MeArm part 2

I got a Mearm for Christmas. Now I have 4 working servos, I have been able to complete it.

I thought the instructions were fine. As suggested, I didn’t fix the claw until I had the servos working. Then, I set the servo controlling the claw to 90% (closed), and then I screwed in the arm to the servo whilst the claw was as closed as I could get it. Inevitably, the claw was not quite closed as I was screwing in the arm, but it closes fine once I set the servo to 95%. The other thing that does not quite work is the way the base connects to the “hip” servo: the bottom of the “box” that contains the arm doesn’t quite sit flat, and, when the arm is sticking out, the bottom of the box bends down to touch the actual base unit, causing it to stick slightly when the hip goes round.

To get the MeArm working, I used this post, and the code at his GitHub site here.

My first attempt, using ForToffee’s arm_cmd.py, was terrible. Requests to move the servos were not being turned into movement (or even any attempt to move the servo), the actual movement of the servos did not match the requests, and when I asked it to move the shoulder, the hip would move too! I then made two alterations, but I can’t tell you which of the 2 changes was the important one 🙂 First, I stopped using the GPIO unit for Python, and switched to using ServoBlaster. ForToffee said he needed to do this for his more demanding code that was looking for joystick movements; I think I needed it just for simple arm movement. Second, I loosened almost every screw.

And it now works a treat with ServoBlaster. The servo on the grip (#1) moves from 95% to about 80%; the servo on the hip (#5) moves from 0% to 100% with 180 degrees of rotation; servo #4 goes from 40% to 100%; servo #3 goes 55% to 100% (and I can’t tell which is the shoulder and which is the elbow!). So far, I have been typing the ServoBlaster code in interactively, but I am pretty sure I can amend ForToffee’s code so we have 8 keys which each move a servo by a small increment in either direction.

Hooray! A Raspberry Pi project that works!

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