Editorial Feature

Robotic Drive Train - DC Motors

Actuation of a robot is powered by a drive train – a mechanical cluster including the motors, gear control unit, driving belts, axles, wheels, and mechanical exoskeletal limbs – all such mechanical components help support the locomotion of a robotic structure.

The motor is considered the heart of a robot and is the main driving force behind moving an artificial structure. Complex robots are constructed with three main types of motors: the direct current (DC) motor, the Servo motor, and the Stepper motor. For the scope of this article, attention will focus on the DC motor, structural components to the DC motor and the functional principles to this component.  

Direct Current Motor

A basic DC motor runs on a direct electrical current supplied from a battery or AC wall current. The direct current provides electrical energy for the motor and helps rotate the drive shaft. The following video animates the principles to a DC motor and demonstrates how this component works.

Video courtesy of Maxon Motor.

At an electromechanical level, the DC motor is designed to convert the electrical current into energy that will drive the mechanics to a robot by applying a certain degree of torque to the motor shaft.

Typically, a permanent magnet DC motor is used in the design and construction of robots. Stators and rotors make up main functional parts to a DC motor. The torque on the motor shaft is manipulated by the electromagnetic flux in the stator.

Permanent Magnet Direct Current Motor

With permanent magnet DC motors, the magnet uses an electrical current to create a magnetic field to drive the motor.

Permanent magnetic DC motors that are made up of an iron core have an armature shaft (an electrical component) that rotates within a certain quantity of magnetism.

The magnet used in this motor is made out of earth alloys, though some magnets used for this purpose can be made out of ceramic and alnico (a type of iron alloy).

An iron rotor designed with slots helps wind the rotor and stator. To put this into context when considering the speed and movement of a robotic machine, as the speed of the DC motor increases, the movement of force starts to fall.

It is quite common for conventional DC motors to use an iron core permanent magnet, but these motors are designed to have teeth to power the winding motion of the DC motor, though this can lead to a torque ripple.

The teeth to the iron core result in a start and stop function to allow for precision in the locomotion of a robot, but this clogging will cause vibration and noise.

Coreless Direct Current Motors

Maxon Motor have designed a DC motor that eliminates the need for an iron core. The Maxon DC motor still retains the three main functional components to the DC motor: the stator, which has a permanent magnet at the centre of the motor; and the rotor that has a winding component connected to a shaft via a plate.

The shaft to this motor is positioned using ball bearings. The winding component moves and collects air gaps between the magnet component and the housing (a component that allows for magnet return).

The brush system to this motor is made out of graphite that feeds conductivity via electrical motor connections.

One of the main advantages to the ironless DC motor is the elimination of magnet teeth resulting in a smoother running of the DC motor at varying speeds with the added advantage of eliminating vibrations and noise – a DC motor allow for better control of rota positioning and better controlled behaviour of the robot.

DC motors are manufactured in a range of sizes and ideal for their application in robotic structures because they are capable of carrying a particular load with a high torque-to-volume ratio.

In order for the DC motor to work efficiently with the wheel and extremities to the robot, this component needs to be connected to a gear box, belt, and chains that will collectively drive robot articulation.

The video below demonstrates an example of how DC motors have been used in WiFi robot construction. In comparison to hydraulic and pneumatic drives, the DC motor can provide clean drives and this can help with precise repeatability of movement.

Video courtesy of Roboteq Inc.

Sources and Further Reading

  • Branwyn, G. Absolute Beginner’s Guide to Building Robots. USA, Indianapolis: QuePublishing.
  • Deb, S.R., Deb, S. (2010). Robotics Technology and Flexible Automation.  New Delhi: Tate McGraw Hill.
  • Maxon DC motor. Permanent magnet DC motor with coreless winding. Maxon motor ag, www.maxonmotor.com/academy.

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