Some Servo Problems

-
I ran into some servo problems a while back that I haven't had time to try to fix. The first one turned out to be a programming error where I was using a byte data type to hold too much data. I'm finding that the C-like programming of the Arduino is very limited, and it's taking some getting used to from my experience with more fully featured Perl/PHP languages that I'm used to.
The second thing was that one of the servo motor brass gear shafts kept popping off the plastic servo output shaft. I fit the 1/4" o.d. brass tubing over the slightly less than 1/4" servo drive shaft by expanding the brass tubing slightly. The problem was that the alignment between the servo and the end of the brass tubing was off slightly causing the tube to "walk" off of the plastic shaft as it rotated. I took my micrometer and measured the vertical and horizontal variations from back to front on all the servos. Most of them were off by .02" at most (which is about the size of a mechanical pencil lead) and shouldn't be a problem, but the one, in particular, was off by .06" and was causing problems. I removed some rubber mounting bushings which allowed a little more wiggle on the servo mounting and now it's within the .02" variance as the others. So far, so good.
I also found that for large movements the servos did well, but for small adjustments (10 degrees in Arduino) it didn't always do well in repositioning. I thought that there might be too much friction in the gear shafts for the servo torque to overcome, so I tore everything apart again and applied a little oil to all the brass. It helped a bit but still didn't solve the problem. Then I remembered that these servos are rated something like 5-7 volts, and I had been running them on 5 volts. After bumping up the power supply to 6 volts, everything seems to be in order.
I wrote a quick program to help me calibrate the servo motors to the various clock positions. I plan to do this initially by altering the low and high PWM values for the servos. The factory recommendation is between 900 and 2100 milliseconds, but each servo is a little different (gotta love analog!) By altering these values slightly I can get the servo start and end positions very close to each other. The next step will be to validate the intermediate positions and see if additional adjustment factors are needed to get each servo to reliably move to each of the 12 clock positions.

Comments

Post a Comment

Popular posts from this blog

V-22 Osprey Project - Design Changes for v3

-
Image
It's been a busy end of 2021, and as we start 2022 I'm getting closer to building version 3 of the Osprey wing.  The single biggest problem I've faced is the amount of backlash in the direct spur gear drive of the tilt mechanism.  It introduces some nasty instability that, left unchecked, would likely destroy the model in flight. I've investigated other tilt mechanisms, but the one I keep coming back to is the rack and pinion design that Tom Mast uses in his model at Rotormast.  His design uses a high-torque digital servo with a small pinion gear driving a rack attached to the rotor nacelle.  The servo has been modified with an external potentiometer that's driven by a gear on the translation tube to give positional feedback to the servo.  I like this design a lot because 1) It works, and his model is proof, and 2) it requires less torque from the servo and has less backlash. I ordered the pair of modified servos, potentiometer, and rack from Tom's site and star...
Read more

Whereabouts Clock Update

-
Image
After a few months, Life360 has gone the way of Apple and locked down the API I used to get location data from the phones.  This is disappointing, but inevitable. I found on a forum for Home Assistant that the app OwnTracks might be a suitable alternative. It's a neat little app that will trigger the phone to send its location data via MQTT to a server that you specify. I had to rework the code a bit, but the new operation is as follows: OwnTracks on the phone will push location data per its configuration to the MQTT server (Docker container) The Mosquitto (MQTT Broker) container will receive the data and forward the data to its subscribers The MQTTWarn container is a subscriber and translates the received data and adds it to the MariaDB database The Apache web server hosts a Python CGI script that is checked each minute by the Arduino microcontroller.  If the data has changed, the hands are moved to the new location. The only exposed port to the Internet is TCP 1883 for...
Read more

Whereabouts Clock: New Clock Mechanism

-
Image
 When I made the first mechanism of the Whereabouts Clock I utilized sail winch servos to get a motion that was at least 360 degrees.  Most RC servos are limited to about 180 degrees before they hit a physical limit.  I always wanted to create a clock with motors that weren't limited in the number of rotations, such as a stepper motor. However, stepper motors don't retain positional accuracy after a power loss.  The motor drivers only know "left" or "right" and a number of steps.  If the motor is moved, or power is lost, it has no idea where the motors are any longer.  In most control systems using stepper motors there are limit switches that are triggered to "home" the motors to a known position. Also, the Arduino Uno is limited in the number of IO pins.  Most stepper motor control requires at least 4 pins and additional lines for motor enable, micro switches for positioning, etc. A while back I purchased an Arduino Mega and a six-pack of 28BYJ48 ...
Read more