15.9.1 Introduction
Linear motor systems started to make an impact on machine tools in the late 1990s for specialist applications such as CNC crank-pin grinding and camlobe grinding [Landis 1999]. More recently, several OEMs such as Danobat (Deba, Spain), have designed machines for speed-stroke grinding with speeds up to 250 m/min and acceleration/deceleration of 5 g. These are based on the superior speed, reduced friction, and inertia obtainable with linear motors.
Linear motors have many advantages. They consist of a single moving part eliminating the ballscrew with its inherent backlash, pitch errors, and compliance. That combined with low inertia and zero friction give a much faster response time and higher accelerator/decelerator rates. The reliability is far greater than a ballscrew while requiring much less maintenance. The system can also run either at low speeds for creep feed grinding or extremely high speeds for speed stroke grinding all with submicron accuracy. In conjunction with direct control through the PMAC via a suitable linear encoder, it is possible to control, position, velocity, and acceleration, which is key for cylindrical grinding of nonround parts.
Linear motors do have disadvantages. First and foremost is cost; the price of a linear motor is still significantly higher for a given power than servodrive systems. However, this is offset to some extent by a faster assembly time into the grinder. The motors generate a lot of heat, especially at high speed, which must be dissipated through internal cooling. The magnets are very high strength and will attract metal swarf unless well guarded or sealed. Linear slides are not commonly used on vertical axes because a counterbalance or brake must be supplied in the event of a power failure. In fact, even for horizontal axes, a capacitor or auxiliary power supply is normally provided to control the axes in the event of a power loss.
Linear motors, like most technologies, had initial teething problems. The slider winding is embedded in epoxy, which is prone to attack by water vapor and allows water to gradually eat its way into the winding and eventually short them. Where linear motors have been applied, the closest attention has been paid to slideway guarding. Many OEMs have tested their first linear motor — driven grinders in the laboratory for several years running prototype production under a range of conditions prior to offering linear motors commercially.
Until very recently, linear motors were still a niche drive for very specific machine movement requirements. If such drives are under consideration it is recommended that the potential buyer confirm with the machine tool builder that they have a successful track record of at least 2 to 3 years.
15.9.2 A Linear Motor System
Figure 15.25 shows a typical closed-loop linear motor drive system. An ac linear induction servomotor is essentially a rotary motor that has been laid out flat (see Figure 15.26). It consists of a track containing rare earth (e. g., samarium/cobalt) high-density permanent magnetic strips embedded in epoxy and fixed to the machine base, and a slider assembly made up of a laminated steel structure with conductors wound in transverse slots. The slider is supported by a pair of linear bearings—ideally, hydrostatic
bearings. Thrust is developed by the ac current in the motor conductors interacting with the permanent magnetic field.