Basics: Servo Drive – Structure, Function, and Design

The servo motor converts electrical power into mechanical power. In servo drive technology, it is usually a permanently excited synchronous motor with a slim construction and high overload capacity; in some cases, it is also a linear or torque motor. Baumüller offers its servo motors in four series, ranging from the dynamic DSD2 and the compact DSC to the DSH1, which features exceptionally low cogging torque for maximum precision.

The servo controller supplies power to the motor and closes the control loops. Depending on the manufacturer, the same device is also referred to as a servo converter or servo amplifier; at Baumüller, this class of devices is known as b maXX.

A servo drive consists of five modules that work together as a functional unit:

The servo motor generates torque and motion; synchronous, linear, or torque motor. In industry, these are consistently designed as AC servo drives; DC versions with brushes are now only used in existing systems.

The servo controller redirects power to the motor, closes the current, speed, and position control loops, and provides safety functions ranging from STO (Safe Torque Off) to SS1 and safe positioning, which can be transmitted via FSoE (FailSafe over EtherCAT).

The sensor system reports back on position and speed; for example, at Baumüller as a resolver or a digital encoder with EnDat 2.2, Hiperface DSL, SSI, and many others.

Mechanical system: transmission, spindle, rack, chain, belt, or clutch; transmits motion to the load

The higher-level (superordinate) control specifies the motion profile and process; usually a PLC or motion control system.

Baumüller manufactures its servo motors in seven sizes ranging from 28 to 132, with rated torques from 0.6 to 715 Nm, and available in uncooled, air-cooled, or liquid-cooled (water or oil) versions. This allows the motor to be precisely tailored to the axle based on its size, windings, and cooling system.

The mechanical system is definitely part of it, even if it is easy to overlook. A transmission adapts speed and torque to the load and reduces its inertia in proportion to the square of the transmission ratio, but introduces clearance and flexibility into the axle. Both of these factors limit the achievable positioning accuracy, regardless of how good the motor and controller are. Servo gearboxes are therefore designed with low backlash and can handle higher loads than standard gearboxes. With linear axes, this detour is eliminated: A linear motor moves the carriage directly, without a spindle and without gear backlash — though at a higher cost.

The process starts on the left. The control unit sets a position setpoint. At the summation point, the actual value of the encoder is subtracted from this; what remains is the deviation. The position controller converts this into a speed setpoint; the speed controller converts it into a current setpoint; and finally, the current controller controls the power module, which supplies the servo motor with current and voltage. The encoder feeds back the position and speed data, and the loop is closed. A varying load torque acts as a disturbance variable on the motor; the closed-loop control system continuously compensates for it.

Why this tiered structure? Because every internal closed loop control must operate at least as fast as the one surrounding it; otherwise, the axis will vibrate. The internal current control is therefore the fastest: Baumüller's b maXX 3300 calculates it in 62.5 µs, while speed and position control each run at 125 µs.

What the controller is actually dealing with — that is, the motor, transmission, load, and their friction — is referred to in control engineering as the closed loop controlled system. During commissioning, Baumüller automatically tunes the controller and system using the ProDrive tool and the Smart Tuning function, rather than searching for each parameter manually.

An important characteristic is overload capacity. A servo motor can briefly deliver many times the torque it can sustain, and that is exactly what it needs when accelerating. The exact value of this multiple depends on the size: Small axes can reach three times their standstill torque (M0) or more, while large drives have lower values because their power electronics are designed to operate closer to their continuous-duty limit. In each case, the weaker link — either the motor or the converter — acts as the limiting factor. The synchronous motor characteristic curve shows the relationship between torque and speed across the entire operating range.

In the design, it is not the peak value that matters, but rather the averaged torque over the machine cycle; otherwise, the drive will be overdimensioned. Equally important is the ratio of load inertia to motor inertia. A ratio of around 5:1 is considered non-critical; with stiff mechanical system and a well-tuned controller, even a 10:1 ratio can be managed. If it gets significantly larger, the load may end up moving the motor rather than the other way around, and the axle tends to vibrate. When the load becomes too heavy, a transmission helps by transferring its moment of inertia to the motor side.

No one has to do this calculation by hand. Using the sizemaXX design tool, Baumüller dimensions the axis step by step, avoids overdimensioning, and identifies potential savings in the drive. This not only saves time, but also keeps energy consumption low during subsequent operation.

A servo drive refers to the controlled axis as a whole: the servo motor, servo controller, encoder, and mechanical system. Its performance stems from the cascade of current, speed, and position control circuits, and it can be verified — rather than merely claimed — based on overload capacity and cycle times. When designing, the averaged torque and the inertia ratio are the determining factors, not the peak value listed in the catalog.

From an economic standpoint, servo technology pays off in applications where cycle time and precision are the deciding factors. Their benefits go beyond mere positioning. Baumüller servo motors achieve efficiencies of over 90 percent — up to 96 percent at peak — and in multi-axis systems, braking energy can be reused via a common DC link instead of being wasted. When it comes to a system's energy efficiency, this is often a more effective lever than the motor itself.