Servo drive

Electric servo drives

Electric servo drives refer to those components of a machine that control its motor to achieve the best possible performance. Servo drives ensure that the machine ramps up to the final speed as quickly as possible, operates at this speed for as long as possible, reduces the speed as late as possible and comes to a stop on time. Servo drives are mainly used in machine and system building, in decentralised systems as well as in positioning systems (rotary, cross and multi-axis systems).

How are electric servo drives controlled?

Electric servo drives receive commands (optional: control commands) from a process control system, a PLC or from a position controller (optional: motion controller). They execute them and report on this process, its completion and any malfunctions. Should the load of the machine change, electric servo drives can regulate the power autonomously and absorb the change.

Which systems are supported?

Modern servo drives support various fieldbus systems, including Ethernet/IP, EtherCAT®, PROFINET, Modbus as well as CANopen and DeviceNet®. Older servo drives usually still have a serial interface.

Festo products

At Festo you will find electric servo drives as servo drive controllers or stepper motor controllers.

Servo drives

The servo drive product range is specially designed for our servo motors, toothed belt axes and spindle axes or electric cylinders. They are perfect in combination with engineering software and also benefit from complete safety solutions for mechanical and drive systems. Together with our automation platform and other comprehensive motion control solutions, the servo controllers provide a virtually unlimited range of solutions for industrial automation tasks. In this, they are supported by innovative software solutions for engineering and configuration. Thanks to the variety of available fieldbuses such as Ethernet/IP, EtherCAT®, PROFINET, Modbus® as well as CANopen and DeviceNet®, the servo controllers are ideally suited to communicate directly with almost all programmable logic controllers (PLCs).

The electric, multi-protocol capable servo drive CMMT-AS is included in our Core Range as a standard servo drive, and is one of the most compact servo drives on the market. It is characterised by precise force, speed and position control. The software makes commissioning quick and easy, and the required fieldbus variant can also be set individually.

Our motor controller CMMP-AS, on the other hand, is particularly well suited to decentralised motion functions thanks to its numerous interfaces and functions. Its standardised interfaces allow easy integration into the mechatronic multi-axis modular system.

Stepper motor controllers

Our stepper motor controller product range offers a simple and cost-effective way of implementing electrical movements; it makes moving and positioning easier than ever before – and is much more cost-effective than conventional electric solutions.

When it comes to tasks with low power requirements, the electric multi-protocol-capable servo drive CMMT-ST is very efficient and has proven itself especially in positioning tasks and point-to-point motion solutions. It is 50% more compact than our smallest servo drive controller CMMT-AS and therefore perfect for use in simple applications.

Motion control

To make a production process as cost-effective as possible, the machines used need to meet a number of important requirements. They need to:

• Be energy efficient: this will reduce energy costs and diminish the impact on the environment

• Reach a high production speed: a yield per machine that is as high as possible

• Be reliable: minimise machine standstill

• Be maintenance-free: maintenance intervals are far apart and maintenance is as quick as possible

• Be accurate: the goods produced have to meet the specifications that have been stipulated, rejects have to be avoided

• Be flexible: be able to quickly switch production, minimise change-over times

These are the attributes in which electric drive systems excel. Simple asynchronous motors, often actuated by a frequency regulator, can be used as the drive system for basic movements and basic functions. But when the movements are more dynamic, with higher speeds, acceleration, deceleration and a motion path needs to be precisely followed, then servo systems are often used. A motion controller actuates the servo drives (also called servo controllers) via a fieldbus, and thus ensures that the different movements in the machine are precisely synchronised.

A motion controller can actuate simple point-to-point movements, but its real power lies in its ability to realise complex motion profiles, which are often coordinated movements between different actuators. This can be soft and noiseless, making two linear movements run beautifully synchronously or carrying out complex CNC tasks, such as for a lathe or a router table for example.

Servo drive

But what exactly is a servo system? Strictly speaking, a servo system controls the position, velocity, acceleration and torque (or force) in a mechanical system (not necessarily these four parameters at the same time).

An electric servo system usually consists of:

• a servo motor with (integrated) motor feedback of the rotor position,

• a servo drive (motor controller),

• actuated mechanics (linear or rotary).

A servo motor is not just one specific type of motor. Every motor that has an encoder and is integrated in a servo loop can be considered to be a servo motor: an asynchronous motor with an encoder, a stepper motor with an encoder, a brushless DC motor (BLDC), a synchronous motor with permanent magnet and encoder, etc.

But because motors (either synchronous or asynchronous) with a slim rotor design are often used for servo applications, they are often referred to as “servo motors”.

The servo drive (motion controller/ servo control) uses pulse-width modulation (PWM) to regulate the frequency, voltage and transmit electric current that is supplied to the motor so that the required speed and torque can be achieved. The position of the rotor is usually measured by an encoder and the measured value is read off by the servo drive. This is how the speed can be very accurately and continuously adjusted, and the actual position can also be calculated and moved to the required position. This is called closed-loop control.

The big difference between servo motors and stepper motors is that a stepper motor can also be used as part of an open loop for positioning applications. A stepper motor doesn’t need feedback (i.e. an encoder) in order to be used as a positioning system, and is moved on a few steps by the stepper motor drive, with one step corresponding to a specific angular displacement of the rotor (for example 1.8° per step). Because of the lack of feedback on the rotor position, this is by definition no longer a servo system.

Servo drives are often used and supplied with 24 or 48 VDC (extra low voltage) for smaller servo systems with restricted power that use stepper motors or BLDC motors.

Servo systems with more power often use servo drives supplied with 230 or 400 VAC (low voltage). This alternating current is first rectified and saved in an intermediate circuit, after which this DC voltage is transformed back into an AC voltage using a PWM switch with a specific amplitude and frequency for actuating a servo motor.

Modern servo drives are often controlled by a higher-order PLC or motion controller via a fieldbus (Profinet, EtherCat, EthernetIP, Profibus, CAN bus, etc.). This is how they receive the required position, speed, acceleration and torque (or force) for simple point-to-point movements, or an isosynchronous continuous stream of setpoints for the position and/or speed if complex, synchronised or CNC-related movements need to be carried out.

Buying a servo drive

It can be quite a complex task to determine what the best components are for designing a well-performing and accurately sized electric positioning system because so many mechanical, electrical and control parameters have to be taken into account.

Fortunately, there are software applications that can help you. A good example is the Electric Motion Sizing app from Festo, an online tool that just needs a few details about the application, such as the mass to be moved, length and positioning time, to put together a complete electric positioning system, including mechanical components, motor, gear unit and servo drive. With just one click on the button a full parts list is generated, complete with the necessary accessories such as motor cables for connecting the servo motor.