Reinventing the motor

Linear motor technology has been around for decades, but the price put paid to its widespread use. Now motor control expert Baldor has come up with a redesign that halves the cost.

Progress usually comes at a price. Though a new technology’s benefits may be patently obvious, it is often harder to justify the costs.

Early adopters, frequently in the US, Germany and Japan, take the initiative and the risks, make the investment and build up the new technology’s sales volume sufficiently to lower the price to a level where lesser manufacturing economies such as the UK can feel comfortable investing in it. This is about to happen for linear motors in the UK.

‘Linear motors overcome most of the disadvantages of the most commonly used ball screws,’ said Simon Smith of linear motion specialist Aerotech. ‘Ball screw systems are subject to screw wear and backlash and cannot tolerate high speeds or acceleration rates. There is a temperature effect on the screw which reduces accuracy. Ball bearings also reduce the smoothness in velocity. Because of wear the characteristics change over time.

‘While the fundamental technology of linear motors has been known for decades, the cost of the rare earth magnets precluded its widespread application. As magnets became cheaper and more plentiful, visionary manufacturers began to perfect the other elements of the motors, such as winding technology, bearings, feedback devices, commutation and motion control.

‘The latest air bearings, for example, take electromechanical engineering to extremes of performance and accuracy that could not have been achieved a few years ago,’ said Smith.

Linear motors normally use magnets at a cost of around E5 (£3.50) per 25mmof track. Tracks can be several metres long, contributing greatly to the overall cost.However, Arkansas-based motor control designer and manufacturer Baldor has just announced that later this year it plans to introduce a mid-range linear motor for half the price you would pay now. The range, called HyCore (hybrid core), has completely eliminated the magnets from the linear motor’s track.

Baldor’s John Heilig, who led the HyCore development team, explained its advantages: ‘The main improvement of this motor is the cost saving that can be achieved by removing the magnets from the magnet track. The reason it is called a hybrid motor is because it is similar in construction to a hybrid stepper motor which would have magnets and windings in the armature. The magnets are now in the moving portion in the coil assembly. Therefore there are a lot fewer of them.’We think there is an extra chunk of the market available that does not need the performance of iron-core cog-free servos but would like to use linear motors for the extra benefits they can get.

‘The HyCore is not intended to replace other linear motors. It gives a cost-competitive alternative to ball screws and belt drives.’

If a linear motor system costs E1,500-2,000 then the equivalent HyCore system’s price is just under E1,000. That is equivalent to a rotary servo motor, with coupling and ball screw system – but with higher performance, better resolution and repeatability.

Baldor expects the HyCore to be used in applications like part transfer, simple actuation, conveyor systems and sorting. Comparable iron-core linear motors 10 years ago cost the equivalent of about E3,000. In a further 10 years, said Heilig, with new technologies and hopefully with growth, it could be E600 – nearly half the price again of HyCore. ‘We will have developed new motors within the next 10 years and those motors will be designed to reduce cost because that’s where the market is,’ added Heilig.

Ten years ago linear motors had a speed limit of 2-3m/s. That’s been improved slightly with cog-free motors which brought speeds up to 5-10m/s. ‘I don’t see the need for more speed than that,’ said Heilig. ‘At high speed you run into lots of mechanical issues like cables falling apart and bearings not being able to cope. Control technologies have improved too – particularly processor speed – allowing greater motor speed.’

The biggest linear motors 10 years ago had a continuous force of about 3,000N but there are linear motors available now that deliver 30,000N. There is still demand for ever greater forces but only in small volumes – for the machine tool industry, for instance.

‘Ten years from now they could be larger but I don’t expect them to be tremendously larger unless some newer technology comes along,’ said Heilig. ‘The major benefit of ball screws is that force range. You can make a ball screw that will give you 100,000N quite easily.’

However, according to a recent study by Frost & Sullivan, in the long term the largest contributor to market revenues is expected to be the machine tools application segment. By 2008 this sector will surge ahead of the semiconductors sector as the largest application market for linear motors, accounting for 24 per cent of revenue shares. So in terms of volume, high-specification linear motors are a smaller market but they will be the largest in terms of sales.

The relatively well-established semiconductors and electronics sectors were the first and second-largest application sectors for revenues in the European linear motors market in 2001. While still significant, their revenue shares are likely to fall as emergent application segments experience higher rates of growth, claimed the report.

‘The uptake of linear motors only started to accelerate in the last two to three years for the machine tools market,’ said Gordon Semple of GE Fanuc Automation. ‘There are two reasons that account for this. One is our own development of motor and drive technology, not just in the design of the linear motor, but in the electronics supporting it. The electronics have come on leaps and bounds in the way that they can process data – particularly when your linear motors are accelerating so quickly you need to ‘churn’ the data and control the current much more accurately. We are certainly controlling them far better than we were 10 years ago and getting their true performance out of them.

‘The other factor is that customers really appreciate how to design them into their machines – or even design their machines for linear motors. People realised quickly that you can’t just take off the lead screw and the rotary motor and put a linear in its place.’

Linear motors are a long way off being the same price as a standard rotary motor at the high-end specification required for machine tools. But as development has progressed, prices have not increased – in fact, they have come down in some instances.

‘What we do now is supply smaller motors down to 300N maximum force,’ said Semple. ‘That has allowed people to design smaller, faster and more accurate machines for certain technologies – for example, cam grinding, where you’re looking for very high reciprocating axes, and for precision machines like lathes. There are Japanese companies now producing very small-footprint, high-production lathes using small linear motors. This was not current thinking 10 years ago. Back then we were looking at ever more powerful motors.’

The big names in the machine tools industry know the advantages of linear drive technology and by now also know how to deal with previous shortcomings. Today an increasing number of machines offer the alternative linear drive on specific axes, so that end users can decide between the conventional ball screw drive or the linear, based on their own preferences and area of application.

One shortcoming, however, which is often mentioned and frequently the subject of complaint, is the lack of compatibility of linear drives from different manufacturers. Because no standards have been established, manufacturers construct their own drives according to their own philosophy.

The differences are not great, but even small discrepancies in length or width mean they can’t be interchanged. Machine tool builders have to construct a separate machine base for linear drives from each drive manufacturer.

‘In rotating machines there are standards for frame size around the world,’ said Semple. There isn’t a standard for linear motors. Certainly in Europe we have to look at what is available on the market and make sure that we are compatible.’

GE Fanuc has now taken the first step to counteract the problem of different motor sizes. The introduction of the new S-series has made it easier for users to employ different drives with the same machine construction from different manufacturers.

With this lucrative market poised for take-off, competitive pressures are intensifying. Both large companies with a broad product range and small businesses with dedicated product offerings are in the fray. Siemens is in the number-one position with its specialist linear motors division producing technology primarily for the machine tools sector. Anorad follows in second position, trailed by Baldor, Aerotech, Etel and Bosch Rexroth.