Move Over P20!

P20 (the robot we built for PiWars 2020) had loads of improvements over P19 (the machine we entered for PiWars 2019).  But most of the improvements were kind of… “incremental”: none of them resulted in a big performance increase.  For PiWars@Home we decided there was opportunity for fundamental change.

So, we present P21; our machine for the 2021 games.

P21, Our New Robot for PiWars@Home

I know, it looks just like P20.  But P21 has one big upgrade: it uses new motors.  The motors have encoders, which, when combined with PID enabled motor drivers, allow much finer control.  And since we’re expecting better control, we’ve gone for a 50% faster output shaft RPM so that the robot is faster too.

This change has taken a long time to complete since it has required some fundamental changes.

  • Our designs employ an Arduino based peripheral control board which perform the low level, timing-constrained, jobs on behalf of the Raspberry Pi.  We have replaced this board with a new one since each motor now requires a couple of digital signals from its encoder.
  • The peripheral controller software has also been re-written.  The motors are now interfaced via PID algorithms; this means that when the Raspberry Pi requests a motor speed (expressed as a percentage of full speed), that is what it gets.  With the previous (pulse width modulated) control scheme, you get a vague approximation.  At low speeds control is particularly poor with PWM systems where no motor speed feedback is employed (because of the friction that is inherent with gearboxes).
  • The chassis has also been redesigned and re-printed since the motors are a different size.

There is one other difference between P20 and P21: the implement interface has been changed.  P20 provided a bunch of servo outputs for implements; so, it is simple to connect servos with no further electronics.  P21’s implement interface provides all the internal voltages (5.2v, 6v and the battery voltage) plus a two-way serial interface.  This change was largely imposed by the toy box lift that uses motor pulses to measure the position of the jaws above the ground; the distance of travel is too much for a conventional servo.  The new interface is far more flexible than the old system, but it requires every implement to have a dedicated control board. There’s no such thing as a free lunch.

P21’s hardware and low-level software is now largely complete.  We now need to work back through the higher-level code in the Raspberry Pi to take advantage of P21’s increased performance.  We’re hoping to see some big improvements soon!

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