Minutes to motion - linear positioning explained

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. Here, 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).

Mechanical calculations

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 options.

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.

Prime mover

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.

Position sensing

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.

Summary

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 options very rapidly and produce precision motion in minutes.

To learn more about linear motion systems please go to www.inmoco.co.uk.

Inmoco

4 Brunel Close
Drayton Fields
Daventry
NN11 8RB
UNITED KINGDOM

+44 (0)1327 307600

info@inmoco.co.uk

www.inmoco.co.uk

More from Inmoco

Miniaturisation in motion control

Posted 5 days ago

New compact actuators deliver high force

Posted 1 month ago

Pumping technologies for precise fluid handling

Posted 3 months ago

High force integrated actuator reduces footprint

Posted 7 months ago

Rod-style electric actuators deliver 222kN thrust

Posted 8 months ago

Smooth motion arc segments, 360-deg rotary direct-drive motors

Posted 9 months ago

Agility Series direct-drive motors feature zero cogging

Posted 10 months ago

Voice coil motors offer precision motion in compact package

Posted 11 months ago

Inductive angle encoders are simple, robust and ultra-precise

Posted 1 year ago

Servo systems cables with improved connectors and labelling

Posted 1 year ago

Low-voltage DC servo motors for high-performance machines

Posted 1 year ago

Inmoco offers alternatives for motors no longer available

Posted 1 year ago

What does the future hold for motion control?

