15.7.1 Introduction
Motion control is the technology required to drive the carriages on the machine slideways and ensure that the motion is correct. Glancing through any used machinery brochure, it is easy to understand the changes that have occurred in the design of machine tools over the last 25 years. In no area is this more apparent than in linear axis motion control. Prior to about 1980, nearly all machine tools were hydraulically driven; today more than 80% of new machines are driven by servomotors with ballscrews. The driving factors for this have been cost, advances in motor, encoder, and machine control technologies, reduction in the number of mechanical machine components for easier maintenance, and the desire to eliminate hydraulics from machines for heat and vibration reasons.
15.7.2 Hydraulic Drives
A hydraulic drive consists of a cylinder with a piston and rod that slides in and out of it. The force produced is the product of the applied pressure on the piston and the area over which it acts. Manual or servovalves control velocity and position by controlling the flow of fluid in and out of the cylinder. The cylinder can be either single action, often used in opposed pairs, or double action. Hydraulic systems usually offer only medium accuracy and are now most often used in surface grinders with a large range of travel such as the slideway grinder example shown in Figure 15.6.
15.7.3 Electrohydraulic Drives
For precision applications requiring only a short range of motion such as some of the dresser infeed examples illustrated in the previous chapter, electrohydraulic drives are used. These consist of a sealed system with two bellows; a small diameter, long-stroke master connected to a large diameter short-stroke slave. A motor-driven screw compresses the slave bellows and actuates motion of the master bellows. The resolution of the screw motion is increased by the ratio of the master/slave bellows diameter.
15.7.4 AC Servo — and Ballscrew Drives
Ballscrew and ac servodrives are the basis of most of the current grinders on the market today and led to the elimination of hydraulics. They consist of several components, namely, the recirculating — ball leadscrew, or ballscrew, ballnut, motor, and positional/speed monitoring encoders as illustrated in Figure 15.19.