| Noise in audio/video and R/C systems of Radio Controlled vehicles whether they be robots or elaborate model boats can be caused by a number of things including poor power ground schemes and poor power distribution, modulation of the batteries by controllers, signal ground loops, bad audio/video shielding, improper signal levels between audio/video components and Radio Frequency Interference. This growing page delves into diagnosis and solutions.
SERVOS: The usual application in an R/C plane
plugs the 8" long servo pigtail right into the receiver and it works fine.
Larger planes and bigger projects use more powerful servos and add extension cords.
Often servo operation becomes unreliable and there are a number of products, from RF
chokes to elaborate optically isolated amplifiers, to cure the problem. VANTEC discovered a characteristic of one servo amplifier
Integrated Circuit that can defeat the best of these solutions and led to a lot of
frustration. Although our work was centered around the Futaba S3302 what we
discovered is common to many IC's and is a possible source of trouble in any three
wire servo that uses an IC. Here's the problem: all the power for the servo is
supplied through the "extension" cord. The best wire has resistance. Four feet
of "smaller" wire like the #24 AWG used for servo pigtails, has considerable
resistance. When the servo is commanded to move, the motor starting currents can cause a
voltage drop across the power wiring. Bear in mind both the "red" +4.8VDC
power wire and the ground wire experience this voltage drop and that the ground wire in a
3 wire servo is part of the command signal. Meanwhile the signal wire source
is referenced to the "real" ground, the ground bus of the receiver. The
result is a negative pulse into the servo IC when the servo motor starts. Monolithic
IC's don't tolerate negative inputs well because of their intrinsic substrate diode; they
momentarily cease to be servo chips during the starting pulse. VANTEC has solved this problem by putting a "negative-input-tolerate" buffer
inside the servo case. At the end of the 8" servo pigtail works too.
Needless to say, our buffer was NOT an IC! Consider:
1. Build your own extensions with #18 or bigger power wire, #24 is fine for the signal
wire. If you use RF chokes make sure the 2 power lead units are rated for 5 times
the servo's rated running current. Minimize the extension length and use
quality connectors.
2. If you use a buffer amplifier avoid IC types. Install the buffer right at the
servo's pigtails. VANTEC has found a common emitter NPN transistor
to be a universal input circuit. Use resistors to limit the current to the base
current and establish a controlled impedance voltage divider. A small 470 pf cap B
to E filters noise and RFI. Complete the buffer with another inverting stage. Optical
isolation is not necessary and can be accidently defeated if attention is not paid to
ground loops. Likewise converting servos to "4 wire" with a duplicate
ground.
3. A very large capacitor to bypass the power wire right at the servo's 8"
pigtails can supply some of the motor's starting current pulse thus lowering the voltage
drop across the wiring. 5000ufd to 25,000 ufd. Unfortunately the capacitor
solution is relatively heavy and bulky. Note that "Farad" memory backup
capacitors may not be suitably rated to supply high currents.
SPEED CONTROLS: Brushes arc creating Radio Frequency
Interference. If your motor is not already so equipped install a .001ufd - .01ufd ceramic
disc capacitor directly across each motor's brushes and between each brush and their motor
case. These are yellowish round parts sometimes labeled "102" -"103
with two leads. If motor operation still interferes with your R/C set (jittering
servo's) RF chokes may also be required at the motor in series with each lead. Use
chokes rated to the DC current of the motor.
NOISE IN PROMOTIONAL ROBOT AUDIO/VIDEO SYSTEMS: Promotional robots
often incorporate a number of sub-systems together which interfere with each other,
particularly when they are operated off the same 12 volt Gel Cell battery. Radio control
systems with servos and PWM-type motor controllers introduce loads on the battery that
modulate the battery voltage, often at an audio rate. Commercially available audio/video
components are sensitive to this modulation on the battery and amplify it. As the
battery runs down its internal impedance rises aggravating the situation. Although
wireless microphone receivers can add noise via RF interference and noise, generally
audio/video noise comes from the battery modulation and poor ground schemes. Some
solutions:
1) pay special attention to the power distribution wiring in the whole robot including
sneak ground paths. Place all the audio/video components in a compact area and
dedicate an isolated chassis mounted with insulating washers for the audio/video
components. Tie the audio/video component power grounds together at a single isolated
ground summing node. DON'T incorporate any other components into this ground, just
audio/video components. Then connect this audio/video ground to the main summing ground.
Remember heavy motor currents impose a "hum" on even the best grounds. The
signal grounds and shields for piping the audio/video from one component to another should
be employed but watch for sneak ground paths (control elements?). DON'T use the robot
metal frame to conduct power or signal although a single point ground for the robot frame
is wise. Wire speakers with individual pairs; DON'T use the frame. Match signal
levels; DON'T put a line level output into a microphone input
2) employ an LC power filter to feed the audio/video components. Putting a
"Farad" filter capacitor on the isolated audio/video chassis and feeding it
+12VDC through a suitable choke can smooth out power line noise.
+12Volt battery-----wire----choke---|---to
audio video components
fused and switched in series |
1 Farad
capacitor
|
|
Negative-------wire-----------------|---audio video ground
battery post
Make the terminals on the capacitor the the summing nodes audio video components: the
positive terminal of the capacitor is the audio/video power terminal and the
negative terminal is the isolated audio/video ground summing node. Automotive/Marine
audio/video components have a fuse incorporated into their power lead; leave these in the
circuit while tying all the audio/video power wires together at the audio/video Power
summing node. DON`T incorporate any other components into this power summing node.
The David Levy Company at (562) 404 9998 has a suitable 15 amp choke CK-3510 for $3.90 and
a One Farad 16Volt capacitor PL1F for $77.00.
3) Use a separate battery for the audio/video/video components, crude but effective.
If not a permanent solution it's a good diagnostic tool. |
