Continious rotation servo rotates only one direction, regardless the pot setting
CuriousOne
Posts: 931
Hello.
I built a basic stamp controlled pan/tilt/focus control system for my telescope. It uses two parallax standard continious rotation servos and worm gear drive, powered by dc motor. Whole device salvaged from car seat motorized adjustment. The schematics are as follows:
On the "main" device, there's 12V 5A switching power supply, which via 7805 IC delivers 5V to Basic Stamp 2P and LCD screen, 1602 type, which is used for position control. The 12V from "main" device goes to "telescope" end, where input voltage goes into DC/DC converter, which outputs stable 12V for dc motor supply. The DC/DC converter is used to compensate voltage drop accross the power cable. Same input voltage goes into 7806, which outputs 6V and is used for servo power supply. There's also a video balun, which gets signal from camera. I'm using standard CAT5E patchcord for operation. Everything was working just fine on the bench, problems started when I decided to move the "telescope" end outside, for real-life operation. I'm using 10ft patchcord of CAT5E cable in this case (at bench I've used maybe 2ft patchcord). The problem is, that with any power "glimpse", or DC motor control commands (I'm using the popular bridge driver IC from STMicroelectronics) servos jerk, and if I touch a wire that leads to control pin of servo, servo starts rotation counter-clockwise! assuming that was some interference to control line, I've soldered 10k resistor at "telescope" end to servo control line and GND. It does not help. Ok, replaced resistor with 1k one, that helped, but now servo rotates only counter-clockwise, regardless which PULSOUT value I feed to it. Playing with adjustment resistor also gave no result, it always rotates counter-clockwise. The circuit is done on PCB, every power line has 220uf electrolytic and 0.1uf ceramic capacitors near all major ICs, ground and power lines are wide, and when tracing PCB I did it in way to exclude stray currents.
What I can do?
I built a basic stamp controlled pan/tilt/focus control system for my telescope. It uses two parallax standard continious rotation servos and worm gear drive, powered by dc motor. Whole device salvaged from car seat motorized adjustment. The schematics are as follows:
On the "main" device, there's 12V 5A switching power supply, which via 7805 IC delivers 5V to Basic Stamp 2P and LCD screen, 1602 type, which is used for position control. The 12V from "main" device goes to "telescope" end, where input voltage goes into DC/DC converter, which outputs stable 12V for dc motor supply. The DC/DC converter is used to compensate voltage drop accross the power cable. Same input voltage goes into 7806, which outputs 6V and is used for servo power supply. There's also a video balun, which gets signal from camera. I'm using standard CAT5E patchcord for operation. Everything was working just fine on the bench, problems started when I decided to move the "telescope" end outside, for real-life operation. I'm using 10ft patchcord of CAT5E cable in this case (at bench I've used maybe 2ft patchcord). The problem is, that with any power "glimpse", or DC motor control commands (I'm using the popular bridge driver IC from STMicroelectronics) servos jerk, and if I touch a wire that leads to control pin of servo, servo starts rotation counter-clockwise! assuming that was some interference to control line, I've soldered 10k resistor at "telescope" end to servo control line and GND. It does not help. Ok, replaced resistor with 1k one, that helped, but now servo rotates only counter-clockwise, regardless which PULSOUT value I feed to it. Playing with adjustment resistor also gave no result, it always rotates counter-clockwise. The circuit is done on PCB, every power line has 220uf electrolytic and 0.1uf ceramic capacitors near all major ICs, ground and power lines are wide, and when tracing PCB I did it in way to exclude stray currents.
What I can do?
Comments
Perhaps attaching a schematic and code will help.
-Phil
Put a hole in your roof and leave it on the bench
What's your pinouts on the Cat5e cable? For the balun that should be great especially at 10' but for power run something heavier. Hopefully you're not using it as a servo extension, the microcontroller is on the telescope right? Servo wires are less than a few feet?
Jim
No, ground does not get disconnected. But yes, electricity here is 50hz.
I did further investigations today. Both servos use same twisted pair (different conductors) for control signal. I've disconnected one servo from cpu side and servo side, hooked both wires in paralel and connected in such way to left over servo. Still the same problem.
Next, I thought maybe servos run out of juice, so hooked servos directly to 7805, which powers BS2P, but used long run for signal cable, same problem - if disconnected, rotates on touch and disregards commands.
Next, I thought maybe power is OK, but problem is in control signal interference, so I left servos connected to power to 7806 as they should be and used short (half feet) cable to run the signal - strange problem. Servo does not jerks any more, but also does not responds to control - when I press a button, I hear it tries to do something, but does nothing. The power to servo applied at that moment is exactly 6V.
Next, servo connected with 2ft cable to BS2P and 7805 - works just fine, no jitter, rotates both directions as commanded.
So, there's something weird, and for workaround, I'm thinking about going RS-485, but still, power should be applied to servo end via same ethernet cable (PoE).
Here's simplified schematics how things are hooked up.
-Phil
Call 1 (800) EXORCISM. Tell them I sent you they'll probably give you a couple bucks off.
first, I switched to Rs-485 for control. This required rewrite of code and additional stamp on servo side. Second, I've replaced wires going to servo with shielded ones. Third, totally isolated dc dc converter is used for servo power supply, no other load connected to it. Things work just fine now even with 300 feet cable, bur budget is ...... :-)