Servo selections

The correct choice of servomotor is crucial in a myriad of industrial applications from packaging to machine tools.

In today’s fierce, multi-national, competitive markets, ingenious designers of manufacturing machinery are creating faster, more accurate and compact equipment to produce a myriad of better, lower cost goods, from toilet rolls to paper mills, cables to embroidery, packaging to stage scenery, and machine tools to robots. The list is endless. To realise the full potential of their conceptions, the correct choice of servomotor is crucial to achieving this aim.

Permanent magnet brushless servomotors excel up to 100Nm. This is due to the fact that they are compact, silent, protected to IP65, have long life, and low inertia with high torque for maximum acceleration. There is plenty of choice, but what is the ideal way to select the best motor for the application?

The requirement is for responsive and accurate proportional servo systems for all speed and position control applications. To achieve this, designers must consider the motor and machine as a coordinated pair and be prepared to match machine to motor as well as the other way around.

Here are a few tips.

Servo Bandwidth

Quick precision movements require a servomotor that has an accurate, high resolution and proportional feedback device. In addition, the drive, motor, and feedback must have a high open loop gain to reduce errors. A wide bandwidth is necessary for a fast response. High gains and bandwidths are difficult to achieve without instability caused by resonance of components. Any delays or phase shifts within the system are likely to severely limit the practical gain and bandwidth that can be achieved.

When selecting a servo motor, these points are to be considered:

Motor construction

To ensure resonant frequencies are set well beyond the desired bandwidth limit, motors must have a rigid frame, a torsionally stiff shaft, and a directly mounted high-resolution feedback sensor. Minimum losses combined with an efficient means of heat dissipation and high winding temperature rating will be reflected in the motor performance figures.

Motor Windings

The quality of the winding is fundamental to the life and reliability of the motor. Reputable motor manufacturers pay particular attention to quality of the wire and the insulation system employed. Modern switched mode servos necessarily have fast edge switching. A badly arranged earthing system creates winding stress and the erosion of the wire insulation that over long periods can cause voltage breakdown. Long cable lengths will increase the effects of voltage spikes at the motor end.

Feedback Sensor

A rigidly constructed motor and load with resonant frequencies well beyond the required bandwidth is necessary to achieve high loop gain (for minimum errors) and high bandwidth (for a fast response). Look for a high-resolution feedback sensor as this reduces delays and increases bandwidth and stability. Resolvers are rugged, but less accurate.

Plug and Play

Plug-and-play motors contain essential parameter data stored inside and can be used with drives of the same family. Drives typically employ three feedback loops: current (torque), speed and position. Pre-set adjustment greatly assists initial installations. Once a system is established, the drive should offer a suitable cloning method for parameter pre-sets.

Gearboxes

Choice of gearbox is the system designer’s responsibility, so check torque output, rating, duty cycle and no load torque / efficiency. For convenience and sometimes for minimal size, integral motor-gearbox combinations can be purchased from some suppliers

Motor Sizing

Performance figures are generally quoted for 400 degrees C ambient. Peak torque and inertia set acceleration limit, but ensure that load/motor inertia ratio is greater than or equal to 3 for optimum stability.

Some manufacturers, like Control Techniques Dynamics, offer free promotional software sizing programs: www.drivesportal.com/portal The user can define the system, plot a motion profile, and match a motor. This useful tool gives the user the opportunity to try different ratios, motor speeds, inertias and alternative schemes for optimisation of system design, thus saving many tedious recalculations. However, most simulators do not fully interpret reality, so be sure to invoke a safety design factor of at least an additional 20% of torque, depending, of course, upon how many considerations have to be taken into account.

In practice, it’s impossible to achieve theoretical acceleration limits, due to drive bandwidth, current rise time and other practical limitations. If the command ramps are set at limit, saturation will be reached with no reserve for load variations.

Thermal Protection

To protect them from the ultimate risk of burnt out windings, motors are conventionally fitted with thermistors buried in the windings. Where thermistors are offered, it is wise to connect them to the relevant drive protection input.

Drives have an algorithm to estimate the motor temperature, based upon the thermal time constant of the motor windings. When properly implemented, this protects the motor and permits periods of operation in the intermittent operating area. How to know the motor temperature at drive switch-on, has been neatly overcome by Control Techniques’ M’Ax drive and Control Techniques Dynamics SL motors.

Drive Switching Frequency

Drive current rating is usually quoted at the lowest operational switching frequency (which might be audible at the motor) whilst conversely, motors may be rated at the higher drive switching frequencies. Drives require de-rating at the higher frequencies, motors at the lower. Choose a compromise based upon conditions and price breaks!

Safety

With up to 680V dc, do not earth the dc link bus supply, leave at +/- 340V dc. Ensure safety for operators and installers alike.

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