Accuracy is defined as the maximum error or difference between the expected value and the actual value. It can be measured in linear or rotary units depending on the mechanical design. Units of rotary position are typically given in arc-minutes or arcseconds, while linear devices are measured in fractions of inches or microns (millionths of a meter).

The resolution of a position feedback device is the smallest amount of movement it can sense. Rotary devices typically have resolutions measured in counts or lines per revolution, while linear devices typically measure the smallest distance change in microns. A benefit of high resolution is the ability to boost servodrive gains without causing instability. This allows faster response and shorter settling time.

Repeatability is the ability of the device to accurately return to the same location. A device's repeatable accuracy may far exceed its fundamental accuracy.

Servomotor feedback devices generally measure velocity or position, often in combination. The role of feedback depends largely on the type of servomotor and amplifier. Brushless servomotors must sense rotor position to control the points at which the drive electronically switches current through the windings in a process called electronic commutation. Often the same position feedback device is used to provide velocity and acceleration feedback. The system calculates those two factors as a function of the change in position versus time.

Feedback devices generate different kinds of output waveforms and voltages. Thus, the feedback device must generate signals that are compatible with the drive amplifier. Alternately, the drive amplifier must support the feedback device the application requires. This is usually not a problem as most amplifiers support a variety of feedback devices. But as a general rule, getting the motor and drive from the same manufacturer assures interoperability and specified motor performance.

Sealing isn't the only common requirement for servomotors. They typically are specified for environmental conditions that include specific temperature, shock, and vibration levels. Of primary concern when specifying motor torque is the ambient operating temperature. Typically, vendors specify torque ratings at ambient temperatures of 25 or 40°C. Continuous torque ratings are based on the temperature rise from ambient to the maximum allowable limit. Temperature rise is related to the power dissipated in the motor. So motors that operate in higher ambient temperatures must be derated. Conversely, motors operating in cooler conditions may boost their torque rating.

Some motors must mount to a minimum-sized heat sink to meet their
advertised ratings. Thus, engineers confronted with a small machine mount and less surface area should get advise from an applications engineer about motor sizing.

Some environmental factors may exceed normal considerations. For example, these extreme conditions may include operation in a vacuum or exposure to nuclear radiation.

Motors working in vacuum dissipate heat only through conduction or radiation. Usually they require significant derating and perhaps different construction materials. Motors operating near nuclear radiation need special insulation, bearing lubrication, and possibly other material changes. Again, designers should contact the motor manufacturer about any special environmental considerations.

Servomotors often come with various options for lead termination: from flying leads to motor-mounted connectors that rotate for more convenient and neater cable dressing. In addition, connection types vary widely in how they are sealed. Sealing ranges from the relatively unprotected flying leads to tightly sealed motor-mounted connectors that resist water jets. Obviously, the choice of wire termination depends upon the environment and the necessary degree of protection. Most manufacturers use International Protection (IP) ratings that identify enclosure resistance to penetration by solids and liquids.

The trend in servomotors to use larger bearings and shaft diameters is an attempt to boost axial and radial load capacity for longer life. Special bearings — such as precision, duplex, or cross roller types — provide greater run-out accuracy or load capacity. In general, servomotor bearings are permanently lubricated and sealed with a standard lubricant that accommodates a wide temperature range. If required, special lubricants are available that perform better under extreme heat or cold.