Motion slickness

Linear motors, which are indispensable in the electronics industry, are fast gaining a foothold in machine tools. Jon Excell reports on a growing trend.

The use of linear motors in machine tools is not a new concept. Over the past few decades, tempted by the prospect of high speed, low maintenance and extreme accuracy, machine tool builders have gone to great lengths to incorporate linear drives into their machines.

However, the perennial problems of heat, magnetic fields, and high cost have all too frequently sent the same designers running faithfully back to the tried and trusted ballscrew.

Recently, many of these problems have been overcome, and today an increasing number of machines offer direct linear drives. Indeed, according to a recent report by business consultant Frost & Sullivan, the machine tool sector is at the sharp end of a rapidly expanding linear motor market. By 2008, claims the report, machine tools will surge ahead of semiconductors as the largest application market for linear motors, accounting for almost a quarter of the European market.

The advantages of linear motors are well known. Fast, accurate and capable of frightening acceleration, they also convert electricity into movement without diversion via mechanical transmission elements. This omission of mechanical components prevents jolting and mechanical wear in the drive chain.

A recent case study from machine tool builder Deckel Maho Gildemeister (DMG) provides some insight into how the traditional problems of linear motors have been overcome.

Linear motors are manufactured in two basic forms: ironless and ironcore. Neither system requires support bearings, bearing blocks, motor coupling or a screw, making application designs simpler.

Ironcore systems are capable of producing greater thrust than ironless systems, making them more suitable for heavy-duty applications. However, neither system is capable of producing the mechanical advantage of ballscrews. As a result, they don’t deliver the same levels of continuous force, and if they’re pushed too hard there is a risk of demagnetisation and failure.

DMG overcame this problem on its DMC85V machining centre by twinning up drives on the X, Y, Z axis. The ETEL brushless high-force motors used are cleverly arranged to prevent torsional twist.

The machine, which uses a total of eight drives, has better positional accuracy than machines using ballscrews and costs only 20 per cent more than a traditional design, according to DMG.

The most interesting aspect of the design is the way everything grows and shrinks in balance (thermo-symmetric), stabilising the machine.

Thermal temperature sensors and feedback systems compensate for local growth, and the motors – separated from the gantry and surrounded by a cooling aggregate – don’t add much heat to the structure either.

DMG chairman Dr Rudiger Kapitza agrees with the report. ‘Due to the far greater performance of linear motor technology compared to the conventional drive technology, they will have a wide range of uses in the future at DMG, which means with virtually all machines, even those in the low-end range.’

Siemens, named in the report as the global leader in the field of linear motor machine tool applications, echoes this view, forecasting that over the next few years 20-30 per cent of all machine tools will be equipped with direct drives.

Its specialist subsidiary Siemens Linear Motor Systems (LMS) has beenmanufacturing linear motors for years.

In the early 1990s demand for increased speed and precision of machine tools — particularly in the automotive industry – led to firms like Ex-Cell-O and Renault Automation pioneering the development and implementation of linear drives.

Siemens LMS became involved in 1997 with its 1FN1 linear motor and was one of the first companies to successfully match the linear drive to the harsh conditions of an industrial environment.

The big problem with replacing a conventional drive with a linear motor is that the drive must be designed into the machine tool from the very start and motor, control and software must be matched perfectly.

‘You’ve really got to look at the complete design structure of the machine,’ said Siemens application engineer Jim Fernley. ‘Really, it’s more suited to the ‘clean sheet of paper’, and it’s only when somebody’s looking at a totally new machine that they would probably make this leap from the existing technology to the newer technologies.’

Siemens claims to be the only manufacturer currently supplying an optimally adapted package that matches linear motor, control and software. The company said that, thanks to a modular component concept, its IFN3 motors can be adapted to specific jobs. For example, various components, such as cooling systems or covers, can be added to the motor. Depending on the design, the motors can be used as single comb or mirror-inverted double comb solutions. The different travels are implemented in the grid of the standardised secondary parts.

This makes linear drives particularly relevant for large machines with long travels. Parallel circuiting of several primary parts ensures that there are no problems with the high-force transmissions. The economical force limit is thought to be around 100kn.

Siemens LMS deals with the problem of heat using an integrated water cooling system. Motors in thermo-sandwich design, in which a temperature rise at the interfaces to the machine can be avoided by two cooling circuits, are available for high-precision applications.

However, the great ballscrew versus linear drive debate rages on, and while linear motors may be perfect for some applications, the ballscrew still has plenty to recommend it. For instance, the ballscrew has one big mechanical advantage not found in a linear motor – the motor invariably has to grow to provide the thrust for the axis to reach the accelerations and speeds required, whereas with a ballscrew thrust can be achieved simply because it is mechanical.

‘People are looking seriously at linear motors, but they will invariably cost more, and although you’re saving on ballscrews and couplings and so on, you’ve got an increase in the motor cost,’ said Fernley.

‘The machine builder has therefore got to look at what other benefits he can get from it. He can get a much more dynamic machine, and if it’s a conventional machining centre you don’t get the quadrature points that you get with linear ballscrews because there is absolutely no backlash. You get a maintenance-free system.’

At the moment machines equipped with linear motors tend to be in the hi-tech, high-priced end of the market, because it is only really at that end of the market that machine builders can justify doing this. However, the signs and expectations are that with time, as more and more machines adopt linear motors, the technology will come down into the lower-priced end of the market.