Radio Control Wiring Hints

Any radio controlled machine with motors has potential problems, and this page of radio control wiring hints hopefully helps to identify and solve them.

The block diagram shows the electrical system of a typical radio controlled machine, such as a robot.

radio control wiring
At one end we have a radio receiver, whose input is a very small signal – maybe 2 microvolts at 40MHz. At the other end we have a motor whose input signal is a PWM waveform of maybe 36v peak, or about 18v rms of 20kHz.

So the input signal voltage is some 9,000,000 times smaller than the output, but at 2,000 times the frequency. You will realise from this that there is some potential for the output to interfere with the input!

4QD controllers are however well-behaved and do not generate a significant amount of interference. The same cannot be said for motors however! A motor contains a commutator and brushes. This arrangement is switching the motor current maybe eight times per revolution of the motor armature. A typical motor can revolve 3,000 times per second. So the current is being switched at a frequency that maybe around 24kHz. The motor current can be as high as 100 amps (or more).

Now any electrical switching arrangement tends to arc and spark. Sparks generate noise. The noise from a spark is broad-band and can have a very high radio frequency component, extending to hundreds, even thousands of megahertz.

You may recall that the original experiments with radio transmission were made using a spark transmitter and with this a signal was sent across the Atlantic! The wining in your robot is not going to be as good an aerial as was Marconi’s original, but the moral should be clear.

So you need to make sure the motor, in particular its commutator and brush gear is in the best possible condition to minimise arcing. You also need to take steps to suppress and noise that is generated and clearly this needs to be done as close to the noise source as is possible.

Then you need to attend to the transmission paths so that this motor noise is not easily fed back either into the controller or into the receiver. Transmission paths are partially along wiring and partially by radiation, so screening should be considered.

Although the controller is a lot better behaved than the motor, and contains internal suppression, you should still be aware of noise transmission paths from controller to radio receiver – these will conduct not only controller noise but also motor noise.

You also need to attend to the aerial so that reception of wanted signal is maximised and unwanted noise is minimised.


Motor Noise Suppression

Motors are electrically noisy! Electrical noise interferes with wanted radio signals, so a noisy motor will cause erratic performance. Furthermore, one cause of MOSFET failures is noise caused by motors (see MOSFET failure mechanisms) so a noisy motor can cause controller failures. Yu therefore need to do three things:

  • Minimise noise generated by the motor
    • Make sure the motor, particularly the brush gear and commutator are clean, free from corrosion and in generally good condition
    • Make sure dirt (particularly metal particles and abrasive dust such as fibreglass) cannot get into the motor, through for instance, the ventilation holes.
  • Suppress noise generated. Motors, however good, always generate some noise. so put a ceramic capacitor of about 10n (0.01µ) across the brushes. See Causes and cures of motor noise.This motor capacitor is also needed if you are using relays to reverse the controller as it helps quench any arcing which may occur if the motor is reversed fast.
  • Reduce noise transmission and radiation. Suppression cannot kill all noise, simply reduce it. So dress the wiring to minimise transmission of noise back to sensitive electronics. See Motor Noise transmission

Motor Noise transmission

Noise, at least the noise which is a nuisance in the type of machine we are discussing, travels by four paths:

  • Along the motor wires.
  • By radiation from motor body and wiring
  • From the motor body through its mountings
  • By capacitive and inductive coupling from ‘hot’ wires to other wiring

All of these paths need to be considered and optimised to get best results.

Along the motor wires

It’s possible to go ‘overboard’ and fit ferrite cores to the motor wires, but this is not often done and is probably not worth doing.

Radiation

Any electrical signal gets radiated as a ‘radio’ transmission. The motor wires are carrying high currents, switched at 20,000 times per second. This background is also overlaid with any noise generated by the brushes. Clearly, you should reduce radiation from thee wires as much as practicable and the best way to do this is to keep them as short as possible and to make area of the open loop between then as small as possible.
Radiate1

In the first diagram the motor wires are widely separated and there is a large area of ‘loop’ between then, indicated by the gray area. This loop will act as an aerial, radiating any interference.

So run the two wires (M+ and M1) close to each other as shown in the second diagram where the area of the loop is very much reduced. If the wires are of a length that makes it possible, twist them together.
Radiate2

Motor body mountings

The motor body is generally a good screen. However there is coupling between it and the guts of the motor, so there will be a noise signal present on it. Exactly what you should do with this will depend on the design of the machine, but it will generally be screwed to the metal chassis and you have little choice of other action.

Capacitive and inductive coupling

Any two wires running parallel to each other for any distance interact because of mutual inductance and capacitance. So consider which wires are likely to be ‘hot’, i.e. are connected to noise sources, and which are likely to be sensitive, i.e. connected to inputs where noise might get in and cause problems.

‘Hot’ wires will generally be motor-to-controller and controller-to-battery wires, and sensitive wires will generally be those associated with the radio receiver and the interface.

Keep these two groups of wires as far as possible from each other. If they must be in close proximity, you may need to consider using screened cable for the sensitive ones.


