What’s really the difference between a dc servo and an ac servo system?

This article provides a history and general guidance on the capabilities of different ac and dc servo technologies.

By Hurley Gill • Senior applications and systems engineer | Kollmorgen

Many of today’s industrial and commercial applications — whether controlling a specific operation or just some specific motion — require electric motors. These ac and dc machines are not just electrical devices; they are also mechanical devices. These electromechanical solutions typically set the specific performance limits and capabilities of every machine axis and thus the operations being performed. That’s true in manufacturing applications and elsewhere.

Performance limits and capabilities affecting a given operation can be enhanced to boost product quality, throughput, and more with the addition of automated closed-loop control.

Designs with such closed-loop control function as servomechanisms typically associated with the word servo — often commanding the specific axis’ force or torque, velocity, and position. A closed-loop system is most simply defined as a driven mechanism dependent upon instantaneous feedback data from the continuous monitoring of a given operation or process resulting in systematic adjustments of that same primary operation for the accomplishment of a desired outcome.

Today, a closed-loop system is generally identified as a servo system. For controlled mechanical motion, this is generally accomplished by the usage of an ac or dc servomotor. A servomotor (whether considered ac or dc) is basically a bidirectional servo actuator under closed-loop control that …

• Uses data from instantaneous feedback monitoring of actual motion (as position, velocity, and torque or force, for example) …
• Against commands or setpoints to cause dynamic-corrective adjustments (± the difference between the two) of a given operation for a precise outcome.

Whether the servomechanism is designed using an ac or dc servomotor, each axis of motion is individually controlled by converted electrical signals based on programmed information. So, the servomotor’s response is continuously monitored for continuous calculations against a desired command (setpoint) resulting in a continuous series of corrective adjustments.

In other words, regardless of the job to be performed by a machine or machine axis, the limiting condition is typically established by the available response times of the electromechanical components from which a given servo-mechanized machine is composed.

A servo (closed-loop) control system allows for the control and manipulation of an axis’ dynamic response relative to the machine operations’ load disturbances and so on. Performance is optimized through the use of a proportional, integral, and derivative (PID) servo controller algorithm. Further optimization of the electromechanical response is possible when the best available technology is selected for the application.

Servomotors have evolved over the years, and various technologies have become common in precision motion control. Initially, these systems were mostly industrial dc motors, driven by an ac motor-to-dc-generator set, with tachometer feedback for continuous velocity monitoring — and when applicable, there was continuous monitoring of axis position by a separate feedback sensor (such as an encoder or resolver).

Later these industrial dc systems were replaced with permanent-magnet (PM) dc brush servomotors driven by:

• Linear drive technologies, then
• Chopper (silicon-controlled rectifier or SCR) drive technologies, then
• Pulse width modulation (PWM) bipolar junction transistor or BJT drive technologies.

Rather quickly, most of these dc brush servomotors were replaced with electrically commutated permanent-magnet motors — including brushless dc (BLDC), ac (PM AC), and even some ac induction (asynchronous) servomotor designs.

The terms brushless dc servo and ac permanent-magnet (PM) servo were first established to promote the adoption of these motors and to promote the idea that ac permanent-magnet motors with electronic commutation could effectively replace permanent magnet dc brush servomotors. Additionally, because of different electronic commutation technologies, the industry developed naming conventions related to counter or back electromotive force (Bemf) characteristics. Today, these widely used names are trapezoidal for six-step commutation (and vice-versa) and sinewave for sinusoidal commutation (and vice-versa).

To be clear, originally the distinctions between BLDC, ac PM servo, and ac servo arose not solely from technological differences and limitations. These names were also part of marketing efforts to dispel misconceptions about the capabilities of electronic commutation in comparison to mechanical commutation of dc servomotors.

For this discussion, we assume servo designs and servo systems refer to complete solutions that include a servomotor, servo drive, and monitoring feedback sensor(s).

In fact, BLDC and ac PM servomotors basically feature the same type of permanent-magnet motor construction. In some cases, they’re just styled with slightly different magnetic designs to further reduce core losses. So actually, the two terms BLDC and ac PM servo describe the commutation method used to control power to the permanent-magnet motor. Standard motors of both designs are three-phase PM — ac synchronous — servomotor designs.

Hence it’s the specified drive’s commutation type that influences which motor construction is most suitable — in other words, for best performance the servomotor is specifically designed for a designated commutation technology. Otherwise, either style servomotor may be driven by either commutation type notwithstanding feedback-sensor differences or servo drive requirements. However, one can expect slightly less energy losses (generally negligible) when the motor’s Bemf design style matches the drive’s commutation topology.

