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R/C SERVO
TESTER
This pocket sized servo
signal emulator makes it a breeze to test and setup your servos. It features
digital accuracy and is easy to build and use.
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Introduction
I recently found myself
having to test a large number of R/C servo controlled devices. My normal
routine is to grab my spare receiver, plug in the servo and battery, then
fire up the transmitter. As simple as this sounds, it is inconvenient for
me, especially when the "spare" Rx is installed in a plane. When the transmitter
nearly fell off the workbench one day, I decided enough was enough.
I thought about buying
a R/C servo signal emulator, but I am a do-it-yourself sort of guy. The various
projects I saw on the internet did not turn me on. What I found were circuits
that were usually based on LM555 timers. They all seemed to use too many
components for too few features.
My goal was to use
a cheap PIC microcontroller and do everything I could to minimize
the component count. The PIC would ensure precise R/C signal generation and
I figured that features could be easily added in firmware rather than by
more components. In the end I met all my expectations. And although I used
junk box parts, the cost to duplicate my work is well under $10.
The R/C Servo Tester
(RCST) is easy to use. Just plug in a 4-cell battery and your servo. The
variable pot allows you to set any servo position you wish, within the 1.0mS
to 2.0mS range of a modern R/C system. With the press of a button, you can
find the precise center to your servo. Flick a switch and the servo will
cycle (run back and forth ) at your chosen speed.
Servo Control Basics
 The
servo signal is a simple digital pulse. It spends most of its time at a logic
low (0 V). About every 20mS it goes logic high (3 to 6 VDC)
and then quickly goes low again. It is this tiny window of logic high time,
called the pulse width, that gets the attention of the servo.
Please refer to the
drawing. The period labeled "A" is called the frame rate. In the example
it is repeated every 20mS (50 times per second), which is quite typical for
most radio systems.
Modern servos define
center as a 1.5mS pulse width, as shown by detail "B" in the drawing. Full
servo rotation to one side would require that this pulse width be reduced
to 1.0mS. Full rotation to the other side would require the pulse width to
increase to 2.0mS. Any pulse width value between 1.0mS and 2.0mS creates
a proportional servo wheel position within the two extremes. The frame rate
does not need to change and is usually kept constant.
The servo will not
move to its final destination with just one pulse. The servo amp designers
had brilliantly considered that multiple pulses should be used to complete
the journey. This little trick reduces servo motor current draw and it helps
minimize erratic behavior when an occasional corrupt signal is received.
To move the servo, you must repeat the pulse every few milliseconds, at the
chosen frame rate. Modern R/C systems use a 40Hz - 60Hz frame rate, but the
exact timing is not critical. If your frame rate is too slow, your servo's
movement will become rough. If the rate is too fast the servo may become
very confused.
Circuit Construction:
My board was point-to-point
wired using 30 gauge insulated Kynar wire. This wire is normally used for
wirewrapping, but works fine with a soldering iron. I just strip a bit
of insulation off and solder it to the parts. I recommend a 40 watt or less
soldering iron (700° tip)
 Layout
is not critical except that cap C3 should be close to the PIC (mine is soldered
directly across pins 1 and 8). Cap C1 should have 2% tolerance for best accuracy.
Use a socket for the PIC chip. If your servo voltage will be higher than
5V, such as a five cell R/C pack, then you will need to add an LDO voltage
regulator (e.g. LM2931-5 Vreg).
The connections to
the battery pack and servo are handled by female servo cables or simple 3-pin
headers. I built it both ways and prefer the header version. You can see
my stacked 3-pin headers on the upper right of the photo.
Check your work carefully.
Do NOT install the PIC chip until you have verified that pin 8 is ground
and the pin 1 has 4.75 to 5.25 VDC on it. Remove power BEFORE you install
the chip. Double check the servo cable for correct polarity before you plug
a servo in. Simple mistakes can destroy electronic parts, servos, and may
generally ruin your day.
