Two-wheeled combat robots have many advantages: simplicity of construction, high spin rate, fantastic maneuverability, and weight savings. Perhaps their greatest problem is their instability in a straight line – they just don’t wanna go where you point them!
A similar problem is associated with remote controlled model helicopters. These flying head-choppers have always been difficult to control. In the 1970’s helicopter pilots first started installing electro-mechanical gyroscopes in their models to sense the rate at which their aircraft rotated and to send signals to the tail rotor servo that kept them pointed where they wanted.
By the late 1980’s, modelers had begun to construct all solid-state devices using piezoelectric accelerometers to sense rotation. These became known as ‘piezo gyros’. With no moving parts they are lighter (less than an ounce), less expensive, faster, and more robust. Just what we need for a two-wheeled robot!
There are two types of piezo gyros offered: ‘rate’ gyros and ‘heading hold’ gyros. What type of gyro do you need for your ‘bot?
A rate gyro senses rotation and gives a brief steering signal to the servo or steering mixer to turn in the opposite direction. This is useful in keeping aircraft pointed where you want them in gusty winds, but because they just ‘guess’ about how much correction is needed they are generally too imprecise for robot control.
A heading hold gyro (sometimes called a 'tail lock' gyro) keeps
track of rotation to sense how far and how fast it has turned. It also tracks how much steering
input has been received from the radio system. If the two don’t match up, it sends a direction
change signal to the servo or steering mixer to correct the turn rate and keeps sending it until
the correct heading is regained. Perfect for getting your two-wheeled ‘bot under control.
You attempt to accelerate across an arena with a two-wheeled remote control combat robot. One wheel gets a little too much power or traction and the 'bot veers to one side. In spite of your best driving efforts, you repeatedly over or under correct and zig-zag across the arena, completely missing your opponent. With a heading hold gyro installed, any unintended direction change would be instantly sensed and corrected – straight as an arrow! You can also hold a smooth constant-rate turn, rather than the common 'point and shoot' driving style.
Tips and hints
The 'gyro' itself exerts no physical turning force on the robot. It acts by modifying the steering signal from the radio receiver. The gyro plugs into the steering channel of your radio receiver, and the steering servo, on-board mixer, or mixer-equipped motor controller plugs into a port on the gyro.
Note: A gyro is not compatible with transmitter channel mixing or two-stick tank steering. If your two-channel motor controller includes mixing you're all set, or you can buy a stand-alone mixer to plug in between the gyro and your controllers for single-stick control.
An invertible tank-steer robot needs a gyro with the ability to adjust sensitivity (gain) via a spare radio channel. A gyro will send reversed steering correction signals when flipped upside-down, but an inverted tank-steer 'bot retains normal steering response. The result is an uncontrollable 'death spin'. A mixer with an 'invert' function corrects the forward / backward reversal but will not cure the death spin; you must shut down the gyro.
Four-wheeled 'bots usually have no trouble maintaining a straight line, but carving a smooth turn can be a challenge. A heading hold gyro can assist 'bots with four or more wheels in maintainig a nice, constant rate turn.
Piezo gyros have a tendency to ‘drift’ with temperature changes and loose track of which way ‘straight’ is. Keep your gyro away from fluctuating temperatures like chilly CO2 supplies and hot motors / controllers.
I’m using an Ikarus Profi Gyro (manual - archived) that has proved to be suitable for robot work. It can be switched between ‘rate’ and ‘heading hold’ functions, has both on-board and remote sensitivity adjustments, and is available on-line for less than $100.
I'll be happy to discuss piezo gyros with robot builders. My email is: firstname.lastname@example.org