Page 18 - Industrial Technology EXTRA 28th September 2020

Operational lifetime
The motion system has a job to perform which is
typically defined in a number of cycles per day and
time per cycle or hours of operation per day and
duty cycle of operation. With these details, we can
determine the approximate number of hours the
motion system needs to complete over the
anticipated product life.
This impacts on the motor choice.
Brushed DC motors have a mechanical
commutation system that wears over time, limiting and control the position of the rotor. Standard
the lifetime of the motor. Brushless DC and stepper encoders offer a range of resolutions within the
motors are electronically commutated and therefore same package size to meet the varying
do not have any wear associated with commutation, requirements of applications. Increased resolution
giving them a longer expected lifetime. can be gained by the addition of a gearbox on the
Another factor to consider for lifetime of the front of the motor. The resolution is multiplied by
motion system is the bearing system. Sleeve the gear ratio, so combining the encoder and
bearings will provide a few thousand hours of life gearbox multipliers can achieve precise positioning.
while ball bearings will generally provide in excess Stepper motors provide positional accuracy
of 10,000 hours of life. Of course, we are assuming based on their mechanical construction. The
that the radial and axial load applied to the motion number of poles on the rotor will dictate the number
system shaft stays within the design limits of the of steps per revolution, providing a step angle for
specification, and that there will not be elevated each pulse given to the motor. Drivers typically can
temperatures that will reduce the lifetime of the increase this resolution by half-stepping or micro-
lubrication. stepping, creating intermediate electrical steps
between the mechanical ones. The addition of a
Mechanical envelope gearbox is an option here too, gaining the additional
A critical first action is to confirm that the chosen resolution of the gear ratio.
motor technology is available in a diameter and
length that will fit in the allotted space within the Duty cycle
application. While the speed and torque Typically, the motor does not run continuously but
requirements can often be met by one or more rather operates for a period (on-time) and then rests
motor technologies, and even by variations within a for a period (off-time). The main consideration with
given technology, the power capability needs to be the duty cycle is the temperature rise of the motor.
reviewed to ensure required mechanical envelope All motors have a maximum rated temperature and
can be achieved. operating the motor above that temperature can
lead to damage to internal components.
System accuracy The amount of current drawn by the motor will
Another factor to review is the accuracy required dictate the temperature rise; the higher the current
from the motion system. Both brushed and the faster the temperature will rise. Since current is
brushless DC motors require an encoder to track proportional to the torque output of the motor, it can

September 2020 • INDUSTRIAL TECHNOLOGY EXTRA • p18