Servo motors are designed to operate bi-directionally and to be used in a servo loop to position a load through negative feedback, in a stable manner exactly where a command requests that load is to be positioned. Rotary servo motors are either Brushed or Brushless.
BRUSHED SERVO MOTORS
Brushed servo motors consist of a rotating armature carrying the motor windings with permanent magnets fitted inside the outer case of the motor. A copper segmented commutator on the end of the armature carries the winding ends in pairs, to a pair of carbon brushes. A DC voltage applied to these brushes causes the armature in the classic way, to rotate and present the next pair of armature/commutator contacts to the brushes and so on. This produces motor rotation in the magnetic field generated by the permanent magnets. This is a simple design and well proven with self commutation. A change of polarity on the brushes changes motor direction. No voltage on the brushes produces no rotation – but probably no torque.
A Brushed servo motor drive or amplifier produces current to drive the servo motor in an intelligent manner and in a selection of modes. In Current or Torque mode a servo amplifier will introduce a Gain in the system where a low voltage swing from +10 to –10 Volts for instance, will command full motor torque/speed in one direction to full torque/speed in the opposite direction. O Volts will command no movement and with a full controlled closed loop will command full opposing torque to counteract any tendency for the motor to move in a direction away from stationary.
Brushed servo motors have high peak starting torque where, for a short duration, that torque is available to break friction/stiction, to start and to accelerate a load to a required velocity. Then when decelerating the load, to act as a brake through regeneration of energy back in to the power supply or dump resistors.
Generally when more torque is required, more copper is required in the motor armature windings, for a given current flow.
When a motor with more copper in the windings operates at higher speeds, then:
- The motor “generator effect” becomes dominant. A larger back EMF voltage is generated
requiring a higher overcoming bus or drive voltage from the servo amplifier.
- Speed and torque are therefore mutually exclusive.
With the armature on the inside of the brushed motor the heat flow from the windings is not
conducted efficiently out of the motor case.
BRUSHLESS SERVO MOTORS
Brushless Servo Motors have a 3 phase winding on the inside of the motor case, therefore, heat
conduction out of the case is more effective. The rotor is a permanent magnet with no windings. By electronically switching current as three phases related to each other sequentially in these windings, the permanent magnet rotor will follow the switched electric field around and create rotary motion.
The switch points around the motor are at 120 or 60 degrees depending on the motor type. Hall-Effect semiconductor switches located within the motor at 120 degree points, signal these switching positions to the electronics in the servo amplifier, as the rotor rotates. This is the simplest form of Brushless motor control and usually the brushless servo amplifier applies current to the windings with a Trapezoidal, rising and falling waveform on each winding.
As in stepping motors, Inductance is an issue with servo motors and a “chopping” or “switching PWM” effect applies higher voltage pulses of short duration to overcome Inductance effect and maintain proper mean current to the motor windings.
There is a variation in mean current applied with a Trapezoidal amplifier and in some applications this “torque ripple” is significant enough to cause variations in velocity which may be unacceptable. Sinusoidally commutated brushless servo amplifiers produce three sinusoidal waveforms to the 3 motor windings, spaced at 120 electrical degrees. This results in lower torque ripple and is a more elegant solution to motor control.
THE COMPLETE CONTROL SYSTEM
The diagram above shows a typical motion system with closed loop control.
Brushed or Brushless motors all operate in this configuration.
- The Controller is usually an intelligent electronic system which can provide a reference control or demand voltage to a Current Mode servo amplifier.
- The amplifier provides a proportional demand current to the servo motor.
- The servo motor moves as commanded with an acceleration to a commanded velocity and resultant position, after a deceleration time.
- The Feedback device, eg. rotary encoder, linear encoder, LVDT, interferometer, provides digital data as pulses back to the Controller, which computes distance, velocity and acceleration of the motor and compares these with commanded data.
- Actual Load position, velocity and acceleration may vary from controlled motor performance and that too can be optionally controlled, as shown above.
- The Controller also monitors system stability, both static and dynamic, through the feedback devices.
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