Sunday, 30 September 2018

If at first you don't succeed ...

After days sawing, drilling, redrilling and redrilling (honestly - some parts are looking like a sieve!) some sheets of 3mm acrylic, and then bolting on all the motors and electronics, Sputnik 0.3 emerged.
The first robot free from lego.

With hope and suspense in the air, the Raspberry Pi (and currently unused Arduino) were powered up. The controller was paired and we were delighted to see the motors turned in the correct direction when we moved the controller without having to swap round all the wires. That has never happened before! Hopes were high.

The robot was put on the ground and it was just as fast as we had wanted - actually a lot faster. Success! Except for one thing - it wouldn't turn! As soon as we put the joystick to the left or right, the motors stalled and the robot was stationary. The motors were quick - but clearly did not have enough torque.

So we had a robot that would probably be OK for the straight line challenge - but not a lot else.

We realised we still have a lot to learn about robotics. Found a good link about robot chassis dynamics on the vexrobotics site which told us that you want a short, wide robot to reduce the scrub forces.

However, we were planning to put distance sensors on stalks outside of the wheels, which puts us close to the maximum width, so we couldn't make it much wider - and the wheelbase is about as short as we can get it.

So to fix this we're going to:

  • Redesign the robot - increasing the wheelbase slightly to put the sensor between the wheels to allow us to move the wheels further apart. 
  • Buy new motors with a different gear ration. 
Its a shame - we thought this build would pretty much be the final configuration so we can start practising and coding, but there's still work to be done. 

So - time to try, try again ...

Lessons learnt this build: 
  • Acrylic is horrible stuff to work with using hand tools. So tricky to get straight lines when cutting out details - and the drill has a habit of moving without realising it - moving the hole a crucial millimetre to the right or left. Also - the only place we have to build is outside in the garden, so am a little worried about the amount of plastic waste produced. Managed to clear up most of it, but still. The next build will be using plywood. 
  • Do more prototyping with cardboard. After the 1st cardboard build, we thought we could build a more compact robot - but then didn't rebuild using cardboard. If we had we would have realised that there really wasn't enough room to put all the ultrasonic sensors we wanted. 
  • Look at the stall torque figures when buying motors - not just the speed. The motors we had on our lego prototype were not the right shape for the final design - but did have enough torque to turn - so that should have given us a clue. 
Watch this space for Sputnik 0.4 ... 

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