Controller Noise transmission

The controller is a lot more predictable and better behaved than the motor but it would be foolish to not minimise the radiation from it. The noise is not random, being generated by the PWM which is closely controlled, so it is a lot easier to study and to understand. Transmission mechanisms are of course exactly the same as for the motor and the steps you have taken already to minimise motor noise will also minimise controller noise from the motor wires: these are ‘doing all the work’ so you’ve already done a lot.

Battery loop radiation

Exactly the same considerations apply to the battery wiring as apply to the motor. Keep it as short as reasonably possible, run positive and negative together to reduce any radiation loop area and twist the wires together if the length allows this.

Measures to reduce battery loop area also reduce the battery loop impedance – which (if excessive) can cause the controller’s capacitors to overheat. See PWM speed control on 4QD-TEC site.


Radio Control Signal

Signal strength

The signal transmitted from a typical radio control transmitter is well polarised and is not directional. Polarised means that it tends to radiate in a flat plane. So it tends to radiate in a sphere from the transmitter and the signal energy is therefore reduced as the square of the distance. At a certain distance from the transmitter, the signal is so know that the receiver cannot separate wanted signal from noise. Doubling the signal (or halving the noise) will only increase effective range by 40% therefore.

The signal received from any particular transmitter is a strong function of the receiving aerial and a good aerial can alter this factor by orders of magnitude. So any measures to improve range will depend far more heavily on having a good aerial than on any other factor.

Noise

Only once the receiving aerial is as good as possible is it worth worrying about noise and if you have followed the hints above – you’ve already done a lot. But any electrical system is ‘noisy’. There will come a point, with increasing distance, where this noise will cause signal to be lost.

The other factor is that an aerial of the type used in a radio control receiver is not a single ‘pole’ – any electrical signal works between two points and the strength of the signal received depends on the signal voltage between the input from the aerial and the effective earth point that the electronics is comparing the aerial input with. So the aerial is in effect one half only of the overall aerial. The other half being ‘virtual’ but a function of the wiring, metalwork and chassis of the vehicle. So in effect you have a dipole aerial with a (potentially noisy) motor and controller (and battery) hanging on one end. If that is significantly noisy it injects the radio noise back into the effective earth of the aerial, where it appears to the receiver as a difference between earth and aerial signals and the poor old receiver cannot tell it apart from the signal coming in the aerial!

Isolation

Exactly what effect this ‘lump’ (motor, battery and controller) has is of course scarcely predictable. But it it’s well-constructed and suppressed, the effect should be minimal. So is a well constructed machine, you don’t need any isolation in the earth. However if the machine is not good, then you want to prevent any noise signal travelling back along the earth wire to the receiver.

4QD’s DCI uses potentiometers mechanically controlled by the radio control servos. With this system there is no electrical path and you have complete isolation.

With a digital interface, unless you specifically prevent it, there is an electrical signal path, as shown in the block diagram above, where the ‘virtual’ second half of the dipole is shown dotted. The earth connections through the individual boards are shown in green.

The best isolation device, should isolation be required, is an opto coupler. There are two places where opto couplers (aka opto-isolators) can be used, indicated by the two pecked yellow lines B1 and B2.

4QD have opto isolated versions of both our single channel and our dual channel digital interfaces and the optos in both are in the B1 position. This has several advantages:

  • It isolates the receiver from the microprocessor (also a potential source of noise)
  • It requires only two channels whereas at B2, 4 would be needed (two speeds, two reverse signals)
  • It enables the microcontroller to be powered from the controller.

The disadvantage is that there is still an ‘earth loop’ in the system. Read on!


Battery Earth loop

From battery negative through controller 1 to C1b, through the multicore cable to C1a which is (and really should be) joined in the interface to C2a. Thence back through controller 2 to the battery. There is a page, complementary to this, on Machine wiring – Good and Bad practise which explains that earth loops are generally undesirable!

In this instance, avoiding this loop is possibly likely to cause more problems than it causes. However there are circumstances when it can cause a problem:

One battery negative wire broken.
If one controller loses its battery negative connection, the earth current for this controller will find another path, in this case through the earth loop. Since the peak earth current is the peak battery (hence peak motor current) something will blow in this event. The inputs to (most) 4QD controllers have a fuse track in the earth line to safely disconnect in this event.
Controllers have vastly different battery negative connections.
In this event the voltage drops in the earth wires will be different and the difference voltage will cause a current around the alternative earth path – i.e. through this loop.
Motor brush fault
A fault that must happen in Robot Wars on occasion. In the event of a high mechanical shock the armature can be displaced with its bearings which can then contact the brushes. This shorts the M+ (which is battery +) or M- connection to the chassis. If the chassis is connected to the electronics, it will be connected then to the battery -ve.
So this motor fault causes a direct short across the battery through whatever path is present. Something will fuse! If it is only the controller’s earth fuse, then maybe that will be the only damage!

Earth Fuses

If you are happy repairing earth fuses: the NCC and Pro controllers are both protected here, see the manuals for more information. However – it may be a good idea to include a fuse in the earth track to each controller ‘just in case’. Use a small fuse, say 500mA and insert in the earth (green wire) at point B2 (block diagram, above) of the multicore between interface and controller.

In the event of a motor fault, both tracks could, of course blow, so two fuses are required.


Other relevant pages

  • Electronically controlled machines. Good and Bad wiring practise.