M O T I O N S Y S T E M A P P L I C AT I O N S
9 factors in selecting a
servo drive
SIZING A MOTOR for a servo application requires evaluating the move profile and torque requirements to determine the mechanical demands of the system, such as maximum velocity and acceleration, RMS and peak torque values, and load-to-motor inertia match. Once the motor is chosen, the next step is to select a drive. Selecting a servo drive (also referred to as a servo amplifier) may seem to be a matter of simply matching the drive’s voltage and current output to the motor’s requirements. But there are many factors that need to be considered to ensure the drive operates satisfactorily. While some applications may require other more specialized functions from the drive, here are key factors that guide the selection of a servo drive for most applications. Motor type: A servo drive can be used with any motor that operates in a closed-loop system — including stepper, induction, and asynchronous — but the two most common types of motors that are paired with servo drives are brushless DC motors and synchronous AC motors. Of these, synchronous AC motors are more common in motion control applications. Commutation: The type of commutation required by the drive depends on the type of motor being driven and the application’s sensitivity to torque ripple. Brushless DC motors can operate with either trapezoidal or sinusoidal commutation, while AC synchronous motors use sinusoidal commutation. Trapezoidal (also referred to as six-step) commutation is the simpler method, using three Hall sensors to determine the commutation sequence. But it produces high torque ripple. Sinusoidal commutation virtually eliminates torque ripple and provides more precise control by continuously varying the current supplied to the motor windings. But it requires a high-resolution feedback device to determine the rotor position. Feedback: Servo systems require feedback to provide closedloop operation and detect and correct any errors in the motor’s actual position, torque, or velocity. This feedback can be provided by Hall sensors, resolvers, or encoders, although most high-end systems use a resolver or an encoder. Regardless of the feedback mechanism, the drive must be compatible with its signal to process it and communicate it to the controller. Voltage and current: The most basic requirement of the motordrive relationship is that the power from the drive — maximum voltage, continuous current, and peak current — must be sufficient to produce the mechanical output required by the motor — torque, speed, and position. Because the operation of the motor and the drive are co-dependent, manufacturers provide torque-speed curves that define the performance of specific motor-drive combinations. Operating mode: Servo control loops within the drive are used for controlling torque, velocity, or position (or a combination of the three). While all servo drives incorporate a torque control loop and a velocity control loop, only digital drives can provide position control.
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DESIGN WORLD — MOTION
drives SERVO – Motion Control HB 08.18 V4 FINAL.indd 48
11 • 2018
This is a Nidec Control Techniques Digitax HD M753 servo drive with an integrated two-port EtherCAT switch for easy integration into centralized motion-control applications.
motioncontroltips.com | designworldonline.com
11/20/18 8:43 AM