Ackermann Drive


VolksBots usually are differential wheeled robots; they change direction of movement by varying rotation speed of their left and right driving units. An alternative is to implement an Ackermann steering unit (A-steering unit) which was invented in 1816 by Rudolph Ackermann.
A-steering is based on different steering angles for left and right wheels of a steering axis. In automotive implementations this is realized by dedicated mechanics shown by the picture on the right side. The steering angle (also called Ackermann Angle) for each wheel depends on dimensions of the vehicle (wheelbase, track-width, wheel distance to steering centre).
In VolksBot we did not apply a mechanical A-steering unit, we used distinct steering drives for left and right steering wheels instead. The calculation of Ackermann angles is done in software by taking robot dimensions as input parameters. This enhances flexibility in robot dimensioning and additionally supports higher steering angles.
In order to achieve reasonable driving behavior with great steering angles, we placed the robot driving units also on the steering wheels.

Our solution on the base of Volksbotelements consists of:
  • Two UDUs (Universal Drive Units)
  • Four wheels 260 x 85 mm
  • Wheelbase: 560 mm
  • Track-width: 436 mm
  • Wheel distance to steering centre: 62,5 mm
  • Max. steering angle: +- 60°
  • Max steering speed: 33°/s
  • Integrated shock absorber
  • Steering drives: 2 Maxon DC motors 22 W, each with planetary- and worm- gear and Maxon EPOS 24/2 motor controllers
  • Driving unit: 2 Maxon EC Flat motors 90 W, each with Harmonic Drive gear and Maxon EPOS 24/5 motor controllers
  • Power-Supply: Two lead gel accumulators, 12 Volts, 15Ah
  • VolksBot Power Unit (SPU) with additional power supply 24V/20A, 12V/5A, 5V/5A
  • Software control by C++ libraries for Windows und LinuX incl. modules for calculating different Ackermann angles and driving speed for left and right side respectively

Videos:
Achsschenkellenkung volksbot volksbot
   
Fraunhofer-Institut für Intelligente Analyse- und Informationssysteme