Servo Motor Control Overview and detailed Function

AC Servo Motor
AC Ser­vo Motor

Ser­vo motors are used, for exam­ple, for pro­duc­tion machines, indus­tri­al robots and tool machines. All these appli­ca­tions are char­ac­ter­ized by the fact that the move­ments are exe­cut­ed high­ly dynam­i­cal­ly. So that the work­piece or the prod­uct can be man­u­fac­tured quick­ly. Ser­vo motors are also used because of their high accu­ra­cy of con­trol, because the accu­ra­cy of posi­tion and speed has a direct impact on the qual­i­ty of the workpiece.

Advantages Servo Motors

Ser­vo motors are AC motors, so they can be induc­tion motors but also syn­chro­nous motors, BLDC motors are of course also includ­ed. Syn­chro­nous motors with per­ma­nent mag­nets are usu­al­ly used. Because these can gen­er­ate a high­er torque in the low­er speed range com­pared to induc­tion motors, with the same size and low­er phase cur­rent. Due to a high­er torque, a robot, for exam­ple, can move a pack­age from A to B faster. Quite often there is also talk of DC ser­vo motors, but these are usu­al­ly also only brush­less DC ser­vo motors, so basi­cal­ly BLDC motors. There are also RC ser­vos for remote con­trolled cars, boats and planes, but these usu­al­ly only con­tain a sim­ple DC motor.

Video about Servo Motors

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Servo Motor Drive System

A ser­vo motor is always a part of an elec­tric ser­vo dri­ve sys­tem, which con­sists of a ser­vo motor, an invert­er and a posi­tion sen­sor. The invert­er is also often called pow­er elec­tron­ics, ser­vo con­troller or sim­ply con­troller. In order to con­trol the posi­tion and speed very accu­rate­ly, the posi­tion sen­sors have a high accu­ra­cy. In com­par­i­son, elec­tric motors for fans or water pumps only have Hall sen­sors or are even oper­at­ed with­out sen­sors. Often, the dri­ve sys­tem also con­sists of a gear­box to increase the torque or to con­vert a rota­to­ry move­ment into a trans­la­to­ry movement.

Servo Motor Drive System Function
Ser­vo Motor Dri­ve Sys­tem Function

Design Servo Motor

What is the dif­fer­ence between a ser­vo motor and a motor for a water pump or an elec­tric motor for an elec­tric car? In order to con­trol the speed as well as the posi­tion, a high torque accu­ra­cy is required, which can best be achieved with exter­nal mag­nets on the rotor. Elec­tric motors with exter­nal mag­nets are also called SPMSM. The mag­nets are also often slight­ly skewed to each oth­er to make the torque curve smoother and there­fore improve smooth speed con­trol. For good torque con­trol, many mag­net­ic poles are used in the rotor, this gives the back EMF more sinu­soidal oscil­la­tions per rev­o­lu­tion, reduc­ing the torque rip­ple. More mag­net­ic poles have the addi­tion­al advan­tage of increas­ing the torque den­si­ty of the motor, which improves its dynam­ic response. Because ser­vo motors are oper­at­ed in alter­nat­ing mode, they have a fan or water cool­ing in only very few cas­es. Addi­tion­al cool­ing would increase the pow­er den­si­ty, but not the torque den­si­ty, which is more impor­tant for ser­vo motors. Instead, ser­vo motors only have cool­ing fins. At the rear is the posi­tion sen­sor, which can be opti­cal, mag­net­ic or induc­tive. Because the sen­sor is mount­ed on the back of the ser­vo motor, it often looks like the ser­vo motor has two con­nec­tors, but the sec­ond con­nec­tor is usu­al­ly just the plug sock­et for the sen­sor. There are also ser­vo dri­ves where the invert­er is mount­ed direct­ly on top of the motor hous­ing or behind the sen­sor. How­ev­er, such ser­vo dri­ves are usu­al­ly used for low­er powers. 
Servo motor with Sensor
Ser­vo motor with Sensor

Servo Motor Control

So a ser­vo motor is not so dif­fer­ent from an elec­tric motor for a water pump. Because of the many mag­net­ic poles, not only a sen­sor with a high res­o­lu­tion is need­ed, but also a ser­vo con­troller with a very fast pro­cess­ing of the sen­sor sig­nals and good con­trol. The con­trol of a ser­vo motor is done by a cas­cade con­troller. In the out­er cas­cade the posi­tion is con­trolled, the set posi­tion can come from a PLC con­trol via a bus sys­tem or via an ana­log input at the invert­er or the ser­vo con­troller. In the mid­dle cas­cade the speed is con­trolled, which must be faster than the con­trol posi­tion. The actu­al speed of the ser­vo motor is cal­cu­lat­ed from the first deriv­a­tive of the posi­tion sig­nal. The cur­rent is con­trolled in the bot­tom cas­cade. Usu­al­ly a FOC is used for this, which makes it pos­si­ble to con­trol the alter­nat­ing cur­rents with a sim­ple PID controller. 
Servo Motor Control
Ser­vo Motor Control