The basics of servomotor axis control

Precise control of axis position is an enabling technology for automatic machine tools. How does it work?

Several ways exist to control axis position, but one of the easiest to understand is a permanent-magnet DC servomotor. The basic element is a DC motor, in which there are loops of wire on an armature that is free to rotate on bearings. The armature is in the presence of a magnetic field created by permanent magnets. When a voltage is applied to one of the loops, a torque is created, which causes the armature to rotate (Figure 1).

As the armature rotates, the torque decreases, but then the voltage is switched to the next loop of wire. The contacts between the loops of wire and the voltage source are maintained by conductive brushes, and the switching between loops is called “commutation.” The rotating armature of the DC motor is connected to a screw. As the screw rotates, it drives a nut attached to the machine tool table, causing the table to slide along the guide way.

The torque created by the applied voltage is used to accelerate the inertia of the motor and screw, and to overcome the friction and load torques. A DC motor in this configuration—called “open loop”—has a rotational speed that is sensitive to the load torque. It directly “sees” the inertia of the rotating armature and screw and the friction in the bearings. It also sees the table mass and the cutting forces, but only indirectly through the screw.

To make the DC motor less load sensitive, it is common to measure the rotational speed, compare it to the desired speed and adjust the command voltage to the DC motor based on the difference. The rotational speed of the motor can be measured with a small generator attached to the rotating shaft. This is called a “tachogenerator.” The tachogenerator produces a voltage proportional to the rotational speed of the shaft; it is like a DC motor, but backwards.

The measured voltage is compared to the input voltage, and the small difference is amplified and passed to the DC motor (Figure 2). In this configuration, the DC motor has a velocity feedback and is said to be under velocity control. This trick makes the DC motor much less sensitive to the external load.