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PWM fried from servo
Hello
Due to a recent discussion on another thread, I have decided to see what I can make with out old Tetrix kit and motors. I have mounted a robotic arm to our robot " ghettobot" which is a test bed. I am not using the bots systems, the arm has its own independent power and controllers, and radio transmitter of the hobby rc type. A Futuba to be exact. Also, using one saber tooth 2*12 motor controller. I ran into a problem, the Tetrix motor runs fine.the arm is a multi jointed arm so the joints all use high tec servos. I cut the end off a PWM and ran it into the screw terminal on the saber tooth, as soon as I powered the system on, the Tetrix came to life " evident by the sudden jerk of the arm" but the PWM cables I used to connect a female PWM to female fried and blew up in smoke. I was told the servos are 5-12v servos, so I see no reason for it to fry like that. I was using a 12v source with a breaker. Please help me, I know I'm probably doing something wrong |
Re: PWM fried from servo
Servos are typically 6V, atleast the one that come with KOP. 12V will fry them.
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Re: PWM fried from servo
Do I understand correctly that the wires burned up? If so, it seems more likely that you had a mechanical short somewhere, possibly created by the "sudden jerk". A failure of the servo unit due to over-voltage would more likely result in the circuit opening; the wires would have been spared.
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Re: PWM fried from servo
Servo has electronics inside, applying 12V will burn them. If its not working, the smell will tell you if its a toast.
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Re: PWM fried from servo
I want to make sure we're all clear on exactly what you did and how you wired things. Here's my interpretation:
1. You connected the Futaba R/C controller to the S1 and S2 terminals on the Sabertooth. 2. You connected 2 wires from the tetrix motor to Motor 1 or 2 on the Sabertooth. 3. You cut the PWM connector off a servo and connected the black and red wires to Motor 1 or 2 on the Sabertooth. Then you powered everything on, and I'm guessing the wires from the servo to the sabertooth melted and the servo went up in smoke, yes? |
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Edit: i cannot get back to the shop until Monday so I cannot inspect the servo and the wiring until then. I'm assuming I did it wrong,I dont think it was the servos fault. When I get back I'll post a photo of the setup I was using ( with replaced wires to show where I put the jumpers ). |
Re: PWM fried from servo
So, what you were supposed to do is connect the servo directly (or through an extension) to one of the ports on the Futaba RC. You shouldn't have connected it to the Sabertooth. Read on to see why.
I'm going to start with a (brief) story so you know that I've been there too, then tell you what you did wrong. Back when I was in high school on a BEST team, we built a small bot with a small arm we wanted to power using servos. To make the mechanical design cleaner, we mounted the servos back to back. Because this was BEST, we wanted to drive both servos with one channel, so we had to figure out how to reverse one of the servos. And I said "It's a motor, we'll just swap the red and the black wires and it'll go backwards!". (Al Skierkiewicz is shaking his head right now if he's reading this. Hi Al!) It didn't work and I ruined a servo and we had to mechanically change the robot and it was a terrible idea anyways. I was completely wrong and ruined a servo for the same reason your wiring was wrong and smoked: Servos are not motors. There are a fair amount of fancy electronics in there including a potentiometer, motor controller, and feedback controller. The red and black wires provide power to the internal electronics and motor controller. That internal electronics is what powers the servo's actual motor. The white wire carries the signal R/C style signal that tells the servo what position to be in. When I swapped red and black, it was like swapping red and black on the input power to your Sabertooth. It's a bad thing and ruined something inside the servo. When you connected red and black to the 12V output of your Sabertooth, it was similar to connecting the power input of your Sabertooth to, say, the 120V coming out of a wall socket. Too many volts not doing what's expected. Thus the smoke. I will tell you that even if you connected the Sabertooth to 6V and the servo hadn't smoked, it wouldn't do what you wanted. Changing the voltage on the red and black servo wires doesn't command the servo to move, changing the signal on the white wire does. And once you told the servo to go in reverse, it'd be just like my mistake and would've fried anyways. Thus you should connect the servo to the Futaba, which provides the correct control signal on the white wire, and should provide the correct power on the red and black wires. I have no idea if your servo is still good, but I'd guess it's not. So find a new one and connect it that way. TLDR; Servos aren't dumb motors. They're smart motors and their smarts can and will be fried if you connect their power (red/black) to too high a voltage or the wrong polarity. Plus the red/black wires don't control what the servo does anyways. |
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Normally, I would not have tried this kind of setup, but I am unfamiliar with this kind of setup.i had to make it I dependant because I didn't want to take a chance of me messing up and frying the control system on the robot its mounted to. I'm guessing the servo is toast. So I just connect it straight to the futuba? Also, can the sabertooths interface with the roboRio in any way so that I won't need to have two separate systems? |
Re: PWM fried from servo
I don't think there's anyone on CD who hasn't done something silly can ended up frying/melting/exploding something before. If they haven't, they probably haven't done much interesting stuff. Just don't make a habit of it, because it gets expensive.
If you had the sabertooth working with the futaba to move the tetrix motor, then the S1 and S2 control signals for the sabertooth should be the same as the control signals for any of the other FRc motor controllers. So yes, you can control the Sabertooth through the RoboRIO. To do so, connect the white wire of a standard PWM to S1 or S2 and the black wire to 0V. Leave the red wire unconnected, but tape it up so it doesn't short the roboRIO to something else. Then plug the female end of the PWM into the RoboRIO just like any other motor controller. Yes, you can power the Futaba off the 0V and 5V connections on the Sabertooth. They say you can use up to 4 standard servos if you do that, so you should be good. Also, it used to be standard practice to run entire VEX robots off 6 AA batteries, so running one servo off a handful of batteries isn't a big deal. |
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