Robot Arm using Potentiometers and Servos

Written by: Alan Parekh at 4:31 am. Filed under Cool Gadgets, Electronic Hacks

This robot arm uses potentiometers to determine where the human arm is and then a servo controls the robot arm to match the position.

“Each joint on the master arm is a potentiometer(a variable resistor). A joystick port basically checks the resistance. By itself, this resistance means nothing, so I wrote a little routine that converts a resistance into an angle that the servos can understand.

To do this on the elbow for example, I straightened the master arm, and checked the resistance. Then I tried sending various numbers to the elbow servo until I figured out what number meant straight. I did the same for fully bent.

With these 4 numbers in hand I was able to write an equation that maps any angle on the master arm to a similar servo angle on the slave arm. I also used these numbers as hard limits on the values I sent to the slave arm.”

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10 Responses to “Robot Arm using Potentiometers and Servos”

Robot Arm using Potentiometers and Servos | Technology Blog Says:
May 30th, 2007
[…] Original post by http://hackedgadgets.com/2007/05/30/robot-arm-using-potentiometers-and-servos/ […]
NGinuity Says:
May 30th, 2007
This is how most of the manual rotor systems for directional antennas work…kind of cool actually.
joebob Says:
May 30th, 2007
still has some bugs to work out. but other than that, good job!
listofoptions Says:
May 30th, 2007
wouldn’t an analouge version with a comparitor work better, it could be less jumpy….
Alan Parekh Says:
May 30th, 2007
Hi listofoptions,

Sure an analogue version would work well, that is actually how the antenna pointers that NGinuity mentioned work. I guess the creator wanted to use a microcontroller. There are also lots of benefits using a controller like speed limiting and position restriction.
Reinaldo Yañez Says:
May 30th, 2007
Hi. You could try to use the following idea “to be less jumpy”

CalculatedSensorPosition = 0.9 * OldValue + 0.1 * ActualSensorPosition

where:
CalculatedSensorPosition - is the calculated value of the potentiometer
OldValue - is the last calculated value of the same potentiometer
ActualSensorPosition - is the actual reading from the potentiometer.

(use CalculatedSensorPosition in your formula, instead of
ActualSensorPosition)
Convex combination coefficients (0.9 and 0.1) can be adjusted.
This is a rather crude “digital filter”.
Best Regards,
Reinaldo.
John Fenley Says:
May 31st, 2007
This project was tons of fun, and I learned a lot.

One benefit of the control system I used is remote operation. Its difficult to tell, but in this video the arm is powered by a battery and control signals are being sent wirelessly from a transmitter (out of view to the left).

I like the “digital filter”. I used a similar method on a later project. It worked great.
Alan Parekh Says:
May 31st, 2007
Hey John,

Thanks for stopping by and clarifying the operation.
zedomax.com Says:
June 1st, 2007
Robot Arm using Potentiometers and Servos

Download latest version of Flash to view video! . Click Here to View in Full Screen Mode
Wow, this is sweet… This could easily be implemented for medical robotic arms.
A master/slave system I built using potentiometers, servos, and an IFI roboti…
lifekludger » Robot arm makes my brain ping Says:
June 5th, 2007
[…] [via hacked gadgets] […]