Some linear motor drive systems integrate the sinusoidal interpolation into the controller. This reduces the effects of noise over long distances, and removes some of the processing burden from the motion controller for higher frequency data handling. Advanced motion control algorithms offer flexibility with such features as on-board interpolation of the sinusoidal encoder feedback.

When OEMs have a machine that must deliver accelerations of more than 3 gs, linear speeds that exceed 5 mps, and positioning resolutions approaching the nanometer range, their prime mover of choice is turning out to be a direct drive linear motor.

However, some recognize that this level of performance is as much a function of the drive electronics as it is the linear motor mechanics. While a linear motor simplifies the mechanical structure – eliminating the nonlinearities introduced by backlash, friction, and compliance – it's the drive electronics that govern the higher stiffness, position accuracy, and overall throughput possible from these motors.

Remove the mechanics

It's difficult to develop a machine with the high dynamics of rapid acceleration and fast machining speeds that also positions precisely. But these requirements are found in many of today's applications.

Linear motors can provide a stiffness that is ten times that of even a highly preloaded ballscrew. They can do so because they eliminate the mechanical components than lessen stiffness.
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Non-linear effects introduced by machine mechanics frequently reduce servo stability, which diminishes the controller's ability to predict and maintain speed. Feedback data will not always make up for this instability because most feedback systems mount on the back of the motor far from the moving load. Thus, as applications require faster accelerations and speeds, the more likely compliance, backlash, friction, and wear will hinder achieving them.

The same goes for requirements of higher accuracies. Transmission compliance, friction, and backlash limit accuracy. Placing a feedback sensor, such as a linear encoder, at the moving load helps correct for backlash and deadband, improving resolution and accuracy. However, this solution moves the problem further up the system – the effects of backlash, friction and low stiffness are now inside the control loops. Regaining stability means lowering loop gains, which in turn sacrifices the system's ability to achieve fast motions.

Only by removing the mechanical transmission entirely, replacing it with a linear motor for example, can designers achieve both high accuracy and high dynamics in the same system.

Focus on the drive

At some frequencies, linear motor systems can offer a stiffness that is 10 or more times that of a ballscrew, which often displays resonant frequencies within 10 to 100 Hz. This capability allows linear motors to handle high position and velocity-loop bandwidths while positioning a load with nanometer accuracy – even in the presence of external disturbances. Any system resonant frequencies are well outside the position loop bandwidth.

There is a drawback with removing the mechanical transmission, though. It also removes any factors that helped attenuate disturbances from machining forces or cross-axis vibration occurring at the load. Now, the drive electronics must compensate for disturbances that act directly on the drive axis.

In a closed loop linear motor, assuming it is not saturating, it's up to the drive electronics to achieve the desired throughput, settling time, servo stiffness and stability, and positioning accuracy. This means you must choose the right amplifier, position controller, feedback device, and particularly the servo amplifier, as well as the right servo control strategies to get the dynamic performance the application needs.

Dealing with limits

With the drive system taking on more importance, you must pay more attention to motion controller, digital control, and feedback device limitations that will affect linear motor performance. These limitations will involve the quality of the current control-loop bandwidth, controller sampling frequency, and calculation delay, as well as the measured position.

Motion controller. To gain the needed quality, the motion controller should provide high sampling rates and servo-loop bandwidths, stabilize current loops, and offer force angle control and immunity to noise and drift.

High sampling rates. Because there's no way to predict external disturbances, the linear system must measure and respond to them as they happen. This requires the controller to sample feedback data almost constantly. Sampling frequencies for velocity loop and position loop data typically begin at 5 kHz. A linear motor-driven axis can have a positioning loop bandwidth five to ten times that of a conventional rotary motor-driven axis where frequencies of 1 or 2 kHz are adequate.

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