up the elevon mix with both throttle and steering channels at 100%, then set up a dual rate on the steering channel with response rates of 50% and 100%. Adjust as needed, remembering that your stick-throwing gets a bit extreme in combat.
Exponential Response affects fine-level control in a different way, and may be combined with 'Dual Rate'. Setting negative values of 'Expo' reduces the sensitivity of the control stick near the center of stick motion for improved fine control, but increases sensitivity as stick motion approaches the limit of travel to ‘squeeze in’ full response range.
Motor Speed Imbalance
Try differing amounts of negative expo on the steering channel to find what suits your driving style. Try a small amount of negative expo on the throttle channel to give more precise low speed control. Drive motors often run a little faster in one direction than the other, sending the 'bot in an arc rather than a straight line at full speed. It's not technically a transmitter problem, but we can adjust the transmitter signal to compensate.
End Points sometimes called 'ATV' or 'Travel Adjust' - set a limit on the throttle of ESC plugged into a specific receiver port. Reducing the speed of the faster motor of a pair can transform a high-speed arcing turn into a straight charge across the arena.
Real World Example
Most transmitters allow independent limits for each side of the center 'neutral' position, allowing correction of both forward and reverse speed imbalance. Q: So I've built a 4wd antweight bot with 4 N20s for drive run by a dual brushed ESC. I've sorted out my own custom mixing for the right stick on my FlySky FS-i6 but I've got two problems with control. 1) If I push the drive fully forward my bot is real zippy fast and I have a little trouble controlling it. Great for rushing, not so great for turning. But if I only push the drive a little forward in an attempt to slow down, one side starts driving before the other and I turn instead of going forward. So I need to either go faster than I'd like, or start by turning away from a straight line. 2) My other problem is that I turn SO FAST. All 4 motors are spinning to turn me around and it's very easy to spin out, especially with my plastic wheels on a smooth floor. I would like to get my motors synced up better and slow down the turn rate, but I'm not sure where to start and I only figured out my current mix through trial and error. Any ideas? A: I'm not sure why you decided to create custom mixes when you could just turn on the built-in FS-i6 Elevon mix but since you went to all the trouble to figure it out we'll just leave it alone and fix your other issues. Both of your problems are common and have fairly easy solutions. Unequal Motor Start-up: 
The FS-i6 Subtrim function adjusts the center positions of individual receiver outputs. Each side of a dual motor controller mixed to a single transmitter stick can be individually adjusted so that both sides power up together rather than one side turning on first.
To set subtrim: prop the drive wheels off the ground and push the throttle forward slowly 'til motors on one side just start to turn. Select the receiver channel controlling motors on the side not yet turning (usually: left is Ch1, right is Ch2) and adjust the 'Subtrim' setting until the drives on both sides of the robot turn at the same slow rate and direction.
Two transmitter adjustments are commonly used to bring this problem under control: 'Dual Rate' and 'Exponential Response' -- but to avoid possible conflict with your custom mixes we'll keep it very simple and set End Points for your steering channel. 
The FS-i6 End Points function is the simplest method to reduce your robot's turn rate and keep your ‘bot composed under hectic conditions. My standard steering response setting is 50%; start there and adjust up/down to your own liking.
If the transmitter tweeks alone aren't enough to get your 'bot tracking and turning as well as you like, you can resort to adding a peizo gyro to the control system. See: Beginners Guide to Combat Robot Gyros. See also: Combat Robot Radio Systems - what functions do you actually need?
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