Your exact PIC choices
have some flexibility. You can use a PIC12C508, PIC12C508A, PIC12C509, and
PIC12C509A. Project Update: You can also use the easier to find PIC12F508
or PIC12F509 instead.
Be sure to verify that
your chip burning system has selected the proper configuration fuses, as
shown below. For example, if your programmer uses the PICALL software, then
press F3 to review the Config fuses.
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WDT: |
Watchdog
Disabled |
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MCLR:
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MCLR Disabled |
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Oscillator: |
IntRC |
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Memory:
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Code Protected |
The PIC's Hex file
is designed to automatically instruct the programming hardware to chose these
values. However, it is always a good idea to check them for accuracy. By
the way, after you program the PIC your programmer will report a failure
if you attempt to verify the PIC again. Do not be alarmed -- everything is
OK. Just ignore the "failure." Whatever you do, do NOT program the chip
twice!
If you have trouble
burning the PIC, then please check your programmer. Whatever the fault, it
is not a RC-CAM hex file issue. The most common problem is that the
user has forgotten to burn the PIC's four configuration fuses, as mentioned
above. More programming information can be found starting
here.
The entire firmware
is written in "C." You do not need to know anything about the C language
to build your system. All you need is the object code (Hex file) to program
the PIC, which is provided at no charge for personal use. A hobbyist can
use the firmware in a personal project at no charge (please review
the readme file in the hex file download).
Sorry, but I will
not provide the text based source code.
Instructions: Servo Signals
for all Seasons
With S1 and S2 turned
off, the variable pot is used to select the servo's position. The endpoints
are software limited to approximately 0.95mS on the low end and 2.1mS on
the high end. The step resolution is 10uS, which allows for very smooth
movement.
To cycle the servo
back and forth, enable switch S2. The cycle speed is determined by the pot's
position. The seven sweep times vary from about twice per second to once
per twenty-five seconds. Step resolution will vary, depending on the chosen
speed. The servo steps are coarse on the faster speeds.
To find the servo's
electronic center, just press and hold S1. The servo pulse will be
forced to 1.5mS and the servo wheel position should move to a nominal center
location. You can also use this feature to help calibrate the knob's tick
marks, since it will indicate where they knob needs to be for a centered
servo.
The pot position controls
an "RC" timing circuit. In effect, R1, C1, and some clever PIC software,
are used in place of an A/D convertor to measure the pot's position. Since
the reference is not voltage regulated, it is affected by battery voltage.
The decoded pot position can vary about 5% over the voltage range of a typical
4-cell NiCD or NiMH pack. However, the S1 (Center Switch) timing is
not affected by the battery voltage and will remain accurate throughout all
expected voltages.
Design Documents:
The technical details
are available as file downloads. There is no charge for the information when
used in a personal (hobby) project. Commercial applications must obtain written
approval before use.
Please be aware that
the information is copyright protected, so you are not authorized to republish
it, distribute it, or sell it, in any form. If you wish to share it, please
do so only by providing a link to the RC-CAM site. You are granted permission
to post links to the web site's main page
(http://www.rc-cam.com/).
Please respect this simple request.
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Schematic
Files: PDF file of the RCST circuitry. The components are from
www.digikey.com.
Hardware Revision: Rev A, dated 02-19-2003 |
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PIC Object
Code: Hex file and license details for the compiled RCST firmware. You
should occasionally check for updates.
S/W Version: V1.0,
dated 02-19-2003. |
The Small Print:
| If you need a part
then please consult the sources shown in the project (see schematics download).
I do not work for, nor represent, ANY supplier of the parts used in this
project. Any reference to a vendor is for your convenience and I do not endorse
or profit from any purchase that you make. You are free to use any parts
source that you wish. |
All information is
provided as-is. I do not offer any warranty on its suitability. That means
that if you build and use this device, you will do so at your own risk. If
you find software bugs then please report them to me. I can only make corrections
if I can replicate the bugs, so please give me enough details to allow me
to witness the trouble. |
