Air bearing stage with linear electric motor and intelligent digital axis controller. Linear servomotors serve well in applications needing high acceleration and deceleration, high velocity, or precise velocity control even at low speed.

The components that make up your high-accuracy positioning system — bearings, position-measuring system, motor- and-drive system, and controller — must work together as well as possible. Part 1 ( PTD, 10/93, p. 51) covered system base and bearings. Part 2 (PTD, 4/94, p. 47) covered position measurement. Here, we discuss stage, drive, and encoder design; the drive amplifier; and controllers.

Stage, drive, and encoder design

Figure 1 shows the three commonly used methods of assembling linear stages when using linear encoders:
• Drive and encoder are positioned in or as close as possible to the center of mass of the slide.
• The drive is located in the center of mass; the encoder attaches to one side.
• The drive is located on one side; the encoder, on the other.

Figure 2 summarizes relative values of general properties of these systems.

The ideal system has the drive in the center of the slide mass with the encoder. However, this is usually impractical. The usual compromise locates the drive slightly off to one side; the encoder, slightly off to the other. This gives a good approximation of a central drive with the motion feedback next to the drive system. Central drives are preferred because the drive force introduces no unwanted force vectors into the slide to cause twisting or cocking. Because the bearing system constrains the slide tightly, cocking would produce increased friction, wear, and load-position inaccuracy.

An alternative method uses a gantry style system with two drives, one on each side of the slide. The resulting drive force emulates a central drive. With this method, you can locate the position feedback in the center. If this is impossible, you can locate encoders on each side and control the table with special gantry drive software.

Drive amplifier

Servo drive amplifiers receive control signals, usually ±10 Vdc, from the controller and provide operating voltage and current output to the motor. In general, there are two types of power amplifiers: the linear amplifier and the Pulse-Width- Modulated (PWM) amplifier.

Linear amplifiers are inefficient and therefore are used mainly on low-power drives. The primary limitations on the output power-handling capacity of a linear amplifier are thermal characteristics of the output stage and breakdown characteristics of output transistors. The power dissipation of the output stage is the product of current and voltage across the output transistors. PWM amplifiers, in contrast, are efficient and are typically used for power capacities above 100 W. These amplifiers switch the output voltage at frequencies up to 50 MHz. The average value of output voltage is proportional to the command voltage. The advantage of this type is that the voltage is switched On and Off, causing greatly increased power dissipation capacity.

Once you have chosen the amplifier type, the next step is to ensure that the amplifier can provide the required continuous current and output voltage at the required levels for the maximum motor rotation speed (or linear velocity for linear motors) of the application.

For brushless linear motors, you can make another distinction between amplifiers. Two types of motor commutation are in general use: trapezoidal and sinusoidal. Trapezoidal commutation is a digital type of commutation in that the current for each of the three phases is switched either On or Off. Hall-Effect sensors implanted in the motor usually do this. External magnets trigger the sensors. However, the relationship between the Hall-Effect sensors, the coil windings, and the magnets is critical and always involves a small position tolerance. The response timing of the sensors, therefore, always occurs somewhat slightly out of phase with true coil and magnet positions. This leads to a slight variation in application of current to the coils, leading to unavoidable vibration.

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