Servo systems are applied in a wide range of applications, from intermittent operations that require high torque output for quick acceleration and deceleration — such as pick-and-place from a conveyor — to processes that call for nearly uninterrupted operation with constant speed and torque requirements — such as printing, roll feeding, and labeling. Given these dissimilar application requirements and the effect that torque, current, and duty cycle have on servo system performance, it’s easy to see why one type of servo drive or motor doesn’t fit all applications.

A key factor in servo system performance is heating, or more specifically, the ability of the motor and drive to dissipate heat to avoid damaging the motor insulation and drive electronics. There are a variety of causes for excessive heat generation, but aside from misapplied or poorly maintained products, operating at peak torque (and, thus, peak current) is one of the most significant factors.

Servo motors and drives have two operating zones: continuous duty and intermittent (peak) duty.
Image credit: Moog Animatics

Recall that the torque curve for a servo motor and drive combination includes two operating ranges: continuous torque and intermittent (peak) torque. The continuous torque range shows the torque the motor and drive can produce indefinitely at a given speed and is the basis for evaluating the RMS torque required by the application. Peak torque is the maximum torque the motor and drive can produce at a given speed and requires maximum current from the drive. To avoid overheating, the peak torque value is allowable only for a short amount of time — typically a few hundred milliseconds.

Accordingly, servo applications often fall into two categories: those that involve very rapid acceleration and deceleration, and therefore, have high peak torque requirements, and those that require good continuous torque characteristics with moderate peak torque demands. The first type of application is referred to as pulse duty, and the second type is referred to as continuous duty. To address the disparity in performance requirements between these different applications, some manufacturers offer two variations of servo drives and motors: pulse duty versions and continuous duty versions.

Pulse duty servo drives and motors are designed to perform well in applications that involve very rapid acceleration and deceleration rates, and in turn, have high peak torque requirements. Accordingly, pulse duty servo drives have a high current overload rating, while pulse duty motors have lower inertia than conventional designs, which reduces the amount of torque (and thus, current) required for demanding move profiles.

Pulse duty servo drives and motors are often used in pick-and-place applications, such as this robotic palletizer, which require high peak torque production to meet demanding accel and decel rates.
Image credit: Bastian Solutions, Inc.

On the other hand, continuous duty versions are designed to produce relatively higher torque at higher speeds on a continuous basis, with moderate peak torque capabilities. Thus, the torque curves for continuous duty drives and motors have larger continuous operation areas.

Continuous duty motors and drives are ideal for printing applications, which require near-constant operation at high speeds with moderate torque requirements.
Image credit: Kollmorgen

RMS, or root mean square, torque is a calculated value based on the torque required during each phase of the motion profile (acceleration, constant velocity, holding, deceleration, etc.) and the duration of each. RMS torque is a time-weighted average — in other words, it is the amount of torque that, if produced continuously, would generate the same level of heating as the various torque levels and durations the motor experiences over its actual duty cycle. The purpose of RMS torque is to ensure that overheating doesn’t occur during normal motor and drive operation.

Feature image credit: Nidec Motor Corporation

Sizing a motor for a servo application requires evaluating the move profile and torque requirements to determine the mechanical demands of the system, such as maximum velocity and acceleration, RMS and peak torque values, and load-to-motor inertia match. Once the motor is chosen, the next step is to select a drive.

On the surface, selecting a servo drive (also referred to as a “servo amplifier”) may seem to be a matter of simply matching the drive’s voltage and current output to the motor’s requirements. But there are many factors that need to be considered to ensure the drive operates satisfactorily within the complete servo system. While some applications may require other, more specialized functions from the drive, here are nine factors that guide the selection of a servo drive for most applications.

1. Motor type

A servo drive can be used with any motor that operates in a closed-loop system — including stepper, induction, and asynchronous — but the two most common types of motors that are paired with servo drives are brushless DC motors and synchronous AC motors. Of these, synchronous AC motors are more common in motion control applications.

2. Commutation

The type of commutation required by the drive depends on the type of motor being driven and the application’s sensitivity to torque ripple. Brushless DC motors can operate with either trapezoidal or sinusoidal commutation, while AC synchronous motors use sinusoidal commutation. Trapezoidal (also referred to as “six-step”) commutation is the simpler method, using three Hall sensors to determine the commutation sequence. But it produces high torque ripple. Sinusoidal commutation virtually eliminates torque ripple and provides more precise control by continuously varying the current supplied to the motor windings. But it requires a high resolution feedback device to determine the rotor position.

Sinusoidal (left) and trapezoidal (right) current waveforms for synchronous AC and BLDC motors, respectively.
Image credit: STMicroelectronics

3. Feedback

Servo systems require feedback in order to provide closed-loop operation and detect and correct any errors in the motor’s actual position, torque, or velocity. This feedback can be provided by Hall sensors, resolvers, or encoders, although most high-end systems use a resolver or an encoder. Regardless of the feedback mechanism, the drive must be compatible with its signal to process it and communicate it to the controller.

4. Voltage and current

The most basic requirement of the motor-drive relationship is that the power from the drive — maximum voltage, continuous current, and peak current — must be sufficient to produce the mechanical output required by the motor — torque, speed, and position. Because the operation of the motor and the drive are co-dependent, manufacturers provide torque-speed curves that define the performance of specific motor-drive combinations.

5. Operating mode

Servo control loops within the drive are used for controlling torque, velocity, or position (or a combination of the three). While all servo drives incorporate a torque control loop and a velocity control loop, only digital drives can provide position control.

Servo drives often have a multi-loop structure, with the current loop nested inside the velocity loop, which is nested inside the position loop. Analog drives can provide current and velocity control, but only digital drives incorporate position loops.
Image credit: nctu.edu

6. Analog or digital

Traditional servo drives were analog and converted ± 10 volt signals from the controller to current commands for the motor to control torque or velocity. In order to tune an analog drive, gain values and other parameters are set via potentiometers. With newer digital drives, the command can be executed by either digital or analog inputs, and tuning is done via software. Along with torque, velocity, and position loops, digital drives can also manage higher-level functionality, such as path generation. Digital drives are also capable of monitoring internal functions of the drive (such as following error) and providing more detailed fault diagnostics.

7. Communication

In order for the drive and controller to “talk” to each other, they must have a common language. To accomplish this, servo drives are offered with a variety of communication protocols, from basic serial links, such as RS485, to more advanced protocols, such as traditional fieldbus networks (DeviceNet, for example) or Ethernet protocols. The choice of communication protocol will largely depend on the required communication speed, whether there is a master-slave arrangement between multiple drives, and in some cases, the type of controller being used.

8. Integrated safety

Functional safety is now mandated on many types of machines and is governed by the EN/IEC 62061 and EN/ISO 13849-1 safety standards. To comply with these standards, safety functions, such as Safe Torque Off (STO) and Safe Stop 1 (SS1), are integrated into many higher-level drives.

When the STO function is activated, the drive stops supplying power to the motor, and the motor coasts to a stop.
Image credit: Siemens AG

9. Mounting

For industrial applications, servo drives are, for the most part, either panel-mounted or PCB-mounted. Panel-mounted drives are often installed in a control cabinet where they are protected from environmental hazards and can be externally cooled. PCB-mounted drives (also referred to as “plug-in” drives) provide a compact solution for direct integration onto a PCB and are commonly used in high-volume OEM applications.

Panel-mounted drives are typically mounted in a control cabinet.
Image credit: ABB