There are two basic types of commutation technologies — six-step (or trapezoidal) commutation and sinewave (or sinusoidal) commutation. The electrical cycle of the former commutation method is effectively defined into six distinct dc-style steps. The latter is defined by its comparatively smooth and continuous ac-style waveforms.

The electronic commutation of a servo drive with six-step (trapezoidal) commutation topology limits the motor’s applied voltage to only two of the three motor phases at any given time. That’s because one of the three phases is always off even though the dc-stepped waveform is basically trying to emulate a sinewave.

In contrast, with sinewave commutation topology, the supply voltage is applied to all three motor phases at the same time (as with any typical 3φ motor). That’s true whether the drive is controlling an ac PM (synchronous) servomotor or an ac induction (asynchronous) servomotor.

These different commutation methodologies lead to the use of different parameter units for the identification of motor and drive characteristics. These parameter units can be differentiated from each other and converted to and from one another by referencing the commutation type to be employed. Discrepancies and misunderstandings about this data and listed units on a motor datasheet can lead to flawed interpretations and suboptimal operation. Understanding servomotor-parameter nuances and their proper application is essential for optimal performance, reliability, and diagnosis of mechanism-operation issues.

For more information, read the author’s white paper titled, Servomotor parameters and their proper conversions for servo drive utilization and comparison online.

Choosing the servo technology, control method, servomotor, and feedback to use for a specific electromechanical design can be daunting for the beginner engineer. However, the best selection for a given application becomes clearer once the advantages and disadvantages of different servo technologies are understood. That’s especially true when a given servo technology will definitely best satisfy an application’s core function and specifications. Here, the servomotor must first satisfy repeatability, accuracy, and flexibility requirements for present and future needs. Then remaining considerations typically relate to component availability, machine life expectancy, operating environment, allowable noise, and target efficiency.

On the other hand, when either servo technology might satisfy a given application’s requirements, one should use discernment and foresight and carefully catalog everything known about the application, possible future needs, and the designer’s experience.

To illustrate, following are three application examples (and their basic requirements) along with a chart of ac servo and dc servo technology capabilities relevant to these examples.

Example application one: A filament or fiber-drawing application often requires a tight short-term and a more open long-term, velocity tolerance. This tight velocity requirement may be best solved with an electromechanical servo system featuring:

• Relatively large rotor inertia to help dampen instantaneous disturbances, with
• Relatively high feedback resolution to help maintain the long-term velocity specification — perhaps even to the point controlling velocity within an effectively infinite position loop (i.e. to the limit of the drive’s capability).

However, if the draw operation requires dynamic smoothing of inconsistent material over short distances, relatively low rotor inertia and high feedback resolution will be a better solution. That’s because needed bandwidth response times can be met at the target production velocity rate.

Example application two: For typical pick-and-place or point-to-point positioning applications, the specific operation dictates the acceptable range of position. So, the dominant attribute is repeating accelerations and decelerations (whether or not there are any constant-velocity traversals during a move). On vertical axes, the motor must also be capable of holding the load against gravity.

For such repeated accelerations and decelerations, mechanism solutions with lower mass or inertia are best — even if the application doesn’t need highly repeatable or accurate final positioning with or without tight settling times.

At this point, the primary design objectives are control stability, operating environment, initial cost, and energy operating expenditures. Industrial or commercialized robot requirements present the ultimate challenge for a multi-axis design often requiring increased axis repeatability or accuracy during positioning, acceleration, deceleration, traverse displacements (and even while holding loads) for the fulfillment of some specification for multi-axis coordination.

Demanding requirements often indicate the use of highly repeatable and accurate feedback devices (which in turn must be suitable for the operating environment).

Example application three: For autonomous mobile and cobot applications, priority requirements are likely safety and energy expenditure. That’s because these designs can potentially share space and have direct interactions with human personnel during normal operation. Plus, these applications — in addition to the common criteria of environmental suitability and ability to carry or hold a load — often have exacting stop-function requirements. That’s because cobots and mobile designs must be able to quickly respond to unexpected events. Such designs (especially where battery powered) must be energy efficient.

These safety and energy requirements indicate solutions with the lowest possible mass or inertia — even when the application doesn’t necessarily need highly repeatable or accurate positioning from the servo-controlled axes.

Kollmorgen | kollmorgen.com