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Linear positioning explained
28 November 2014
Automation is essential if UK manufacturing is to compete against the low-wage economies of the developing world and linear positioning is often a fundamental part of automation. Gerard Bush of servo specialist INMOCO explains the design basics for linear motion systems.
The simplest form of linear motion is single axis, and even the most complicated multi-axis system can be thought of as a series of synchronised single axes. The main considerations for linear motion are load, speed and positional accuracy. These are dictated by the application and in a multi-axis system may require several stages of calculation.
A linear motion system is made up of one or more actuators, which usually also serve as structural members. For this reason, the actuators need to be stiff enough to support the dynamic loads experienced during accelerations and decelerations. They are usually made of square section aluminium extrusions that are strong enough and rigid enough for most applications.
For particularly demanding applications, there are actuators made of steel and iron and even granite ones for nanometre-precision requirements. Alternatively, standard aluminium actuators can be mounted in rigid load-bearing frames.
Each actuator contains within it a drive mechanism that generates linear motion, and there are a number of different formats. The most common is the toothed or timing belt drive, which is low cost and accurate enough for many applications. Belt drives can achieve high-speed movement, while their robust simplicity provides a long, low-maintenance life.
The next two most common drive mechanisms are probably rack-and-pinion and lead screw, both of which are slightly more expensive than belt drives, but can work with heavier loads. For the most demanding applications, there are the options of precision-ground ball screw and linear motor — both of which provide high-accuracy positioning.
The various forms of screw drive are limited by the screw’s critical rotary speed and may also exhibit backlash, wind-up and pitch cyclic errors. Further, high-linear velocities require low-screw pitches, but this restricts the mechanical advantage (which can be overcome by using a higher power motor, but this increases costs).
Therefore, when designing linear positioning systems, there are a number of mechanical calculations to be made relating to the type of drive mechanism in use. It is very important to remember that the actuator’s drive is usually connected to the motor via a coupling, which will have mechanical characteristics that also need to be modelled during the calculation phase.
Further mechanical calculations are required because linear systems usually include bearings. There is a wide choice of bearings, but they can be largely classified as ball or roller. Ball bearings are the lower cost choice but, because they provide only point contact, concentrate the load. Roller bearings spread the load over a wider surface area, so are preferred in high-load applications. The raceways of the bearings ensure accurate linear tracking and may need to be protected against damage.
There is an important variation on mechanical linear actuators: linear motors have been available for many years and are slowing becoming more popular. They provide a direct-drive option, which can lead to elegant and simple positioning systems that are cost comparable with conventional solutions.
There are many different types of linear motor, but a detailed description is beyond the scope of this article. Suffice it to say, the operating principle of a linear motor is essentially the same as an ‘unrolled’ conventional motor and in positioning systems they are most likely to be unrolled servos or steppers.
However, linear motors have a characteristic that could be a drawback in some applications, namely there is an air gap rather than a direct connection between the track and slider (equivalent to the stator and rotor in a conventional motor). This is maintained by electromagnetic forces, so will collapse in the event of a power failure. While this is probably a minor issue on horizontal axes, it could be a major problem on inclined and vertical axes.
While a linear motor system is self-contained, mechanical actuators need a motor to drive them. In general, linear systems use servo motors, although in some cases steppers are preferred. In the past, servos were significantly more expensive than steppers, but nowadays there are many ranges of low-cost servos that compete virtually head to head with steppers.
A stepper motor is based on the principle of having many, many poles (100 or more is not unusual) or pairs of electromagnets in the stator and rotor. When the magnets are energised they repel one another, causing the central output shaft to spin. Significantly when power is removed, the shaft stops turning and the poles line up, hence, the ‘stepping’ characteristic which can be used for positioning.
Each stepper has an electronic controller that can be programmed to tell the motor to spin through a given number of poles or steps, then stop. This will define the position in which the actuator stops the load. Technically there is no need to have a feedback system that confirms the stop position (although this also means there is no way to confirm that the target position has been attained).
A servo motor has far fewer poles, but is highly suitable for use with a high-resolution feedback system that constantly confirms the position of the actuator. Another major advantage of servo motors is that they are based on very powerful magnets, so provide a lot of power from a compact size. Servos also need a controller to execute the required movements.
Motors, whether stepper or servo, tend to be end- or side-mounted onto their actuators. The former requires a coupling, the latter often uses a small belt drive or gearhead, which gives the option for a turndown or step-up drive ratio.
While steppers have the apparent advantage over servos of not actually needing positional feedback, in practice, most linear systems use them. There are several options for the type of sensor to use, including trip switch and proximity sensors. However, the type most associated with accurate and multi-axis linear systems is the encoder.
Encoders consist of a glass disc (rotary) or slide (linear), onto which marks are etched at regular intervals. The passage of the marks past a set reference point is monitored by an electro-optical system to provide a highly accurate position signal. A rotary encoder would be used to precisely monitor the rotation of a motor shaft, from which position can be calculated, while a linear encoder measures position directly.
Precision linear systems’ engineering can appear to be complicated and to require advanced engineering components. But in reality, they are relatively easy to construct and commission, helped in large part by the standardisation of the key components - actuator, motor and feedback. A practised linear systems’ engineer can often develop solutions very rapidly and produce precision motion in minutes.