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:
Unequal Motor Start-up
- Asymmetric power application at motor start-up;
- Unintended interaction between throttle and steering;
- Over-correction due to excessive steering sensitivity; and
- Differing motor top speeds.
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 accelleration. 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.
Subtrim adjusts the center positions of individual receiver outputs. Two motor controllers mixed to a single transmitter stick can be individually adjusted so that both motors power up together rather than one motor turning on first.
Running an 'aileron' channel mix with the US normal 'Mode 2' channel stick assignmentsputs 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.
Excessive Steering Sensitivity
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'.
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.
Motor Speed Imbalance
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.
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.
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.
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.
Most transmitters allow independent limits for each side of the center 'neutral' position, allowing correction of both forward and reverse speed imbalance.
See also: Combat Robot Radio Systems - what functions do you actually need?
More robot help: Robot Pneumatic Weapon Systems -- Futaba T6XAPS Programming -- Ask Aaron - Combat Robot Q&A