A variety of factors can complicate the task of picking the right servomotor.
Wood Dale, Ill.
Edited by Robert Repas
Brushed and brushless servo-motors are found in many diverse areas including medical, industrial, home, aerospace, defense, and robotic fields. And each application potentially requires a different motor. It can be tough to pick the right motor given the virtually endless choices in types, styles, and configurations.
Fortunately, manufacturers work hard to ensure that certain motor parameters meet some standards that simplify the task of picking a motor. Torque, speed, and voltage ratings have always been the first critical parameters to consider. Motor torque and speed are based on machine load and motion profile as well as load acceleration and inertia. Motor sizing software from many manufacturers lets designers enter the parameters they need. The software then tests each motor against the parameters to determine its suitability to the application. Some software, such as Motioneering from Danaher Motion, automatically takes into account possible performance deratings like those caused from thermal limits of optional equipment or the added inertia of a fail-safe brake.
Factory equipment and machinery continues to shrink. Motors keep pace by boosting torque densities the amount of torque a motor develops for its size. But as torque densities climb, so do motor temperatures. Motor makers try to deliver that power while keeping motor temperatures below maximum ratings typically 100°C above the specified ambient. Lower weight and inertia also benefits the dynamic response of the system. Higher torque density motors yield higher torque-to-inertia ratios to create faster accelerating machinery. The net result produces more products per hour.
Higher-voltage motors typically produce less torque for the same package size, thus have lower torque densities. This is because higher voltages need thicker insulation compelling the use of small-diameter, high-resistance wire. The high resistance wire boosts I 2 R losses generating more heat. Lower-voltage motors need less insulation, so they can use a larger diameter wire that handles greater currents without overheating.
So as not to burden lower voltage motors with high-voltage insulation that leads to lower torque density, some manufacturers create both low and high-voltage versions of the same motor. In contrast, Dana-her makes servomotors 58 mm and larger with high-voltage insulation that still has the same torque density of lower voltage motors. The motors run on 75, 120, or 230 Vac but contain 480-Vac Class F insulation across the line. Consider a case where a machine calls for a 120 or 230-Vac motor on one axis and a 480-Vac motor in another. For some manufacturers, this would require different motors to meet identical torque specifications. Dana-her, however, can generate the same torque for both voltages with the same motor.
Danaher abandoned the typical shuttle winding system used by most manufacturers in favor of a servocontrolled laminar scheme. With the laminar design, the turns of each coil lie much closer to one another. This packs more copper into the winding and, thus, boosts torque density. The technique produces a smaller motor for a given power rating or a higher-power motor in an equivalent package size.
A common cause of motor failure is insulation breakdown from excessive voltage or temperature. High-power-density motors use higher-grade insulation to better withstand the elevated temperatures of operation. In addition, the entire stator is enclosed in potting material that lowers its thermal resistance. Heat conducts quickly to the surface of the stator and dissipates into the atmosphere.
There are more than 75 industry-standard motor mounts currently in use worldwide. Two of the more common ones come from the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC). Manufacturers typically make motors with a common set of castings that fit a wide variety of standards such as the Japanese metric, European metric, and North American NEMA standards.
For example, the casting for the basic 58-mm AKM motor from Danaher also fits the Japanese 60-mm standard and the North American NEMA-23 standard. The required mounting-hole drill modifications take place during machining before final assembly. One casting, one process, and one procedure now make 75 different motor mounts among seven different motor sizes. This method simplifies quality control and manufacturing processes. The large variety of mountings adapt easily to different mechanical components or machine configurations.
A flange mount is the most common form used with gear-boxes. Flange-mount motors, like many motors that mount directly to machinery, have a raised circular surface on their face concentric with the output shaft. This is called the pilot. A pilot recess is machined in the mounting surface of the machine or mechanism. The pilot and pilot recess guarantee that motor and machine shafts align accurately.
Occasionally an output shaft needs modification to mate with another manufacturer's mechanism or specific load. It might require a special keyway, different diameter or length, a spline, or a taper. The factory should make these modifications to ensure the machining is accurate.
Any reductions in shaft diameter should get scrutiny from engineering. Small diameter shafts can fail prematurely when subjected to peak torques. In addition, reducing shaft diameter also reduces shaft torsional resonant frequency (TRF). Lower TRF can degrade overall system response. Designers may have to reduce drive gain to prevent exciting resonances that can produce excessive vibrations. Similarly, longer shafts require engineering evaluation with regard to bending limits and bearing loads.
Seals protect servomotors against solids or liquids that can enter and hamper performance or life. Seals guard the motor body and electrical connections as well as the output shaft. Motors exposed to liquids should use seal materials that do not deteriorate in their presence.
Motor seals reverse their function in clean-room environments like those in semiconductor manufacturing. They protect the environment from possible contaminants the motor emits. Brushless motors have an inherent advantage over brushed servomotors in clean environments. They don't produce carbon dust from brush wear.
A brake holds an axis of motion in position without servocontrol. The most common application is a vertical axis driven with a ball screw or other mechanism that can fall when the servo is not energized. But brakes can provide static parking and emergency braking on any axis. They are considered fail-safe, meaning they mechanically engage when they are electrically off. So power failures automatically engage the braking system.
Unlike automotive disc brakes, fail-safe brakes are not intended for repetitive operation in a dynamic mode. They do not have the surface area to endure nor the heat sinking ability to dissipate a lot of energy continuously. An application needing a dynamic brake for more than an emergency should have an external brake designed for that purpose.
The most common types of feedback devices used for servo-motors are resolvers, incremental encoders, and sine encoders. They come in wide ranges of accuracy, resolution, and repeatability.
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.
Though there is little standardization in the servomotor manufacturing industry, motor manufacturers work hard to overcome this limitation by making their product offerings flexible. Regardless of your application complexity, it is always good to talk to a factory rep to confirm what will work best for your design.