Power 1501MG Servo Apology and First Robotic Leg Test                    

Speed PWM modified servo

My apologies to the Power Servo company about my complaints concerning their 1501MG servo. I discovered that the six volt bench-top power supply that I had been using to power them, could not deliver the required current. I spent the last few weeks tinkering with the internal servo circuits and twice building replacements. I got the same screwy results with my circuits. When I used the servos to test the vehicle’s on-board 6 volt supply, suddenly everything worked fine. There was never anything wrong with the servos and I will be using them just as they came out of the box. Servo PWM speed control is an interesting idea but the generic servo is going to be less complex to use.

The newly installed leg calipers can stop the legs from swinging at any position. I wrote a short Arduino program to test the concept of rapidly swinging the legs front and back, powered only by the stored potential energy of the knee movement. The action is similar to how a child pumps on a swing to make it go. It looks like this concept is going to work fine. Check out the video.

A concern at the moment is that sometimes there is harmonic motion in the piston movements that can cause a leg to shake. I have to think of a way to dampen this kind of movement.

Tinkering with the Servos, a New Catch and More Sensors          

M53660L

I converted the Power 1501MG servos to have their positions digitally read and then sent speed PWM signals to them instead of position signals. This method works much better for fast moves and now the servos do not accidentally loose their positions. But another quirk of the 1501MG’s internal logic seems to be that for slow speed moves, there is a built-in delay of up to two seconds!***  I am going to scrap the electronics inside these servos and replace them with standard M51660L servo controller chips (eBay $2.50).  

Ultrasonic distance sensor.

I installed an ultrasonic distance sensor (ebay $3.98) on the cart frame to monitor how high off the ground the legs have lifted the cart. It uses two Arduino digital channels, one to trigger the ultrasonic pulse and another to detect the echo.

Break switch.

I made a break-peddle activated switch to tell the Arduino that I’m pressing on the break.

Hall effect wheel speed sensor.

I made hall-effect wheel sensors to detect how fast the cart wheels are turning. It sends a positive pulse to the Arduino every time the cart moves forward eleven inches; that is ¼ of a turn of the cart’s wheel.

Caliper

I am redesigning the leg catch system. In addition to freezing the position of the leg at the hip when the leg is all the way back, I may want to lock the leg in the forward position as well. I made pneumatic calipers that ride on the front of each leg that can grab on to an aluminum bar and are able to stop the leg swings at any position. The calipers are cheaply made out of a 2” PVC fitting and a piece of rubber inner-tube. A couple of 24 volt pneumatic MAC valves from the junk box activate the calipers. Unfortunately, I have to do quite a bit of disassembly of what I’ve already built to make the change.




*** The 1501MG servos are good. See July 19, 2014 entry for explanation.