The position of these mechanisms is measured through the rotary servomotor’s encoder. The encoder may indicate the load has settled with a low settling time. What that means is the encoder has stopped moving. The load may still be in motion and not yet settled or experiencing vibration and oscillation. Rigidity and backlash interferes with the measurement of settling time through the encoder of a rotary servomotor. However, in the linear motor, the encoder is fixed to the load itself, reporting the true settling time of the load.
Backlash and rigidity also contribute to a mechanism’s positioning accuracy and repeatability. Accuracy is a measure of the deviation from the ideal; in this case, position. If the machine is commanded to move 4 mm, does it move exactly 4.000 mm? Or if you were to measure it externally, did it only move 3.999? What’s often more important is repeatability, also called precision. Because if the machine can move 3.999 repeatedly when you command 4.0, then adjust the command until it does move to the required position.
As with settling time, accuracy and repeatability are a function of machine rigidity and backlash. The control system measures position at the encoder. Rigidity and backlash add an element of uncertainty to those measurements.
The manufacturing process of a belt, ballscrew, or rack and pinion will affect the accuracy and repeatability. Only the linear motor by natural design measures the load directly and moves it without the backlash and compliance problems that can plague mechanical actuators.
So why not compensate for the shortcomings of ballscrews, belts, and rack and pinion mechanisms by adding a linear encoder? This is possible, and one term for this in the industry is full closed-loop operation. This allows the position loop of the rotary motor to be closed by linear encoder feedback.
However, adding a linear encoder adds significant costs and complexity. The linear scale must be accurately placed. The read head clearance to the linear scale must be accurately aligned. A cable track for cable management is needed on the carriage. Most servo amplifiers don’t support a second encoder input without additional hardware cost. Servo tuning also will be affected and must be completed again.
If the performance of an existing system must be improved, then full closed-loop control is a viable option. It is generally not a significant cost savings compared to using a linear motor in the first place. Adding full closed-loop control will improve repeatability and accuracy, but does not do much to improve rigidity, settling time and wear. For new applications, it is important to consider a linear encoder on a belt, ballscrew or rack and pinion will really save money in the long run.
Wear and maintenance for linear actuators
All mechanisms wear down. It’s inevitable. Users should understand what part of the mechanism wears and what happens to the system’s performance. In the linear servo, only the linear bearings wear out. But until they utterly fail, that stage will position accurately, at least in the linear direction. Pitch, yaw and carriage roll cannot be detected by the linear encoder and will deteriorate over time.
Some linear bearings require regular lubrication, but self-lubricating linear bearings are commonly available. The same principle applies to linear bearings on a ballscrew, belt or rack and pinion. However, these mechanisms have other moving parts that wear out. The screw itself, the nut, the bearings on the screw, motor bearings, belts, rack, pinion and gearboxes all eventually wear out even with regular lubrication.
From day 1, the performance of these mechanical systems begins to decline. The backlash and stiffness get a little worse every day. Therefore expect position settling time, accuracy, and repeatability to continually degrade as time goes on. The linear motor bearings do eventually wear out, but quite gracefully by comparison.
Environment for linear actuators
Different mechanisms are best suited for different industrial environments. Consider the magnetic field of the magnet track of a linear motor which may attract magnetic particles. A bellows protection system will help keep particles off the magnet track. While linear motors don’t require a cleanroom, they do their best in clean environments free of excessive dust and oil. The linear scale can still work with some interference. However, users should consider a linear motor with an inductive or magnetic linear encoder for very dirty environments.
The ballscrew is used commonly in machine tool and other dirty environments, although the screw also must be protected. Belts and rack and pinion also work well in many environments.
Clean rooms are on the other side of the spectrum, where it is critical to reduce the number of microscopic particles emitted by the actuator.
Linear motors can be easier to make clean room compatible as the wear of linear bearings is the only source of contamination. The wear points of ballscrews, belts, rack and pinion or other mechanisms with mechanical power transmission are generally less desirable for clean room operation due to the number of particles ejected during operation.
Selecting the right actuator
When selecting the right actuator for an application, it is important to consider long-term cost and performance. There are applications with specific constraints which logically eliminate a linear motor from consideration. Other applications have long been the strength of linear motors – long stroke or high speed, fast acceleration, precision and accuracy.
There’s a middle ground where linear motors are often overlooked in favor of the traditional solutions of ballscrews, belts, or rack and pinion. Solving these applications with linear motors can save money in the long run while providing exceptional and stable performance over the life of the machine.
Matt Pelletier, product training engineer, Yaskawa America Inc. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, email@example.com.
Keywords: linear servo motors, motors and drives, linear actuators
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