Basics: Servo controllers – intelligent intermediaries in automated servo systems

Three terms refer to the same device, but with different emphasis:

• Servo amplifier focuses on amplifying the signals. The term dates back to a time when the component did, in fact, primarily serve to amplify.
• Servo inverter emphasizes the conversion of electrical energy for the motor.
• Servo controller is the most accurate term, because modern devices control rather than simply amplify or convert. It is precisely this wide range of functions that is embodied in Baumüller’s b maXX servo controller family.

In practice, all three terms are used, depending on the manufacturer and tradition. Anyone looking for a servo amplifier is usually referring to the same drive controller.

However, a servo controller is not a frequency inverter. This confusion happens frequently, but the difference is nonetheless crucial. A variable-frequency drive controls conventional induction motors by adjusting the frequency and is suitable for uniform rotational movements, such as those required by pumps or fans. A servo controller, on the other hand, operates with sensor feedback in a closed-loop control system and is capable of position control: it moves to defined positions with high dynamic performance and maintains full torque even when stationary. The frequency inverter therefore primarily controls the speed, whilst the servo controller also controls position and torque, and does so with significantly greater precision and dynamic response.

A servo controller is only as good as its integration into the system. Modern devices communicate with the control system and other components in real time via industrial fieldbuses. The relevant drive profiles are described, among other things, in the IEC 61800-7 series of standards.

In new machines, real-time Ethernet systems have largely replaced traditional fieldbuses:

• EtherCAT – the widely used real-time Ethernet bus for highly dynamic multi-axis systems with very short cycle times.
• PROFINET, EtherNet/IP and POWERLINK – established real-time standards, depending on the control environment.
• CANopen and PROFIBUS – traditional fieldbuses that are still widely used, particularly in existing installations.

Baumüller’s b maXX servo controllers support all common bus systems, including EtherCAT, PROFINET IRT, EtherNet/IP, POWERLINK, VARAN and Modbus TCP. Safety functions can be controlled directly via the bus using FSoE (FailSafe over EtherCAT).

In addition, there are the sensor interfaces through which the controller receives feedback from the motor. At Baumüller, alongside the traditional resolver, digital encoder protocols such as EnDat 2.2, Hiperface DSL and BISS are available; with Hiperface DSL, the feedback is even transmitted via a single-cable solution alongside the motor cable. The servo controller also integrates sensors, limit switches and brakes directly via additional digital and analog inputs and outputs.

This means that a b maXX servo controller can be integrated into almost any automation environment, regardless of the manufacturer of the higher-level control system.

Depending on the task, a single controller drives different types of motor: rotary synchronous servo motors, as well as linear motors for direct linear movements or torque motors for high torque at low speeds. This range makes the servo controller a flexible solution for a wide variety of machine designs.

The situation is similar in metalworking: drilling, bending, various surface treatments and cutting using plasma or laser technology all require an equally high degree of precision and dynamic control capabilities that can be digitally parameterised and implemented automatically.

Horizontal stackability

Baumüller’s stackable servo controllers make it easy to arrange several servo motors side-by-side, which can then also operate in asynchronous mode. This is how highly customized systems are created. Subsequent adjustments and integration into existing concepts can be achieved very flexibly. This flexibility really pays off, particularly in multi-axis systems.

Precision

The servo controller allows speeds and torque to be determined precisely. This in turn makes it possible to precisely navigate towards and maintain predefined targets. The controller also adjusts any increase or decrease in speed to the target value in a fraction of a second.

Safety

The servo controller can switch off electric motors from a central location, which enhances safety. Unlike the individual shutdown of separate motors, in an emergency the system can be stopped immediately and simultaneously. Modern devices come with built-in safety functions, such as STO (Safe Torque Off) in accordance with IEC 61800-5-2, so that the drive can be safely shut down without having to disconnect it completely from the mains. Higher-level functions such as SS1 can be controlled via FSoE (FailSafe over EtherCAT). These options are offered, for example, by the Baumüller servo controllers in the b maXX 5000 and b maXX 6000 families.

Efficiency and cost savings

High performance with low energy costs: the efficiency of modern servo controllers significantly reduces operating costs, while the initial investment remains low. With its Drive Connect System, Baumüller offers the possibility of installing and commissioning servo drives particularly cost-effectively. Maintenance requirements are low and the service life is long. Where multi-axis systems operate together in a group or network, energy recovery allows additional braking energy to be fed back into the grid, thereby further increasing overall efficiency.

Intelligence in the controller

Functions that used to be handled by a central control unit are now carried out within the drive itself. On the one hand, this means connectivity via bus systems; on the other, it means automation. The servo controller can be used to generate and process data within the drive system, which can then be used, for example, to optimize the process.

Servo controllers cover a wide power range, from compact units for small axes to high-performance controllers for large main drives. The choice of series depends on the current, power and design of the application.

Baumüller has grouped its AC servo controllers into the b maXX family, which together covers a power range of up to 400 kW, thereby offering scalable solutions for different power classes. The compact b maXX 3300 series is designed for small axes up to around 5 kW. Modular single- and double-axis units such as the b maXX 5300 and the new b maXX 6300 cover the mid-range up to around 97 kW of motor power, while stand-alone units such as the b maXX 5500 (up to 315 kW) or the b maXX 6500 (up to 400 kW) handle the large main drives. Dual-axis modules save space in the control cabinet because two axes share a single unit.

When making your selection, it is worth considering not only the pure power range but also the required interfaces, encoder evaluation and any safety functions. The b maXX servo drives product range provides an overview of the suitable drives.

A prime example of the use of servo systems is the industrial manufacture of textile products. Complex multi-needle lockstitch machines can, for example, achieve extremely high sewing or lockstitch speeds thanks to a suitable drive system, leading to a significant increase in productivity.

As such processes require a very high degree of precision, the servo motors used in them must be capable of performing extremely fine movements with response times in the microsecond range. Baumüller servo motors, together with the appropriate controllers, meet the requirements of this type of manufacturing.

Once the selection has been made, commissioning and operation follow. During commissioning, the controller is tuned to the motor and mechanical system: the controller parameters for current, speed and position are set; at Baumüller, this is supported by the ProDrive commissioning tool and the Smart Tuning function, which automatically determines the controller and system parameters.

A typical error message during operation is a following error (“lag error”). It occurs when the actual position of the axis deviates too far from the setpoint, meaning that the controller is not responding quickly enough to the setpoint (is lagging). Common causes include excessive acceleration, a mechanical jam, incorrectly configured control loops or a problem with the encoder. If you want to rectify the following error, first check the mechanics and the load, then the encoder feedback, and finally the controller parameters. It is often enough simply to smooth out the motion profile or to fine-tune the speed and position control.

As an intermediary between the control system and the motor, the servo controller offers a flexible and dynamic means of adjustment. They are used in virtually all manufacturing sectors where a particularly high level of mechanical precision and flexibility is required. Changes to the overall system and adjustments to safety requirements can also be implemented easily at a later date without altering the existing set-up.

Whether referred to as a servo controller, servo amplifier or servo inverter, the function remains the same. Baumüller’s b maXX servo controllers cover everything from compact single-axis drives to networked multi-axis systems.