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Servomotor users should not be seduced by power density

Servomotors are high-performance devices and represent all that is best in motor technology. Gerard Bush of Inmoco offers some thoughts on how to select the optimum servomotor for a given application and how the technology will develop in the future.

Servomotors operate over a wide range of speeds, provide torque at zero speed and can operate at low speeds for a long time without overheating. They are always used in a closed loop system with a controller (or servo amp) and feedback sensor which between them constantly optimise the motor input current against the position of the output shaft.

There are several issues to consider when selecting a servomotor. The most obvious one is output speed or speed range (which may lead to the inclusion of a speed reducing gearhead). It is important to note that the various manufacturers quote their motors' speed performance differently, so when choosing a motor it is best to compare the speed/torque performance curves for various motors at the operating voltage required.

Other issues include the torque and the maximum current capacity of the motor (without overheating). For all of these, continuous and peak requirements need to be considered.

Another major issue in many applications is space and shape constraints. It is in the nature of many servo projects that the motor may have to fit into a predetermined space, while if the motor is to be mounted on, say, a moving robot arm, or is for an airborne application, lightness is a very desirable virtue.

Once a servomotor is installed it has to be tuned to the needs of the individual application. It is often wise to seek expert assistance from the supplier at this stage, particularly if the application is unusual or demanding. Alternatively, the servomotor controller may include an auto-tune function that speeds up the process considerably.

First time servo-users are often perplexed by the many different types of servomotors, their relative merits and suitability for particular applications. For instance, permanent magnet and shunt wound motors provide constant speed with varying load, making them ideal for machine tool drives, while series wound motors have high starting torque, so are preferred in high-load duties. There are many other variations on the basic servomotor too, such as compound wound units, frameless and pancake motors - each has attributes that are attractive in certain conditions.

In the first instance, the specifier is looking for a balance between price and performance, then at other technical issues. At this point a look at some examples of real world servomotors may help illustrate the advantages of different designs.

Current servomotor examples

The top performance capabilities of servomotor design can be demonstrated by Inmoco's AKM motors. These offer extremely high torque density and rapid acceleration and are available with a wide range of mounting, connectivity, feedback and other options. To meet a wide range of application needs the AKM motors comes in eight frame sizes (40 to 260 mm), 28 frame-stack length combinations and 117 'standard' windings.

The torque performance for the range is 0.16 to 180Nm continuous stall torque (1.4 to 1590 lb-in) in 28 frame/stack combinations. Further, specific torques are often available from multiple frame sizes to optimise mounting and inertia matching capabilities. Speeds up to 8000rpm mean most high speed application requirements can be met, while windings tailored to lower speeds are also available.

AKM motor windings are specifically tailored to 75V DC, 120, 240, 400 and 480V AC. Special windings mean they can be used with all standard global voltages. In all configurations the AKM provides a smooth output with low-cog, low-harmonic distortion. Another reason for the low speed windings is lower current consumption, allowing a smaller servo amplifier can be chosen and therefore cost saving for the complete axis.

A full range of feedback options can be used, including resolver, encoder (commutating), Sine-Absolute encoder or SFD (Smart Feedback Device).

A second Inmoco range of servomotors, which balance performance and competitive pricing is the VLM units. These cover the torque 0.5Nm to 16Nm (peak) and incorporate the features most often needed by users, providing a motion solution that is suitable for a wide range of applications with mid-performance requirements (various kinds of handling, transport applications, low performance cut-to-length applications, rotary knife, machine tool change, etc). These motors are of modular construction and include 12 standard windings and the most popular motor sizes.

Further user-selectable options allow a motor to be specified for easy integration into the host machine or system. The motors are suitable for use with many competitively priced feedback devices and flying leads are supplied.

Inmoco also offers the KBM Series of frameless brushless servomotors, which provide a cost-effective, physically compact solution to achieving positional accuracy in a small, reliable motor package. It gives system designers the flexibility to specify a motor that fits the specific performance and dimensional requirements of the application, without incurring expensive engineering costs. The range encompasses 14 frame sizes, each of which is available with up to five stack lengths. The flexibility of the KBM series is extended by a feature enabling the rotor hub dimensions of the motors to be customised within certain constraints, enabling machine builders to tailor the KBM to their unique physical requirements.

The flexible design and performance attributes of the KBM frameless brushless motors make them an attractive option for use in medical, metal processing/cutting, packaging, printing, aerospace, defence, and oil and gas equipment applications that demand a highly optimised and tightly integrated motion solution.

Servomotor technology has been around for several decades and is well-developed. In fact, further significant developments will be difficult unless new more powerful super-magnet materials are found. However servomotor makers still have busy development departments, where instead of working on more powerful motors the focus is on creating new configurations and concepts that solve the real world issues being addressed by today's machine and system builders.

Intelligent Motion Cont