Posted 1 year ago

Simple Co-Engineering options for Kollmorgen AKM 2G servo motors

Posted 1 year ago

New stepper motor controller from Inmoco

Posted 2 years ago

Next-generation servomotors raise machine performance

Posted 2 years ago

Next-generation servo motors deliver 30 per cent more torque

Posted 2 years ago

Component compatibility can make or break a good system

Posted 3 years ago

High-performance actuator range is compact and rugged

Posted 3 years ago

Helicopter actuators may also transfer to other applications

Posted 3 years ago

Miniature encoders help advance surgical robot capabilities

Posted 3 years ago

Compact actuators improve aircraft motion systems

Posted 3 years ago

Encoders step up to help miniaturise electronics

Posted 3 years ago

Linear actuator design enables automated maintenance

Posted 4 years ago

Advanced motion cards provide precise actuation

Posted 4 years ago

Compact DA series actuators deliver quality and reliability

Posted 4 years ago

DA99 linear roller screw actuators increase reliability

Posted 4 years ago

DA Series high-performance electric actuators

Posted 4 years ago

New high-performance actuators available in UK

Posted 4 years ago

Servo motor with Ethernet interface connects directly with CNCs

Posted 4 years ago

Heavy duty linear actuator has high-performance controls

Posted 4 years ago

Single cable, multi-axis servo systems gets more power

Posted 5 years ago

Precision motion chips for dedicated controllers

Posted 5 years ago

Hygienic motor range has new feedback and connector options

Posted 5 years ago

Inmoco launches KOLLMORGEN safety feedback option

Posted 5 years ago

MC58113 Series programmable motion control chips

Posted 5 years ago

MicroE Veratus Series encoders: smart and reliable

Posted 5 years ago

Smaller, smarter encoders from Inmoco

Posted 5 years ago

Servo controller doubles power

Posted 5 years ago

Maintenance-free linear motors delivery high power density

Posted 5 years ago

PCB-based options are mighty minis for motor control

Posted 5 years ago

Vacuum-rated encoders from Inmoco

Posted 6 years ago

Servo users should not be seduced by power density alone

Posted 6 years ago

Inmoco introduces hygienic motor-gear combination

Posted 6 years ago

Servo technology is moving to a new paradigm

Posted 6 years ago

Compact washdown motor offers excellent performance

Posted 6 years ago

Maintenance-free lifting columns are easy to fit

Posted 6 years ago

Electric actuator optimises machine design and saves energy

Posted 6 years ago

Encoder offers value and performance plus flexible mounting

Posted 6 years ago

User-friendly compact encoder gets new side mount option

Posted 6 years ago

Reduce cabling with distributed servo drives from Inmoco

Posted 6 years ago

New functions make servo super-safe

Posted 7 years ago

Power storage modules act as servo UPS

Posted 7 years ago

Inmoco expands indexer family with motion features

Posted 7 years ago

Motion card integrates amps with full motion control

Posted 7 years ago

Wizard aids easy set-up of embedded motion controllers

Posted 7 years ago

EtherCAT master module for CTC 5300 Blue Fusion controller

Posted 7 years ago

OPC server gives instant access to automation controllers

Posted 7 years ago

Atlas single-axis motion amplifier is multi-talented

Posted 7 years ago

Rotary encoder establishes absolute position at start-up

Posted 7 years ago

Inmoco goes back to BASIC with new programmable servodrive

Posted 7 years ago

Mid-performance, high torque per pound servomotor

Posted 7 years ago

Stepper motors: advanced techniques

Posted 8 years ago

How to calculate point-to-point SCARA movements

Posted 8 years ago

Trombone servo drive range now has increased power

Posted 8 years ago

M4-X DC servomotor range is rugged and high performance

Posted 8 years ago

Achieve smaller, faster, smarter motion control

Posted 8 years ago

Expandable motion control system for extreme conditions

Posted 8 years ago

Elmo Gold Drum HV extends servo drive range to 65kW

Posted 8 years ago

B-Hornet military servo for high-power burst applications

Posted 8 years ago

O.P.S. Optical Encoders can be customised

Posted 8 years ago

ORMEC XD indexer family of servo drives expanded

Posted 8 years ago

Free automation systems control app for iPhone and iPad

Posted 8 years ago

Inmoco's Elmo Gold Servo Drives gain EtherCAT compliance

Posted 8 years ago

Inmoco introduces Elmo's DC Trombone digital servo drive

Posted 8 years ago

Robust linear actuators with integral force measurement

Posted 9 years ago

Energy-efficient IP67 stainless steel servo motors

Posted 11 years ago

Servo drives can be networked with IEEE 1394b

Posted 11 years ago

Wider choice of combined motion control and PLC units

Posted 11 years ago

Electronic cam controller suits labelling and capping machines

Posted 11 years ago

High-resolution encoder offers exceptional versatility

Posted 11 years ago

Elmo Duo controller integrates twin Whistle servo drives

Posted 11 years ago

Compact Guitar servo drive delivers 5kW of power

Posted 11 years ago

Ethernet/IP and Modbus options for multi-axis network manager

Posted 12 years ago

Elmo Solo Whistle saves servo application design time

Posted 12 years ago

New three-axis stepper modules for programmable controller

Posted 12 years ago

Electric linear actuators incorporate force measurement

Posted 12 years ago

Miniature ChipEncoder measures just 7x11mm

Posted 13 years ago

Cost-effective alternative for linear positioning and control

Posted 13 years ago

Rotary actuator includes servo motor, amp and position control

Posted 13 years ago

CANopen and absolute encoder options for digital servo drives

Posted 13 years ago

Programmable encoders feature miniature sensors

Posted 13 years ago

MicroE Systems' Mercury II digital encoder 'sets new benchmark'

Posted 13 years ago

Inmoco introduces ServoWire Motion and Logic Controller

Posted 13 years ago

Inmoco now offers Servotecnica slip rings

Posted 14 years ago

Inmoco launches "world's smallest" 50-nanometre encoder

Posted 14 years ago

High-resolution encoder is claimed to be the world's fastest

Posted 14 years ago

More technical articles
11 hours ago
New bifold constant torque hinge improves user experience
Southco has expanded its popular range of position control hinges with a new version that has been specifically designed for fold-out tables in passenger transit interior applications
12 hours ago
New FDA compliant cable entry plates
Foremost Electronics has introduced new KEL-DPZ-HD FDA compliant cable entry plates from icotek, specially developed for the use in the food and pharmaceutical industries
4 days ago
Boxing clever
A new chainflex cable box means big shipping cost savings for customers
4 days ago
New industrial vision online channel sees the light of day
Industrial vision provider IDS is making its expertise as a manufacturer of digital industrial cameras available free of charge and readily accessible on a new platform
4 days ago
Ball screws help protect buildings from earthquakes
Ball screws from NSK are helping to protect structures and people in earthquake zones
4 days ago
ATEX certification for new pneumatic valve island
Pneumatic process control offers numerous benefits, especially in hygienic applications. Bürkert says that its new Type 8652 AirLINE valve island provides users with improved safety features, communications and diagnostics
4 days ago
ABB launches condition-based maintenance service for robots
New service enables users to plan ahead and optimise production performance
4 days ago
What the UK-EU trade deal means for UK vehicle manufacturers
Now the UK has left the EU, the transition period has ended and the two parties have established a new trading relationship, UK-based vehicle manufacturers have some clarity over what they need to do to serve three key markets: Great Britain (England, Scotland and Wales), the EU and Northern Ireland
4 days ago
Farnell publishes Industry 4.0 ebook
New ‘Industry 4.0 Interviews’ ebook is available for free download from Farnell and showcases the opinions of leading global experts on the future of IIoT and Industry 4.0
4 days ago
Can DC motors be used at high temperatures?
There is no need for a heated discussion about it as maxon’s motor expert Andrew Gibson offers a view on the subject