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Help me build a servo Controller for a robotic arm

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Junior Member level 1
Apr 15, 2002
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Hi There
I am trying to build a robotic arm but don't have strong enough motors(stepper or servo) the regular servos can handle about 45oz(3kg) and steppers can handle a lot less. I have some strong gear head motors that I want to use as servos (because with servos I can have absolute position control) but can't find any circuitry to convert them to servos. I also want to know if it is possible to take the circuit from the 45oz servos (futaba s148 etc.) and run the bigger gear head motors with them. Any help is very much appreciated.


servo controller schematics

They are a bit expensive, but I recommend you look at the SmartMotors from Animatics Corp.

They offer a combination brushless motor/encoder that has a RS232/RD485 addressable controller that will drive the motor to a specific position, at a constant speed, at a rate, as if it were a stepper, etc. Basically anything you might want to do with the moter can be commanded over the serial port. All the closed loop stuff is handled for you (after you tune it).

They offer a range of motor sizes & the controller by itself.

We replaced a number of applications with the smartmotor solution because of teh flexibility & easy of customization.


Presuming you are trying t do this on your own on a low budget, try to pick up some "pancake" motors from a surplus shop. You can drive them with simple Dc power transistor amplifier.

lm628 circuit

Presuming you are trying t do this on your own on a low budget, try to pick up some "pancake" motors from a surplus shop. You can drive them with simple Dc power transistor amplifier.

servo driver circuits

:roll: you may to use the moment driver. It is more powerful vs servo with same size. If you do not afraide this solution, mail me.

build a servo controller

You should be able to modify a servo with a bigger motor. The only catch is you have to modify the ciruit to handle much higher current to the motor. A servo is a closed loop system. If you take apart one of the R/C servo, you will see a pot on it as the feedback. (It was done that way many years ago, don't know if it still is.) This feedback gives the position of the gear. When an position is inputted into the system, the loop circuit will drive the motor until the position of the gear matches what the input is. I don't have experience on the actual circuit itself. You should get a schematic and study the drive circuit a bit. If you are lucky, all you need is to modify the drive amp with a higher power one and you are done. But keep in mind that since it is a closed loop system, any changes may cause the system to go nuts (unstable).

Hope this helps a bit. You may get lucky with some study and experiment.


lm628 cost

Building a servo using DC motors will imply some circuitry like:
- position encoder;
- motion controller;
- motor (H-Bridge) driver;
- eventually, an interface to the main controller.
You may start reading the documentation of a motion controller like LM628/629 from National Semiconductor. This will give you an idea where to begin, and how to design your servo. Personally, I used this circuit for a small project -- it did a great job for me.


national semiconductor servo control

I thing your gear head motor is DC motor. You can use National

These IC's are good PID servo controller.

You can use the LMD18200 H bridge driver.

lmd18200 circuit sample

How does LM628-LM629 or LMD18200 cost each one?I think it's expensive.

lm628 lmd18200

help me schematic of LM628 .

Added after 2 minutes:

Datasheet và Program

servo controller absolute

i'm using lm628 too but i'm still newbie for this ic.. can someone show some circuit and example to program it.. maybe using PIC16F877a.. thanks in advance...

closed loop servo controller driver low cost

anyone? please help me..

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