The prime mover is possibly the most
important part of the whole drive system. The role of this main component is to convert some
form of energy, be it from a type of fuel or electrical, into a rotational movement that will
be capable of enabling the whole robot to move. For example, converting the energy from a
battery into rotational movement as in the case of an electric motor. The choice and sizing of
the prime mover was a vital role within the project and in the following sections there are some of the
processes used to finalise the design of the drive system for the Robot.
After the initial sizing of the motors that would be required for the robot application many
motor distributors and manufacturers were approached in order to obtain as much data on
different types of motors as possible. The companies were contacted via e-mail but with
limited success, although there were a few companies who were particularly interested and not
only was a reply obtained but also a number of brochures detailing the companies' products were
After comparing the information that had been received from the companies with the requirements
that had been worked out, several of the motors that were available to choose from were deemed
to be suitable. Initially talks with one of these companies looked promising. However a conflict
of interests arose due to the company also sponsoring another Robot Wars entry and the offer of
supplying the drive motors was revoked. Finally another suitable choice was decided upon and
an order was sent.
The company chosen to supply the motors was Motion Control Products Ltd.
The motor that was finally chosen and ordered from Motion Control was the 75PX985. The choice of
this motor was influenced by several factors. Firstly it was compared with the calculated motor
requirements (the comparison will be discussed further on in this section). The other factors
that swayed the decision were properties such as the mechanical dimensions of the motor, (the
most important dimensions are shown in the figure below). Also the weight of the motor was important
and at only 7.6 kg each they were a considerable amount lighter than other motors of the same
power. The only draw back found with the motors was the price however a consultation with the
treasurer and the project leader concluded that the extra expense was balanced by the advantages
of the motors.
The following tables are used to show respectively the comparison of the selected
motor characteristics with the calculated motor requirements.
As shown by the tables the shaft speed of the actual motor is three times the required speed,
which suggested that a gearbox with approximately a 3:1 ratio would have to be used to obtain
the required speed. Using a ratio of around 3 to 3.5:1 also meant that the minimum motor torque
required by the robot would be produced by the motor. Also when the 3.5:1 ratio was applied to
the peak torque there was more than sufficient output from the motor to handle the required
peak and minimum motor torque requirements.
The diagram below represents the speed-torque characteristics of the D.C.
brushed permanent magnet motor that was selected to run the robot.
The estimated peak operating position (shown on the diagram as the diamond shape) is situated
within the operating limits of the motor, however it is outside the continuous operation zone.
This means that the motor will experience heating effects above its normal operating temperature
, but as the contest are only approximately 5 minutes in duration the exploitation of the
transient thermal rating of the motor can increase the available output power of the motor.