FEATURE DRIVES & MOTORS
DESIGN BASICS: A quick guide to servomotors
Andrew Fallows, managing director of Motor Technology, goes back to basics with servomotors and looks at some of the latest developments and advantages of modern systems
T
he term servomotor derives from the Latin, servus, meaning slave or
to serve, and is meant to indicate the use of a motor whose main function is to respond to commands or to perform specific tasks. The origin, or first use, is generally
believed to have come from the French, Le Servomoteur, coined by J.J.L. Farcot in 1868 with his reference to hydraulic motors and steam engines used in the steering mechanisms of ships. Today, modern servo systems are used
to perform many motion control tasks – from the relatively simple to the highly complex and dynamic requirements of demanding applications. Fundamentally a servo system consists of a motor, a feedback device connected to the shaft of the motor, such as an encoder or a resolver, and the electronic control. The main benefit of these systems is the ability to precisely control speed, torque and position through closed-loop control.
SERVO ADVANTAGES Servos offer distinct advantages over other types of motors on systems that require highly accurate control of speed and motion. They operate with
an extremely high degree of precision and can accommodate complex motion profiles on a wide range of demanding applications. Just like stepper motors, brushless servomotors evolved from brushed DC motors. On brushed DC motors permanent magnets in the motor housing form the stator field, and windings on the rotor, or armature, react with this field by turning when an electric current is applied. To keep the shaft of the
Direct drive motor
turned inside out, so that the fixed field is now on the rotor and the windings have been moved to the stator. The commutation is performed by an external switching device known as the drive. The drive supplies the power based on the commands of a motion controller, and together they provide the electronic control of the system. Brushless servomotors are highly
reliable and virtually maintenance free since the arcing that occurs on brushed motors between adjacent segments of the commutator, resulting in erosion, is eliminated. With no windings on the rotor they are not subjected
to centrifugal forces and, because the windings are supported by the
housing, they can be cooled by conduction, requiring no
airflow inside the motor. This, in
turn, means that the motor’s internal elements can be entirely enclosed and protected from dirt and the ingress of other foreign matter. Brushless motors can be specified as
motor turning the magnetic field
in the rotor is continuously switched, and this supply of current, and the switching, is done mechanically by the brushes and the commutator. On a brushless servomotor there is no
commutator or brushes, and the original motor design has been more or less
Construction of a DC motor
AC or DC, which can be confusing as both AC and DC drives produce an alternating output. The difference is in the shape of the alternating current waveform derived from the drive into the stator windings: DC is designed to produce a square wave or trapeziodal output and back emf; and AC is designed to produce a sinusoidal output and back emf. Brushless DC systems are more associated with analogue drives and lower-cost or small motor systems. They can be more dynamic but suffer from low speed cogging and are therefore more difficult to control at low speed. AC motors are smoother and are more associated with digital systems. With high bandwidth and high resolution drives they can be equally, or even more dynamic than their DC counterparts.
POPULAR SOLUTIONS In more recent times Torque motors, also known as Direct Drive motors, have become increasingly popular with the need for greater reliability, higher accuracy and improved efficiency. The direct drive motor is essentially a brushless servomotor with a greater number of windings on the stator. Standard motors have four, six, eight or even ten poles, whereas direct drive
12 JUNE 2014 | DESIGN SOLUTIONS / DESIGNSOLUTIONS
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