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Servo Motors: The Muscle for Automation Tasks

15 April 2009

Used in a wide variety of applications, these special motors drive robots in the automotive industry, position portals in furniture manufacturing, and cut fish fillets in food processing applications.

Longitudinal section through a synchronous servo motor with resolver and brake
Longitudinal section through a synchronous servo motor with resolver and brake

A servo drive system can be considered as having components of the servo motor and a servo drive. The servo drive converts electrical power from the mains in a controlled manner into power for the motor. It consists of power electronics and control electronics for the regulation, set point generation, and monitoring of the components.

The servo motor converts the electrical power into movement. It consists of torque generating components, the sensor for angle and feedback and in some cases a holding brake to maintain position at zero current. Generally the operation of servo motors is characterised by frequent changes in speed and torque, operation at standstill to hold positions, and short-term operation with a high overloads.

One example of a servo driven application is a machine to unwind, process, and rewind films. In addition to the two winding drives with three-phase asynchronous motors and frequency inverters, the machine has five servo drives positioning, transporting and processing the material. CLICK HERE TO SEE THIS EXAMPLE

SERVO MOTOR CONSTRUCTION

Servo motors are generally characterised by a slimline design with a high power density, low inertia, and high efficiency. They offer optimal drive behaviour with high dynamic performance and accuracy. They are tailored for operation with servo drives.

In comparison to standard three-phase motors, servo motors are considerably more compact and have significantly lower moments of inertia. Hence they accelerate much faster and provide the power for automation tasks within a small space.

The first illustration shows a cross-section through a typical synchronous servo motor with the active components for torque generation, a resolver for measuring angles and speed, and a brake for holding position at zero current. It can be clearly seen that the active components for torque generation only occupy a small proportion of the total volume. Generous roller bearings, the holding brake and the shaft position sensor make up the majority of the motor volume. To ensure compliance with safety specifications, large insulation clearances are required which are shown as free spaces in the longitudinal motor section.

Plug connectors allow fast and error free connection of the motors. In contrast to three-phase AC motors without speed feedback, the servo motor not only has three power terminals and one earth connection, but also at least six connections for the shaft encoder and two connections for temperature monitoring. The large number of connections can easily lead to errors in the connection process. Plug connectors mean that connection errors are reduced to a minimum and time can be saved on commissioning.

Servo motors have a sensor for angle and speed, whose signal is evaluated by the drive. This enables precise and dynamic control of the speed and position of the motor. The setting range for the speed and the achievable dynamics are considerably higher than when using a frequency inverter with a three-phase AC motor.

Servo motors are also often equipped with a brake, in particular for applications where gravity can cause the machine to move. The brake is a normally-on device which will hold the position when the power supply of the drive is switched off.

Speed-torque characteristic for synchronous servo motor rated at 7.5Nm.
Speed-torque characteristic for synchronous servo motor rated at 7.5Nm.

Servo motors are available in both asynchronous technology (for example the Lenze MCA series) and synchronous technology (Lenze MCS series).

Asynchronous servo motors exhibit a wide range of speed with reduced torque available above the rated speed due to field weakening operation. Because of their higher mass inertia compared with synchronous servo motors, they are particularly suitable for less dynamic mechanics, for example travel drives with toothed belts.

Synchronous MCS servo motors have a rotor with high energy magnets. They achieve a lower inertia than asynchronous servo motors that have the same rated torque, are smaller in size and do not require any magnetising current. The result is a higher dynamic performance with greater acceleration.

OPERATING CHARACTERISTICS

When paired with a servo drive, servo motors have a range of speeds and torques in which the combination can be operated. The second illustration shows an example of synchronous servo motor rated at 7.5Nm. The operating range is principally limited by the available current. It determines the upper part of the torque curve, and in this illustration is based on a 24A servo drive. Different drives will produce different curves.

The maximum speed that a synchronous motor can reach is defined by the output voltage of the drive, which is in turn dependent on the supply voltage. In this illustration there are three curves on the right hand side for 360, 400 and 440V mains voltage.

