New Servo Follower Valves                                                        

New servo follower valve

I made new servo follower valves and installed them on the legs. The holes in these valves are of a narrow triangular shape, such that as the leg is nearing it’s target position the flow of air in or out of the piston cylinder is restricted. The slower movement minimizes the overshoot oscillation motion which was giving me a problem.

The video shows a leg test where the legs take turns going up and down rather slowly. There are still occasional sudden unexpected movements that are worrisome, but I think I will go ahead with the programming of the legs and cart just to see what will happen.

Leg Harmonic Motion Problem                                               

I attached the legs to the cart thinking that when the legs were compressed by the weight of the cart, that it would dampen the harmonic motion that occurs at times. Wrong; it got worse, as can be seen in this video of a right leg test. The leg even becomes airborne at times, which would be handy if I was building a running bot, but this one is supposed to stay on the ground. In the test I make the right leg extend and contract several times and then I extend the leg incrementally. 


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Reasons for the harmonic motion may include:

1.    When a leg is compressed, more air is added to it’s piston causing upward movement (as intended). By the time the leg reaches it’s correct position, there is upward momentum that carries it past nominal, now causing air to be released from the piston. The leg contracts and the downward momentum causes it to overshoot the nominal position again. And so it goes.

2.    When the leg is nearly straight, small changes in piston length cause large changes in the knee angle. The resulting rapid lower leg movement, carries a larger amount of kinetic energy, making the harmonic movements more extreme.

3.    When the leg is nearly straight, the piston cylinder has the greatest volume. Large air volumes take less force to compress per unit of piston travel, so the leg is softer(more easily compressed) when nearly straight.

4.    When the leg is nearly straight, the lower leg’s mechanical advantage to compress the piston cylinder air is at the maximum. This also makes the leg easily compressed when it is extended.

Ways to dampen this harmonic movement:

1.    I thought adding mass to the leg would slow or stop this movement. The leg will bounce even with me sitting on top of it. (50lbs legs + 150lbs me = 200lbs of load!)

2.    Making the valve holes small would slow the movement speed. The leg would probably not overshoot it’s target position. But I need the legs to move fast if they are to walk.

3.    Changing the shape of the valve holes to taper-off the air flow as the target position is neared. This would probably help.

4.    I could monitor the knee position in real time and add or release air with a servo driven proportional valve. The speed could be calculated so as to not overshoot the target. This could take up a lot of microprocessor time and a timing lag could also cause harmonic motion.

So, I am going to rework the current servo-follower valves to make the air holes of a tapering tear-drop shape.  If this does not work, I will abandon my servo follower type of valve and instead, make proportional valves that regulate the airflow under direct microprocessor control.

Wiring  Pinouts                                                 

Someone's wiring mess.

It looks as though very one of the Arduino Uno’s pins are going to be used. There are 6 analog and 13 digital I/O. I added a 4x20 liquid crystal display (I2C LCD eBay $11.95) and a game controller four button left-right-up-down rocker switch to the project. It turns out that this LCD must use analog channels A4 and A5.

At the moment the Arduino pinout looks like this:

       Digital Pins    0     I/O       USB TDX

                            1     I/O       USB RDX

                            2     Out      Caliper valve R

                            3     Out      Caliper valve L

                            4     Out      Ultrasonic sensor Trigger

                            5     Out      Servo PWM R

                            6     Out      Servo PWM L

                            7      In        Break switch

                            8      In        Wheel speed sensor

                            9      In        Ultrasonic sensor Echo

                          10      In        Up momentary switch

                          11      In        Down momentary switch

                          12      In        Slower momentary switch

                          13      In        Faster momentary switch

    Analog Pins   A0      In        Pressure sense  R

                         A1      In        Pressure sense  L

                         A2      In        Rotation sensor R

                         A3      In        Rotation sensor L

                         A4      I/O      I2C display SDA

                         A5      I/O      I2C display SCL

I am using a 13 wire cable with quick connectors that came out of a retired car’s CD changer, to carry the connections between the legs and the Arduino in the cart. The cable pinout looks like this:

  0. Shield                               Ground

  1. Orange                              5 volt for sensors

  2. Pink (big)                          Not used

  3. Black (big)                        Ground

  4. Yellow                              Rotation sensor right

  5. Red in Brown shield          Pressure sensor right

  6. Dark blue                         Servo PWM right

  7. Brown & Grey shielding     Ground

  8. Purple                             Rotation sensor left

  9. White in Grey shield         Pressure sensor left

10. Grey                                Servo PWM left

11. Brown                              Not used

12. Powder blue                     Not used

            13. Green                              Not used

An additional four conductor cable was needed to power the caliper valves and to supply power to the servos. The servo power lines caused sensor noise when they were inside the 13 wire cable. All is well now.

           Black   –   Ground                        Red      –    6 volts for servos

           Yellow -   24 volts Left caliper      White   –  24  volts Right caliper

Project Electrical Layout.