Encoder improves interference immunity

Edited by the Engineeringtalk editorial team Jul 23, 2007

Sick's VFS60 high-resolution incremental hollow shaft encoder offers excellent concentricity and exceptional robustness.

A new encoder technology, specifically designed to cope with the idiosyncrasies of AC motor mounting, is now available from Sick (UK) .

With a larger bearing spacing, an isolated shaft and a metal encoder disc, the new VFS60 will set new benchmarks for encoder performance in a great many industrial applications.

The VFS60 is a high-resolution incremental hollow shaft encoder in a 60mm housing.

Excellent concentricity and exceptional robustness are achieved thanks to the large spacing between the ball bearings, which support the encoder shaft and, with electrical isolation between motor shaft and encoder, the VFS60 substantially improves interference immunity and reliability when mounting on AC motors.

Most incremental encoders used on AC induction motors today were originally designed as standalone units for use on machinery, with small mechanical modifications to enable mounting to AC induction motors.

Often, issues such as run-out, bearing currents and extremes of temperature found in motor applications were not considered.

All incremental encoders have two shaft bearings and in most encoder designs these are positioned very close together.

This makes it very difficult to produce an encoder without run-out.

While this type of construction is suitable for stand-alone encoders, motor feedback encoders require an altogether different construction as any run-out will be seen as ripple on the speed feedback.

For this reason the bearings in the VFS60 have a large separation to ensure that run-out is kept to a minimum.

Another feature of most incremental encoder designs is the fact that the bearings of the encoder are not isolated.

As a result, it is possible for stray currents in the shaft of the motor to pass through the bearings of the encoder and down the encoder ground connection.

Over a period of time, this causes the encoder bearings to become rough and pitted, leading to eventual encoder failure as the bearings lose their integrity and the disc inside the encoder starts to make contact with the pick-up electronics.

When used as a stand-alone encoder on a machine, this is rarely a problem, as the drive shaft does not have stray currents flowing through it.

However, when used as a feedback system for an AC induction motor this weakness often becomes apparent.

The VFS60 has an isolated shaft to prevent bearing currents and guarantee reliable long life operation.

The final feature, which sets the VFS60 apart from its peers, is its metal encoder disc.

In conventional encoder designs, either a glass or plastic disc is used.

The traditional glass disc enables the creation of highly accurate signals and is stable at extremes of temperature.

However, glass is fragile and therefore limits the application areas of the motor and encoder unit.

In an attempt to overcome this weakness some encoder manufacturers have turned to plastic.

Encoders using plastic discs have the benefit of being more robust than their glass disc rivals but cannot achieve the same levels of accuracy.

In addition, they have a restricted temperature range, as the plastic tends to deform at higher temperatures.

Using a new pick-up system incorporating a metal code disc, the VFS60 produces a signal quality as good as a glass disc encoder but with the robustness of an encoder with a plastic disc.

In addition the VFS60 has an industry leading operating temperature range of -20 to 100C.

The new VFS60 incremental motor feedback encoder for AC Induction motors offers users all the features needed for key applications including multiple through hollow shaft mounting options (10, 12, 14 and 15mm).

From within its IP65-rated enclosure it offers a high level of dynamic performance, with up to 10,000 lines, minimal run-out; electrical isolation of the shaft; a high operating-temperature range; and a robust construction.

The Sick Stegmann VFS60 also meets the requirements of RoHS and is CE and UL approved.