Below the maximum torque limit is a curve for thermally permissible operation. The points on this S1 curve represent the working points for continuous operation or continuous stand-by operation. If the working point of the effective torque is below this curve, operation is thermally permissible. However in most servo motor applications, the concept of S1 continuous operation is meaningless. Mostly the torque profile is highly variable with peaks and periods of low torque. A RMS value is calculated and this needs to be within the S1 limit.

Compared to the characteristics of synchronous servo motors, the asynchronous servo motor does not have a fixed speed limit (see the third diagram). The wide field weakening range of the motors means that they can be operated at speeds high above the rated speed but with low torques. The maximum torque depends on the current output from the servo drive, similar to synchronous motors.

SHAFT SENSORS

Servo motors are operated in a closed control loop for speed and angle of rotation. To provide feedback, a sensor is required to record the actual speed and the position angle of the rotor. The requirements on accuracy depend on the application in which the servo motor is used. In order that the servo motors can be optimally implemented in different applications, a variety of sensor systems are available. Synchronous servo motors can be equipped with resolvers, incremental encoders, single-turn Sin-Cos absolute encoders, or multi-turn Sin-Cos absolute encoders.

Speed-torque characteristics for asynchronous servo motor rated at 4.0Nm.
Speed-torque characteristics for asynchronous servo motor rated at 4.0Nm.

Resolvers are sensors that act magnetically. They use the angle-dependent coupling between the windings in the rotor and those in the stator. The angle and the speed are determined from the relationship of the induced currents. They are characterised by a very high level of robustness. Resolvers retain absolute angle values within one revolution. The output can also be used to generate the commutation information for current regulation in the servo drive.

In incremental encoders, a rotating disc with an incremental slot is optically scanned. The optical signal is used to generate rectangular signals, usually with 2048 or 4096 pulses per revolution. Two signals are generated with a phase difference of ½ pulse length, which enables both angle measurement and direction of rotation. Encoders only supply incremental phase signals, which means that the commutation information required for operating synchronous motors is not provided. Incremental encoders are therefore particularly suitable for asynchronous servo motors.

Sin-Cos absolute value encoders also contain a disc that is optically scanned. The optical signals are used partly to generate incremental signals, which return Sin-Cos voltages with 512, 1024 or 2048 periods per revolution. In addition the absolute angle is also determined and made available to the servo drive via a serial interface. The Sin-Cos signals are interpolated by calculation of the arc tangent, meaning that a very high angular resolution is achieved. This high resolution leads to excellent smooth running performance of the motor at low speeds. Single-turn encoders determine the absolute phase information within one revolution, so that all signals for operating synchronous motors are provided similar to the resolver. Multi-turn encoders often have an additional gearing and additional sensors in order to distinguish up to 4096 revolutions directly. This means that a machine can start immediately after it is switched on and does not first have to be homed.

HOLDING BRAKES

Servo motors are often equipped with a brake that is used to hold the position if the power supply to the drive is switched off. Servo motors with brakes are used in particular for applications involving lifting and lowering, or for holding a position against a particular force.

Normally braking during operation to bring the motor to a stop is performed using the servo drive and is a wear-free process. The brake is only used in emergency stop, e.g. in the case of mains failure, and for holding duties.

Two types of brakes are used: Spring-applied brakes apply the braking force using spring pressure through an armature plate and onto the brake rotor. Permanent magnet brakes generate the braking torque by exerting the power of the permanent magnets onto a rotating armature plate.

The advantage of permanent magnet brakes is their backlash free properties. The braking torque is transmitted to the shaft using a metal disc with no play. This means that high positional accuracy can be achieved. On the other hand spring-applied brakes have lower inertias, and can continue to operate even after there has been extensive wear from repeated dynamic braking of the drive.

—Dr.-Ing. Carsten Fräger, Team Leader Portfolio Roadmap Management, Servo Drives & Motors, Lenze Hameln.


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