Stabilising giant TVs

A piezo electric actuator available from Unimatic Engineers has resolved the issue of poor image quality on projection-type televisions and thus stands to open up substantial new markets in the domestic consumer, leisure and entertainment and public information sectors.

Projection TVs typically have a far larger screen than their traditional counterparts. But to date their uptake has been restricted because they tend to suffer from poor image resolution and unpleasant digital optical effects.

Developed in America, and soon to be available in Europe, the new rear projection TV is built on  the expertise of several companies. With Carl Zeiss providing its renowned optical components and Thomson the electronics, it was realised that the image degradation was caused by mechanical problems – the tiniest vibrations would desynchronise the beams that create the colour image on the screen.

Analysis of some prototype models showed that vibrations were microscopic and often very high frequency. These couldn’t be damped out, so a dynamic solution had to be identified.

Martin Stevens, MD of Unimatic, takes up the story: 'The vibrations’ frequency and wavelength were in the same range as exploited by piezo technology and an Elliptec Resonant Actuator was soon identified as offering the best solution.'

Piezo actuators are made from a laminate of two dissimilar ceramic materials, which undergo an atomic realignment, which are subjected to a microvoltage. This has the effect of expanding the actuator by a precise amount; the expansion is reversed upon application of a second microvoltage.

'In the Elliptec resonant actuator, a chopper circuit applies a constant stream of microvoltage bursts, which set up a precision back and forth actuation motion. In theory then it was only a matter of tuning the actuator’s output to the resonant vibration in the TV and the problem was solved.

'In fact a second issue is also addressed: to stabilise the image completely, it is projected as two vertically-interlaced half-images. These are generated alternately and the piezo actuator oscillates a mirror at the same frequency so that they are separated and positioned accurately with respect to each other.

'Because the piezo was reacting to the vibration and also merging the two half-images it took months of calculations, modelling and experimentation to get right. But there is now a body of knowledge that will be suitable for all future similar development work.'