Archives for posts with tag: holonomic

The large EV3 motor has a different form factor then the NXT motor. I had to redesign the legs of Agilis to make use of these new motors.

EV3HolonomicLeg1

EV3HolonomicLeg2The new design is smaller and much prettier to the eye. The smaller size and all the triangular shapes makes the leg very stable. The new leg can be fitted on the same triangular frame that was used for Agilis.

Here are the building instructions for the new leg.

If you only have two large EV3 motors you can easily modify the leg to use the EV3 medium motor. This motor can be attached directly to the drive axis of the wheel and to the underside of the frame. You will have no gearing. But as the medium motor rotates a bit faster you will still have plenty of speed in your robot.

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In case you might wonder how fast or accurate Agilis is, here are some numbers.

Setup

  • The gear ratio of the motors to the wheels is 1:2, making the wheels rotate twice as fast as the motors. (It is possible to change the gear ratio to 1:3).
  • Prototype orange Rotacaster wheels. This is the hard compound. There isalso a medium compound (gray) and soft compound (black) available.
  • The batteries were rechargeable NiMH, 2500 mAh batteries. These were not fully charged.
  • The test surface was a clean linoleum floor.

Speed

  • Reliable top speed is about 60 cm/sec, equivalent to 2.16 km/h or 1.35 mph. At this speed the robot is still accurate as there is ample margin for the PID controllers of the motors.
  • Unreliable top speed is about 75 cm/sec, equivalent to 2.7 kmh or 1.68 mph. At this speed the robot is not very accurate, especially the heading.

Accuracy

  • The test track is a square with sides of one meter each. During each run the test track is traveled 4 times. Making the total distance of the test track 16 meters.
  • The robot finishes the test track on average within 10 cm of its starting position. Expressed as a percentage of the total distance the error is about 0.6%.
  • The movement error is systematic. The robot always ends up above and to the right of the starting position.
  • The robot is more accurate at slower speed and acceleration settings.

The images shows the result of the accuracy testing. For each test the robot was placed exactly on the origin (bottom left in the picture). It then traveled a square with sides of one meter for four times, making the total distance traveled 16 meters. The finish location of the robot was then marked on the floor. This test was repeated three times for a single set of settings of speed and acceleration. Three different dynamic sets were used,  speed: 50 cm/sec and  acceleration at 100 cm/sec^2, speed 50 cm/sec and acceleration at 750 cm/sec^2 and speed 30 cm/sec and acceleration 60 cm/sec^2.

foto (7)

I want to repeat the tests with a 1:3 gear ratio and also with the black Rotacaster wheels.