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Biology - Electronics - Aesthetics - Mechanics

Z-Walker mk II


Project status : Open
Last update : 15 Jan. 1998

After the terrible lost of the Z-Walker mk 1, life went on. I wanted to build a better walker. I took some time to make a list with weak and strong points of the previous design and then i added the things to the list that i like to see in my next design.


Weak spots

  1. Leg drift
  2. Leg design
  3. Leg connection to the shaft
  4. Power to motors at start-up
  5. PCB layout
  6. Could not backup
  7. No sensors



Strong points

  1. It walks
  2. It looks great
  3. Simple circuit
  4. Robust
  5. Good test platform



Planned improvements

  1. Smaller design
  2. Better legs
  3. No power to the legs at start-up
  4. Limiting leg DOF with switches
  5. Backup ability
  6. Improve turning ability
  7. Solarizing
  8. Sensors
  9. Sound
  10. Platform to do further experiments
  11. Optional : NiCd re-charging



I've done allot of experimenting lately. Just to see if it work and how it work. My goal is to lessen to number of components needed for the complete design. I am especially busy with the backup mechanism. I've tried several combinations so far. A 74x244 with a H-bridge, but i didn't find this option stable enough. I've tried using the 74x139. This option work very well. But you still need to switch wires to backup. This means again extra components. A relay is out of the question because a relay use too much current. Backup the walker using bi-lateral switches looks also good. I am familiar with that design and my favour will go for it. If i decide to use that approach then the walker will have one 40106 as a MicroCore, two 4066 for the backup switching and twelve transistors for the H-bridges. That last piece is bothering me, too many parts. I still have to try if i could drive the motors directly with a 74S139. I have tried a 74LS139 but the LS type do not produce enough output current. However i could measure a plus/minus voltage change at the outputs. I got an 74F139 and i tested it. The result can be found here Zoelen-bridge Improving the turning ability will become probably a mechanical solution. I will keep you informed about the last one.

GRAPHICS
ASCII
 
Planned design so far
+-----------+   +-----------+
|           |---|           |
| MicroCore |   |  Sensors  |
|           |---|           |
+-----------+   +-----------+
    |    |
    |    |
+-----------+   +---------------+   +----------+
|           |   |               |   |          |
|  Backup   |---|  2 H-bridges  |---|  Motors  |
|  switch   |   |               |   |          |
|  2x  4066 |---| [transistors] |---|          |
|           |   |               |   |          |
+-----------+   +---------------+   +----------+
    |    |
    |    |
+-----------+
|           |
|  Sensors  |
|           |
+-----------+ 

Fig. 1


To do:

What holding the project is that i still have to check if the backup circuit can be placed between the MicroCore and H-Bridges. I still haven't a good design for a solar engine to power the electronics and i have to try a new SE design first. Also i still don't feel right about the backup design. I am not really happy with it.



24 Dec. 1997

The design start to get shape now. The full schematic has become far to complicated to convert it to a PCB by hand. To design the PCB i use a tool called easytrax. [freely available on the net.] The printed PCB was then draw by hand on the copper. The size of the PCB is 53mm x 78mm. I also descided by now that its time to port PCB drawing using photographic layers. Below i've include a schematic i planned to use. If you look well you will notice that there are some major differences with the previous block design!

I use now two adjusted servo motors. The end stop is removed, because if the motor is blocked then you get a enormous waist of energy. I plan to use end switches switching the enable line on and off. The servo control print is replaced by a Tilden H-bridge. A Zoelen bridge alone would do perfect also. I designed the PCB in such way that it is possible to drive the motor with a Z-bridge only. The motors can be removed and exchanged with other types if needed.

The PCB is already prepared for solarizing. A timer/oscillator is onboard. I've changed the design a little because there is a diffence between the time a walker can move in full light and in the shadow. However the standard design has a fixed time. With the two photo-diodes i hope to be able to dynamically adjust the timer values according the light intensity.


I hope to expand this page further soon.

Bram



Copyright 1998, A.A. van Zoelen. All rights reserved.

A.A. van Zoelen / vsim@mail.com
Updated: 26 Oct. 1999