How It Works

Radio Control (RC) is the use of radio signals to remotely control a device such as a model aircraft or robot.
For ground vehicles the transmitter (Tx) transmits a radio carrier wave in the 27 or 40 MHz bands. (35MHz band is reserved for model aircraft).

The 27MHz band has 13 channels, which are like different radio stations. It is prone to interference by CB (Citizens Band) radios that use the same band. The 40 MHz band has 34 channels. Each unique channel frequency is controlled by a crystal in the transmitter and a matching crystal in the receiver. Some cheap toys have ‘splat’ transmitters that transmit over (and interfere with) a number of adjacent channels and are identified by Band 1, Band 2, Band 3 etc.

The transmitter looks at the position of each control stick and sends this information to a receiver which is tuned to the same frequency. There are 2 methods used by the transmitter to superimpose the control stick information onto the carrier wave:

• Amplitude Modulation (AM). This suffers from interference.
• Frequency Modulation (FM). This suffers much less from interference but is more expensive.

To avoid two robots being on the same frequency, channels can be altered by changing the crystals in both the transmitter (Tx) and receiver (Rx). You must ensure you use the right type and make of crystal in your Tx and Rx as different manufacturers crystals are incompatible. The Skysport 4 radio control uses Futaba crystals. AM and FM crystals are not interchangeable and ‘single conversion’ crystals are not the same as ‘dual conversion’ crystals. Crystals are quite delicate and should not be dropped.

Pulse Position Modulation (PPM) is normally used to "multiplex" the control channels together onto the single radio channel. In the Transmitter the stick position for Channel 1 is read and the information sent to the Receiver. Immediately afterwards the stick position for Channel 2 is read and that information is sent. This repeats for all of the channels, one after the other followed by a longer gap. The sequence is repeated about 25 times a second so that it appears that the servos instantly know you have moved a control stick.

With Pulse Position Modulation, the position of the pulse within the sequence tells the Receiver which servo/motor to send the pulse to. The first pulse after the long gap is sent to Servo Channel 1, the next pulse to Channel 2 etc.

Another system called Pulse Code Modulation (PCM) sends the stick position data from the transmitter to the receiver in a digital format. PCM in conjunction with a special ‘Dual Conversion’ receiver has by far the best performance, is resistant to interference, but is also the most expensive system.

The receiver converts the PPM or PCM signals into Pulse Width Modulated (PWM) outputs to the individual servos or the electronic motor control boards. The way the stick position is conveyed is by varying the width of a pulse. Typically, a pulse width of 1.5ms (milliseconds) will centre the servo or keep the motor stationary. If the pulse is reduced to 1.0ms, maximum reverse speed is demanded and if it is increased to 2.0ms, maximum forward speed is demanded with intermediate positions available.

The motor control boards then convert these PWM signals into the motor speed and direction of rotation.
Always turn on the transmitter first, then the receiver. When turning off the system always turn off the receiver first. The object is never to have the receiver on by itself or stray radio interference may activate the robot motors in an unexpected and dangerous way.

Control.

We have decided to use differential skid steering which is also known as ‘tank type’ steering. This means that the robot is turned by varying the speed of wheel rotation on each side of the robot. If the left wheel is going forward and the right wheel is in reverse, the robot will turn on the spot. (This is known as a zero turning circle).

Method 1 – No Mixing:
The speed and direction of the two motors can be controlled independently from the left and right control sticks, but this requires considerable skill to demand precise turns. To go in a straight line you must move both sticks forward exactly the same amount.
Left stick controls left motor, Right stick controls right motor

Method 2 – Control Mixing:
It is easier to mix the channels such that one control demands straight line speed from both motors equally and the second control modifies the straight line demand such that one motor goes faster than the other in order to turn.

Your transmitter and receiver has four separate functions (or channels) controlled by two sticks. Your robot uses two of these control functions and which ones you use is a matter of preference.
You can therefore implement control mixing by using two separate sticks or one stick moved in two directions. e.g.

• Left stick left/right controls Direction (Chan 4), Right stick forward/back controls Speed (Chan 2)
• Right stick left/right controls Direction (Chan 1), Right stick forward/back controls Speed (Chan 2)

The control mixing can be done within expensive computerised transmitters, but to minimise the cost of the radio control equipment we have done the mixing ourselves within the motor control boards. Wherever the mixing is done, the effect is that moving one control lever on the transmitter forward commands both motors to run forward at the same speed. Moving the other (or possibly the same) lever sideways causes one motor to go forward and one to go in reverse. If selections are made at the same time the robot will turn whilst moving forward or in reverse. The turn demand is added to the straight line demand to provide the required mixed command to each motor. The robot then turns in the direction of the slower wheel.

Most drivers prefer that speed and direction are controlled by separate control sticks (Method ‘a’ above). You can easily implement single stick control (Method ‘b’ above) by moving the lead plugged into the receiver channel 4 slot to the receiver channel 1 slot.

Exponential Control Law

To provide precise control at slow speed for accurate manoeuvring it is useful to have the control sticks more sensitive around the central position. This can be achieved by having an ‘exponential control law’ which allows the sensitivity around zero to be adjusted according to an exponential curve. Expensive computerised transmitters generally have this facility. To minimise the cost of the radio control equipment we have incorporated an exponential control law for both input channels within the motor control boards provided.

From the diagram you will see that moving the stick half way forward (Green Line) will demand half speed with the linear law (Blue Spot), but only quarter speed with the exponential law (Red Spot). At full stick movement both laws demand full speed.

Burning Rubber

Cordless electric drill motors are quite powerful and it is easy to overcome the static friction between the wheels and ground and ‘burn rubber’. If you select full speed from a standing start the wheels will slip.

Theory indicates that the most efficient way to accelerate is to apply the maximum power possible without the wheels slipping and then increase power gradually as the robot starts to move. Static friction is greater than dynamic (moving) friction so if the wheels start to slip it should be most beneficial to reduce power to regain traction and then increase power again.

You should experiment yourselves as Rampaging Chariots seem to defy accepted theory.

Next

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