Are you up to speed on all the changes in digital ac drives and the latest advances they have brought to machine tools? Some may have escaped notice as the rapid pace of development enables designers to leap over the incremental improvements of the past.

Today's machine tools rely primarily on two distinct classes of ac drives; feed drives (or servos) and spindle drives.

The latest servos offer:

• Four quadrant operation

• Current loop bandwidth of 5,000 rad/sec or more

• Velocity loop bandwidth of 600 rad/sec or more

• Smooth torque (no ripple) even at low speeds

• High static and dynamic stiffness in closed position loops

The latest spindle drives offer:

• Maximum speeds to 40,000 rpm, controllable to zero rpm in contouring mode

• Orientation of the cutting tool with a four-factor increase in rigidity

• Recovery from load changes 33% faster than earlier designs

The digital age

Such improvements are the result of the latest developments in digital control, including feed forward control, friction compensation, absolute feedback, and adaptive control.

Feed forward - that is, feeding the position command into the velocity loop - provides position loop gain control while moving. (Feed forward increases the apparent gain after a valid command to move. It does not affect stability at standstill.) By increasing the feed forward gain, you can reduce following error (lag), which otherwise would be large during high-speed machining. Lag is directly proportional to the gain and inversely proportional to the feedrate. So, it's possible to have near infinite gain with zero lag. Increasing feed forward reduces the following error, letting machine tools cut truer circles and, with look ahead and feedrate change control, cut sharper corners with minimal shock to the machine.

Friction compensation corrects for poor performance from components, such as sticky slides. Friction, servo delay, and backlash can cause flats or other imperfections at the quadrant points where the servo must reverse direction. Compensation functions correct these conditions.

All machines have some friction. The few with a lot require high torque motors. Otherwise, once movement starts, any overtravel in the axis can result in oscillation. Servos are ideal for controlling torque buildup and preventing such overshoot.

The machine may also have a resonance at a low enough frequency to cause vibrations during operation. A notch filter will suppress a fixed resonance. Other feedback functions suppress such sources of vibration as compliance in the lead screw. Plus, the drive can monitor cutting loads and feeds, and initiate alarms for conditions such as a broken tool.

Precise digital control reduces stresses to the machine without sacrificing response. Linear acceleration and deceleration control grants the fastest ramps for a given torque, reaching speed in 30 msec or less with a low inertia motor. Bell curve acceleration imparts a softer start when a machine can't tolerate faster speed changes.

In tapping, the feed amount of the z-axis for one rotation of the spindle should equal the pitch of the tap. Precise digital control and accurate feedback controls the spindle rotation and z-axis feed so that they are always synchronous. This permits rigid tapping at speeds up to 4,000 rpm.

Digital control and improved feedback techniques let the same spindle controller and motor handle contouring. Therefore, axis resolutions can reach 0.001 degree, at spindle speeds to 200 rpm for contouring and 25,000 rpm for velocity control.

Absolute position feedback, which is in wide use in the automotive industry, eliminates machine re-referencing following a power loss. This reduces downtime.

Once referenced, the absolute encoder tracks all motion even during power off. On power up, the control reads the machine position and updates the position registers. Absolute encoders typically have a range of up to 5,000 inches.

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