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The Servocore 2-motor walker circuit
The Servocore walker electronics (a 2-motor walker using un-modified hobby servos) by Wilf Rigter

The original Servocore circuit indicated how a high frequency PWMcore could be modulated by a low frequency microcore central pattern generator. Straight from the idea cauldron, it required some time to digest (food for thought?). More than 2 years later here is a regurgitated uServocore version 5 complete with turn and reverse. It uses two 74HC14 chips and is rather more complex than its raw progenitor circuit. While still a thought experiment, the design incorporates an number of some interesting (innovative even) bits to point out.

uServocore5 schematic

PWMcore = tricore
The PWM loop is a tricore, a single delay stage separates the active high PWM pulses for both servos. 

Microcore to Bicore translator
There is an advantage in using continuously active PWM signals. The main reason is that the asynchronous frequencies of the PWMcore  and microcore can sometimes result in a forshortened PWM pulse. The Sevo reconizes this as a command for a new positions and twitches toward that position. This makes the motion somewhat jerky on the original uServo4 circuit.  In this application the microcore is actually used as a quasi-bicore. The microcore outputs are connected through 4 diodes to give the equivalent of master / slave bicore waveforms but with individual control of the rising and falling phase angles. Since that can be used for turning a conventional "bicore" walker (but not the uServo walker), this technique may be exploited in other walker applications as well. 

Normally the microcore generates 4 individual pulses from Nv1 - Nv4.  The master / slave bicore waveforms are two ovelapping quadrature waveforms. The translator synthesizes the bicore waveforms  in two parts using diodes D3 - D8  to OR the microcore outputs.  During forward motion the output of Nv 1 is not used   So when Nv1 is active, the PWM pulses for both S1 and S2 are not influenced through diodes D3-8. and the PWMs generate the  longest pulses driving both servos to CCW. When Nv2 is active it controls duration of the first half of the S1 CW position, Nv3 sets the second half of CW for S1 and the first half of CW for S2. Nv4 controls the second half of CW for S2.

Servo Rotation Sequence
The rotation sequence for the S1 and S2 servos is controlled by the active high microcore Nvs as follows:

S1 (front)
S2 (rear)
CW (Nv2)
CW (Nv3)
CW (Nv3)
CW (Nv4)

Reversing the walker

If the outputs of Nv1 and  Nv3 can be alternately selected, that would provide a 180 degree phase shift of the PWM signals required to reverse the walker.

The microcore to bicore diode network also part of the reverse circuit to select whether the rear legs lead or lag by 90 degrees and therefore reverse the walker. The reverse circuit is a multiplexer to select either the output of Nv1 or Nv3 similar to the Mx circuit but using two 74HC14 inverters  and diodes to "steer" Nv1 and Nv3 pulses and to set the reverse delay. During forward motion Nv1 output is blocked by D10 while the output of Nv3 is enabled by D9. Conversely when reversing, Nv1 output is enabled by D10 while the output of Nv3 is blocked by D9.

The rotation sequence for the S1 and S2 servos to reverse the walker is  as follows:

S1 (front)
S2 (rear)
CW (Nv1)
CW (Nv1)
CW (Nv2)
CW (Nv4)

The duration for the microcore Nv 1 - 4 pulses should be checked  to be equal and slightly longer than the time for CW and CCW rotation. The 74HC14 diode multiplexer / reverser may be used for other microcore applicatons.

Turning the walker
A somewhat primitive turning circuit has been used to add a left, right or neutral bias on the rear motor PWM pulse generator causing the walker to veer left, right or go straight.  The 1M pot (R9) is adjusted the turning radius limited by the stability of the walker. The next revision will use the remaining spare inverters for a more positive turning circuit however for now, lest I suffer some serious mental indigestion, that's it. 

For more information...

Wilf's original presentation of the ServoCore V5 material came in Yahoo group post #32315 here. Meanwhile, the presentation of ServoCore V6 came in Yahoo group post #xxxxx here.

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Page author: Wilf Rigter
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