Servomotors - Definition and function

The servomotor is a type of electric motor that has the fundamental and defining feature of enabling extremely high-precision control, with all the attendant possibilities that this entails. It allows the precise control of angular position, acceleration and velocity. In addition to the servomotor itself, a compatible rotor position sensor is part of the fundamental architecture that allows it to meet these requirements. Also known as a position sensor, rotary encoder or motor feedback system, it is able to determine the position of the motor shaft at any given time.

A module containing the control electronics is responsible for processing position information and making any necessary positioning adjustments; this is the so-called servo controller, which is mounted outside the servomotor itself. It compares the current and target values and corrects them as required in a control loop. This is a continual process, whose purpose is to minimise or balance out differences between the target and actual values. In general, however, the rotor of the servomotor retains its fixed position; only under the impact of load, pressure or other general conditions do any changes in position actually occur.

As a regulatory system, the control loop plays a major role in the conception of a servomotor. Its operation can be controlled by either torque, speed or position. It is also possible to link these variations in cascades in complex control systems.

Combining a servomotor and a servo controller into a servo drive

The combination of a servomotor and a servo controller in a closed control loop is referred to as a servo drive. It is only of subordinate importance what construction type the electric motor has. By virtue of its operation in a closed control loop with a servo controller, the servo drive ranks as a compact drive unit, whether it has the form of a DC motor, or a synchronous or asynchronous motor.

It differs from an actuator, (e.g. a stepper motor) in terms of the way it is activated. While the servomotor operates in a closed control loop, stepper motors only operate in not controlled conditions.

In some cases, a gear unit is employed with a servo drive to match the speed of rotation and the torque to the respective application. In practice, the servomotor and controller must be matched with each other in order for them to work efficiently. Thanks to the flexibility in their configurations, servo drives made by Harmonic Drive AG – including the CanisDriveÒ and the LynxDriveÒ – display a high level of compatibility with just about every servo controller on the market. With the servo controller in the YukonDrive® series, which is specially coordinated to match the requirements of Harmonic Drive® servo drives, a preconfigured drive system, complete with servomotor, is available from a single source. In the case of specialised applications, preconfigured Harmonic Drive® drive systems offer an individualised solution.

General components of a servo drive

  • Servo inverter complete with power and control electronics
  • Servomotor with measuring systems and an immobilising brake
  • Gears for transforming rotational speed and torque

What the servomotor is all about - the precise determination of position 

Not only are a variety of servomotor types available, but there are also several measuring methods by which the position of the rotor can be determined. Three basic types of measuring devices are used for positional determination: incremental, single-turn absolute and multi-turn absolute.

One fundamental distinguishing characteristic in this respect is the physical method that is employed to generate a position. The basic measuring procedures are based on either optical, magnetic, inductive or capacitive factors. The raw signals are processed by the rotor’s position sensor and transmitted to the servo controller as either incremental, single-turn absolute or multi-turn absolute position values.

Depending on the type of rotary encoder chosen, the signals transmitted can be purely analogue, purely digital, or analogue and digital. The data interface used for digital signals in Europe is typically EnDatÒ, HIPERFACEÒ, BiSS or SSI Interface.

Application areas and forms of servomotor

Servomotors are primarily used in applications that require enormous precision and dynamics. They are found, for instance, in industrial systems and automated production facilities, as well as in a wide variety of applications, from packaging machines to machine tools. The latter benefit in particular from modern control techniques that enable the machining of work pieces with complex and sophisticated forms.

A further application field of servomotors is in military defence technology. In tasks demanding fast response times and a high target attainment probability, military land systems rely on servomotors with highly dynamic and robust control capability and power electronics availability. They are frequently implemented in display apparatus. However, the range of the servomotor’s applications extends far beyond military and industrial use. On a human level, servo drives can also be employed in so-called exoskeletons, where they have the capacity to perform at a particularly high level. Several such motors can be installed in a specialised support structure that is designed to help people with paralysis, muscular dystrophy or Parkinson’s disease to perform everyday activities. The exoskeleton is placed on the body, where the mounted servomotors support precisely determined movement patterns, sometimes even creating the conditions required for enabling walking and standing.

Servomotors as the basis of robotics

In addition, servo drive technology provides the fundamental basis for the overriding field of robotics. More and more users are taking advantage of the capabilities of intelligent robots and placing extreme demands on dynamics and availability. To meet these constantly rising challenges, the robot production industry makes use of the unique characteristic of servomotors as controlled electrical drive units. Whether employed in medicine or industry, advances in the field of robotics go hand in hand with the innovative development of servo drive technology.

Other application fields include the automobile industry, mechanical engineering – in particular equipment, machine and fixture construction – and transport and handling systems. Thanks to their excellent performance density and energy efficiency, the fields of application are expanding constantly.

Hollow shaft drives from Harmonic Drive AG.