Absolute encoders for linear motor drives

Absolute encoder technology provides accurate solution to linear motor drives on camshaft grinder.

Working in conjunction with a linear motor to power the workhead slide of its latest twin-spindle CNC camshaft grinding machine, Landis Lund of Keighley has selected Heidenhain absolute encoder technology to accommodate the higher positioning speeds of the workhead and angular relationship requirements of the wheel position to the rotation of the shaft.

It is claimed that the application of linear motor drives in such a machining process is a first and the use of Heidenhain LC181 linear and RCN723 absolute angle encoders is confirmed by John Pickles Landis Lund's Manager Electronic Systems: "They are, as far as I know, the only devices capable of providing the positioning information in a linear motor application such as this.

Heidenhain encoders are used by us on other applications due to their quality and reliability but on the camshaft grinder, this is very much leading edge technology!" Landis Lund, once part of Litton Industries and currently a subsidiary of the American group Unova, employs around 350 people in a 190,000 sq ft facility at Cross Hills, Keighley, West Yorkshire.

Here it manufactures high precision, high production grinding machines, principally for European customers such as Ford, Daimler Chrysler, General Motors, Fiat, VW, Renault and BMW, as well as certain customers in the Far East.

Its grinding machines range from single wheel to complex multi-wheel dedicated and flexible CNC grinding systems.

Although some relatively standard machines are supplied into the toolroom market, the bulk of production is specifically built to meet customer requirements in terms of tolerances and production levels.

A machine will often spend its working life dedicated to manufacturing a single component or closely similar family of parts which means flexibility is therefore less of an issue.

However, speed, reliability and accuracy are major criteria.

The demand for greater output and productivity by the automotive sector is also coupled with an increasing requirement for flexibility.

Fifteen years ago, bringing a new variant of camshaft into production was a laborious and error-prone process involving the manufacture, almost by hand, of master templates which typically took six months or more.

A changeover of camshaft type could easily absorb an entire shift on the grinding machine, while using the latest technology is a simple matter of keying in the appropriate digits into a CNC control.

Says John Pickles: "Our latest control systems are PC-based comprising a pair of PCs set back to back.

One PC provides the 'Man Machine Interface' while the other, acts as the NC control linked to the machine drives via a SERCOS (serial real-time communication system) fibre optic ring.

This means the absolute encoder in our view is mandatory." Absolute encoders are able to provide the absolute position of a slideway (linear) or shaft (rotating) immediately after switch-on or a disturbance without any prior movement of the axis.

The encoder photoelectrically scans coded patterns on a glass graduated disk to obtain the position.

When a linear encoder is used on a linear motor it supplies the actual value both for the positional and velocity controller and this forms the basis for the servo characteristics of the linear drive.

They have low deviation of position during acceleration, have high tolerance to acceleration and vibration in the lateral direction and also provide high quality sinusoidal incremental signals.

On an absolute angle encoder the integral stator coupling ensures higher system accuracy and during acceleration of the shaft, the coupling takes up only the torque resulting from bearing friction.

Signals from the absolute encoder are interfaced via the bi-directional Heidenhain EnDat interface which allows information such as operating parameters, encoder parameters and the machine builder parameters to be transmitted.

EnDat also supports monitoring and diagnostic functions before and during operation.

When grinding a camshaft there are two principal motions involved.

The cam profile is specified from a table of polar co-ordinates comprising the radial dimension from its base circle, and the angle.

As the camshaft is rotated in the workhead, the wheelhead slide is moved in and out according to the angle and radial co-ordinate.

Depending on customer and application of the camshaft, the overall accuracy required may be as fine as 0.0025mm.

To achieve this the Landis Lund method creates the wheelhead motion from a linear motor with hydrostatic slideways rather than the traditional ballscrew and ways.

This means with no metal to metal contact on the slideways, inaccuracies from wear and 'stick-slip' are eliminated, although thermal effects still have to be taken into account.

In conjunction with the linear motor, the Heidenhain LC181 absolute linear encoder monitors the relative position of the slide and feeds the data back to the control.

The encoder uses the Heidenhain DIADUR glass scale with seven tracks of different grating periods and measuring steps between 1.0 and 0.1microns which have defined thermal behaviour.

It will handle traversing speeds up to 120m/min.

Drive to the workhead C-axis at 500revs/min is via a frameless motor which is essentially a linear motor rolled up into a cylinder.

Again, an absolute device is required to commutate the motion and, for this, Landis Lund selected the Heidenhain RCN 723 angle encoder with integrated stator coupling to provide the high accuracy angular positioning.

The choice was further influenced by the 60 mm hollow shaft construction of the RCN 723, allowing hydraulic pipes to be run through to the chuck which simplifies assembly and improves machine presentation and makes it easier to keep clean.

According to John Pickles: "In terms of development, machining tolerances achieved are probably about as fine as any customer is likely to require, but there is a relentless pressure for increased production rates as carmakers seek to get more parts for a given floor space." John Pickles observes that the linear motors coupled with the Heidenhain encoders allow higher workspeeds without compromising accuracy and repeatability.

He points out that generally, these components will probably not be gauged after machining, so any undetected variability would be costly in final assembly.

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