Servomotors: the basics

The Kollmorgen AKMH hygienic servomotor includes FDA-approved food-grade materials and meets IP69K, EHEDG, and 3A standards. The motor helps food processors reduce food-recall risks and decrease cleaning time. The motor can withstand 1,450 psi spray and pH 2 to pH 12 chemicals.

Servomotors are used on machine axes that need to make complex moves or position loads with really high precision. Servomotors can also run at zero rpm while holding torque to keep a load at a set position.

A servomotor (or servo system) is characterized mainly by the use of closed-loop control. The system takes an error in position, speed, or torque and corrects it through the use of a feedback device. A controller compares the system’s actual performance with its commanded performance and takes corrective action to eliminate the error.

A servomotor can be built around one of several types of motors. Common types include dc brushed and brushless motors as well as ac induction motors. The advantage of brushed dc servomotors is their linear and predictable performance that makes them easy to apply. Brushless motors usually run applications needing more torque; the only catch here is that their drives are more complex because commutation is done electronically and not mechanically as in brushed dc motors. What’s more, industry categorizes motors in part by their number of electrical phases. Brushed dc servomotors as well as voice coil motors are in fact single-phase motors, whereas brushless servomotors most commonly have three phases.

There are also instances of classifying induction-motor-based designs running off vector controls as servomotor setups where the design incorporates feedback (usually from an encoder) to track and control speed and sometimes even position. These induction motors typically adhere to NEMA or metric standards, whereas other servomotor offerings are less uniform.

Manufacturers classify motors for constant-speed tasks by horsepower or torque at base speed. In contrast, servomotors operate over varying speed ranges and aren’t rated in this way. Instead they have speed-torque curves that express continuous torque capabilities (that won’t threaten to overheat the motor) and intermittent or peak torque for acceleration.

Many non-direct-drive servomotors have top speeds up to thousands of rpm. To better leverage their full capabilities, designers will often combine such motors with gearing to trade an increase in output torque with lower output speed. Much of the time, this gearing takes the form of planetary or harmonic gearheads, precision arrangements with high accuracy and efficiency. In many instances, gearing even lets machine builders use smaller motors on some axes. This equates to cost savings that may even offset the price of the additional gearing.

One common factor that can impact servomotor performance (and thus accuracy and precision) is the presence of electrical noise in a system. The sources of electrical noise are varied, but one of the most common is from the high-frequency signals generated by variable frequency drives (VFDs.) This noise, called electromagnetic interference (EMI), can be picked up through cabling.

Cables themselves can be manufactured with special shielding to guard against EMI issues. The purpose of cable shielding is two-fold. One, it prevents electrical signals emitting from the cable, and two, it prevents other signals or electrical noise (EMI) from entering through the cable and disrupting other electrical equipment down the line. Shielded cable options include copper braiding, steel braiding, or combinations of these and other materials.

Another common way to minimize cable noise issues is to use twisted pair wires. This also reduces the amount of electromagnetic radiation of signals that could effect other equipment or unshielded cables nearby.

Optimizing the cable will go a long way toward eliminating noise issues. For instance, one simple fix (and what should always be a key design consideration) is shortening the length of cable used to connect equipment together. The longer a cable is, the more it acts as an antenna, making electrical noise problems worse.

The IDEA motor from Haydon Kerk Pittman integrates a brushless servomotor with a drive and feedback connections, reducing motion system component count and simplifying machine troubleshooting. The integrated motors are designed for real-time embedded motion control and are suitable for autonomous precise execution of complex single-axis motion.