Spanish machine tool builder Unamuno has nearly 50 years experience in manufacturing automatic lathes used in a variety of industrial sectors including the automotive industry, electrical products manufacture, medical orthopaedics and the sub-contract trade. To adapt to new market demands Unamuno decided to enter the world of multi-axis CNC and in 2003 started work on the design of a numerically controlled lathe specially designed for bar-turning.
The twin-spindle multi-axis turning centre requires seven axes of linear feedback and linear encoders are specified for position sensing. In previous designs glass based linear encoders had been used but were known to have problems with contamination reaching the encoder's moving parts and proving the cause of a potential failure mode. Unamuno turned to Newall Measurement Systems and to the company's latest design of inductive linear encoder, which has no internal moving parts, as a solution to the traditional problems of space limitations and contamination associated with glass linear encoders. “Unamuno is a family business which has been making machine tools for nearly 50 years. We are proud of our reputation for producing quality machine tools”, said Philip Wilkinson, project manager with the company. “This is a newly developed machine which is still under evaluation. A rigorous testing programme is under way and so far tests have been very good. The main gain is the lack of problems due to contamination”, said Wilkinson. “Normal coolant pressure (on CNC machines) is 10 bar. We have done the machining tests at 50 bar and we have not had a single problem with contamination of the encoders”.
The MHG family of linear encoders provides an industry standard differential quadrature output at RS422 TTL levels. The device is very compact with a scale diameter of just 5.75 mm and is compatible with the majority of digital readouts and controls and features IP67 sealing as standard so contamination is not an issue. Newall supplied samples for evaluation and these were subsequently fitted onto development machines and subjected to a rigorous test programme. Approval of the MHG-TT linear encoders has since been issued as 'fit for application' and the encoders now feature on the new range of Unamuno machines.
Newall Incremental encoders provide quadrature square wave or sine-cosine feedback signals that allow for direct integration to servo driven applications. Newall encoders operate on the principle of electromagnetic induction, which means that all electrical and electronic components are sealed from any harsh working environments. Inducing a 10kHz sinusoidal current through a single drive coil within the reader head generates an electromagnetic field. This field interacts with the nickel chrome elements contained in the scale. A set of four pickup coils detect variations in the induced field that are then combined and processed by the electronic circuitry to generate a signal that varies as the head moves along the scale.
Depending on the position of the reader head as it passes over each element, the phase shift of this pickup signal relative to the drive signal will vary between 0 and 360 degrees. A high-speed digital-signal processor (DSP) converts the analogue signal to the differential quadrature signal and also generates the periodic reference marker pulse.
The design of these linear encoders allow installation in almost any position, unlike glass scale linear encoders, which usually need to be fitted with the lip seal facing downwards to prevent contamination. Optional self aligning fixing brackets allow virtually effortless scale mounting, needing only a single hole for each set of brackets.
Why choose inductive?
Encoders or position sensors can be broadly categorised into two families, DC operation or AC operation. In the former class lie optical and magnetic encoders both rotary and linear. Devices that use AC excitation are either inductive or capacitive. Examples of rotary inductive devices are resolvers and synchros whilst linear devices include LVDTs, Inductosyn and Newall encoders.
In AC systems, the signals containing the positional data are modulated AC signals at the fundamental operating frequency of the device. In DC systems, the signals are slowly varying DC levels.
DC signals are particularly subject to offset errors, drift and low frequency noise.
Offset errors can be countered by the use of techniques such as chopper stabilisation which, effectively, converts the signal to AC to eliminate the offset and then converts back. In AC systems, the nulling of offset errors is inherent in the AC coupling used and no complex techniques need be applied.
Drift is a problem in DC systems, particularly optical where the lamps, LEDs or solar cells are subject to long-term ageing. Inductive systems are inherently stable being based on fixed physical properties such as turns ratios and permeability of the encoder parts. These do not change with time.
Low frequency noise, particularly mains power frequencies, can interfere with DC signals and cannot be blocked without severely degrading the system's response time. AC systems, working at a precise, fixed frequency, will employ low and high frequency filters without impacting upon response speed.
A criticism often aimed at inductive encoders is that their relatively long pitch length requires a much larger interpolation level for a given resolution than for an optical grating. This is true, but it is not mentioned that accurate interpolation is much more easily achieved on AC systems than DC. The accuracies and resolutions that can be obtained from resolvers match those of their optical rotary counterparts.
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