Motion System Design
Driving linear motors

Driving linear motors

Are the drives that run rotary motors capable of driving linear motors? Depends on the application. Some requirements for motor speed and accuracy make demands that the average drive can't meet. But that's about to change with the latest advances in drive electronics.

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.

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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Servo loop bandwidths. Servo loop bandwidth is a function of every element in the control loop, including the data processing speed of the motion controller and the response of the feedback device, as well as the motion control strategy.

The linear motor needs a high loop bandwidth because of the absence of mechanical attenuation. Linear systems using air bearings are frictionless and have almost no inherent damping to help reduce settling time at the end of fast moves. Thus the bandwidths on all three control loops (position, velocity, and current) should be two to three times higher than those of rotary systems.

Stable current loops. Current loop stability is the most important factor of a direct drive system. Without mechanical transmission, the system relies on the feedback signal for positioning resolutions and accuracies. For linear motor-driven axes, resolutions can be well below 0.1 μm, and even into the nanometer range. The quality of the current loops determines whether the linear motor can hold a position stable within this resolution.

The velocity loop cannot compensate for a poor current loop. Noise, unbalanced currents, low currentloop sampling rates, ripple, and low resolution (if digital) must be controlled to achieve the accuracy and resolution linear motors can deliver. Digital current-loop bandwidths should be more than 2 kHz.

Force angle control. You can achieve high speed or accuracy with a traditional rotary motor and a belt drive or large-pitch (for fast speed) or smallpitch ballscrew (for resolution). However, you cannot attain both in the same application. Linear motors can with a technique that enables them to handle high and low speed travel.

Low speed travel typically requires a motor winding with a high force constant to compensate for disturbances where inertia has little effect. To prevent the motor from exceeding the amplifier bus voltage, high speed travel requires a low motor-winding back emf. Linear motors deal with this apparent contradiction by permitting commutation angle adjustment on the fly as a function of speed. Changing this 'force angle' from its optimum of 90 degrees e lets you extend the motor's speed range (and the achievable acceleration) beyond the apparent bus voltage. This is an important feature for large force linear motors, such as those used in machining applications.

Immunity to noise and drift. Digital amplifiers should be immune to environment noise and component drift. Storing compensation and tuning parameters in the amplifier's software also reduces performance variance from unit to unit and, therefore, from machine to machine.

Digital Controls. These devices easily handle the necessary sampling frequencies and update rates. Software for them makes much of the setup and programming easy.

For example, tuning direct-drive axes can be more involved than rotary axes. Most digital control software, though, lets you adjust multiple variables with a few keystrokes or clicks. You'll also find these packages include progressive control algorithms, such as advanced pole placement (APP) and pseudo-derivative feedforward (PDFF) that can increase bandwidth, reduce following error, and improve stability. Some of the algorithms let you enter the units as V/m/sec instead of V/rpm; a feature not found in drives used for rotary motors.

Some drives use software commutation, letting you eliminate additional Hall-effect feedback and its cabling. This commutation is possible if the motion controller first polls the motor and position feedback device through auto initialization during startup. Because linear motors usually mount into a machine separate from the position feedback, it isn't practical to manually align the feedback to the motor phases. The initialization will find the phasing and location of the position feedback with relation to the motor phases. The accuracy of this feedback has a direct bearing on the velocity ripple and thermal efficiency of the motor.

Auto-tune procedures that use an automatic estimate of system inertia can help tune the servo system. But more work needs to be done to perfect them.

Feedback. The position system must feed linear motors good data fast. A high closed-loop servo bandwidth depends on it. Thus, a linear system needs at least 10 times the typical position resolution to maintain stable control at the desired position.

To achieve such a resolution, a feedback system needs a high input frequency bandwidth and integrated sine interpolation.

High input frequency bandwidth. Slewing at more than 2 mps while resolving to a position within 1 μm is not an uncommon application of linear motors and requires at least 2.5 MHz per channel. A typical rotary motor running at 3,000 rpm with a 2,000 line encoder will never generate more than 100 KHz of pulses per channel.

Integrated sine interpolation. Most linear encoder feedback outputs are sine and cosine signals with a pitch equal to the encoder line grating. An intermediate electronics box samples these signals and transmits quadrature digital signals at a frequency increased by the interpolation factor to the motion controller. At some point, however, the needed output frequency exceeds the capabilities of the interpolation box and the input frequency of the motion controller.

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Using integrated interpolation in the motion controller avoids this frequency limitation. It also eliminates many of the noise problems common to analog sine and cosine signals traveling over long distances. In addition, it reduces the amount of data the motion controller must process, enabling it to react even more quickly to the dynamics of the system.

The next stage

While awareness is increasing that some applications need a drive system designed for linear motors, developers are already focusing on the next round of control needs. Coming soon will be drives with higher loop bandwidths, lower cost position feedback as well as alternative approaches to such feedback, and more sophisticated auto tuning algorithms to aid servo system setup.

When less is more

In a motion system for a machine tool, removing the mechanical transmission components:
• Simplifies the mechanical structure.
• Increases mechanical stiffness.
• Eliminates belt stretch, ball screw twist, compliance, and backlash.
• Increases linear speed and acceleration. The only limit comes from the mechanical bearings.

The need for high frequency

Asimple calculation shows how much information a motion controller receives and must process.

For a required speed of 2,000 mm/sec, multiply it by 25 encoder lines per mm and 50 counts per line interpolation to get 2.5 Mhz per channel. This number of encoder lines and interpolation allows a positioning resolution of 0.31 μm.

John Floresta is director of engineering, Kollmorgen, Radford, Va.

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