4WS with Servo Hold Mod on 3 CH TX's without mixing functions

[EQMOD]

Here is one interesting experiment for implementing 4WS Four wheel steering on a micro crawler with a limited 3 channel control TX. The TX i am currently using doesnt have 4WS mixing except that it has a two-state push button for the third channel.


One idea is to use this push button assigned to channel 3 to activate/de-activate (or power down) the rear servo using a servo board. The servo board with its H-bridge motor output connected to the V+ line of the rear steering servo allowing the driver to power-up / power-down the rear servo and maintain its "last state" or "last position" using the TX's two-state 3rd channel push button.





Here is the setup with the rear steering servo connected to a servo board's motor output. The centering pot is used to determine to which part of the PWM signal will the motor output have a + signal for the rear servo's power source.




Secured using a strong double adhesive tape;



With the shell on;





TEST VIDEOS showing 2 Wheel Steering, 4 Wheel Steering, and the "sideways" CRAB CRAWL and this is done through the TX steering wheel and 3rd channel push button.

Demo:

http://www.youtube.com/watch?v=alg0x89NSH0

Test Run 1:

http://www.youtube.com/watch?v=NB5yBTySsRY

Test Run 2:

http://www.youtube.com/watch?v=9fRyz0jmwQE

[EQMOD]

Here is to complicate things - er i mean to provide more options to the crawler operator . I;ve modified the schematics and provided an option to activate-deactivate (hold position) the front steering servo as well using the unused H-bridge output line on the added servo board;



To retain the original config, i've added an external switch (SW1) located at the RX side and make it "config" selectable. All in all with the two-state SW1 on the RX side and the two-state button (SW2) on the TX side there are four steering modes;



Location of Switch SW1 -



Location of Switch SW2 -



Servo control board with a tap on both H-Bridge outputs allowing the user to power up either the front or the rear servo or BOTH depending on the switch state combination.





VIDEOS testing all FOUR Steering Modes;

http://www.youtube.com/watch?v=8f6OfkuQkPo

http://www.youtube.com/watch?v=8KM5Dui2OXY

[EQMOD]

a little robot crawl ...

http://www.youtube.com/watch?v=wne21Svc3wc

[CrawlerSmith]

Hi, I have to say this is an awesome write up. Excellent Mod! you YouTube videos are making me jealous.

I am trying to do this myself and I am not getting the results. I am simply trying your V1.0 wiring.

After everything is wired up and in place, I am getting irratic behaviour from the rear servo. If I turn the centering pot one way the servo quickly moves to its end of travel, if I turn it another way it does nothing. Both positions have no movement when pressing the 3rd channel button on the TX.

So im stuck... all the wiring is correct (I have checked and rechecked several times). If I connect the positive from the rear servo straight to the positive from the 3rd ch receiver the servo works perfectly and moves exactly as the front servo (using the signal wire from the front servo). So it seems that when I am switching the 3rd channel button on the TX there is something wrong at the 'servo control board'.

I have tried 2 different servo boards incase of any board failure. No success.

Hope you might be able to help?

I am using a FS-GT3 transmitter.

[CrawlerSmith]

In addition to my problem... I am wondering if there is not enough voltage passing through the control board? Do servo boards down rate the voltage on the motor output?

When voltage is low on normal servo setups i get irratic behaviour and this is exactly what is happening even though the voltage is fine.

?

[CrawlerSmith]

Originally Posted by CrawlerSmith

In addition to my problem... I am wondering if there is not enough voltage passing through the control board? Do servo boards down rate the voltage on the motor output?

When voltage is low on normal servo setups i get irratic behaviour and this is exactly what is happening even though the voltage is fine.

?

Anyhoo, after some fiddling managed to do it a little differently using a Turnigy On/Off Switch... Check it out >