ATOM encoder enables optimisation of 5-axis measuring head

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Renishaw's innovative ATOM miniature encoder has enabled the company to develop its second-generation REVO multi-sensor system that makes it possible for CMM users to perform five-axis measurements on a three-axis co-ordinate measuring machine (CMM).

ATOM encoder enables optimisation of 5-axis measuring headThe REVO multi-sensor system measures thousands of points per second and operates at speeds up to 500mm/s. As the head is much lighter and more dynamic than the CMM, changes in the part geometry can be followed quickly without introducing harmful dynamic errors.

Renishaw's REVO-2 is an improved version of the five-axis measuring head; the REVO-2 and its new CMM controller, the UCC S5, build on the successful REVO multi-sensor system with enhanced power and communications capability to carry the latest REVO sensors such as the RVP vision measurement probe. The head has an increased range of movement in the negative tilt axis, which can improve part access and reduce the complexity of stylus setups. Infinite positioning further ensures that RVP vision probes can be placed in front of features at any angle.

How the ATOM encoder helps

The REVO redesign has implications for the original onboard encoder, which was a custom design with a fine pitch 12um phase scale developed before highly accurate miniature encoders had been brought to market. The REVO-2 (pictured right) incorporates Renishaw's latest ATOM incremental rotary encoder with RCDM rotary (angle) glass scales on both of its axes (yaw and pitch). It is the first product to have the ATOM encoder designed-in at the concept stage.

ATOM was launched in 2014 as the world's first miniature encoder with advanced filtering optics and features the lowest cyclic error in its class. Each 20um-pitch RCDM glass disc (68mm diameter) is face-read by a dual-readhead setup that helps to optimise the REVO-2's performance by eliminating rotational eccentricity error. The ATOM readheads are connected, via an analogue filter and analogue-to-digital converter (ADC), to the REVO-2's electronics. One of the reasons why the designers selected the ATOM was its advanced, highly-automated manufacturing processes that minimise process variability to assure the best quality and the shortest lead times.

In this application, ATOM offers leading-edge accuracy and speed that enable excellent servo-loop gain levels for outstanding position holding and accurate surface scanning of parts or components. The ATOM system was also chosen for several design features including:

  • Its relative mechanical simplicity and optical disc alignment method using a microscope camera system for improved installed accuracy.
  • Ease of setup in conjunction with the REVO-2's electronics, allowing both incremental signal calibration and auto-phasing of reference marks without oscilloscopes or external equipment.
  • Availability of a chrome-on-glass rotary scale, with a graduation accuracy that enables the REVO-2 to achieve a resolution of 0.002 arc seconds, delivering high precision over the full operating temperature range.

In all, the ATOM helps to streamline the REVO-2 manufacturing process, while still providing exceptional metrology performance.

How the ATOM encoder saves time and money

ATOM RCDM disc scales undergo acceptance testing before delivery, and they are available in a comprehensive range of size options for every type of application. This has enabled Renishaw to bulk-buy RCDM scales with absolute confidence in its manufacturing readiness level (MRL).

Installation during REVO-2 manufacturing has also been streamlined by well developed procedures for mounting the ATOM readhead, with mounting brackets and shims available as orderable parts.

ATOM systems provide two different methods for disc alignment, one electrical and the other optical, and the REVO-2's dual readhead set-up enables either type to be used. In this case, optical alignment was chosen to give a simple and highly repeatable way of setting disc runout - to help minimise process variability. This technique uses a microscope, connected to a camera, to monitor the movement of the alignment band as the disc is rotated. The disc is adjusted until the total alignment band movement is within design specifications. It had previously taken up to an hour or more to complete this operation and align and lock the custom readhead in place. Now, however, the ATOM readhead and scale can be mounted and aligned within a couple of minutes.

Another area of improvement has been in the setup calibration of the encoder once mounting and alignment have taken place. Prior to the REVO-2, the reference signal was output to an oscilloscope in order to phase the reference marks on the scale manually; this procedure was time-consuming and required a skilled operator. In contrast, the ATOM's set-up LEDs enable visual calibration and setting of encoder parameters in a way that is said to be unparalleled in its class. An optional diagnostic kit is also available to support set-up in situations where visual assessment of the readhead is difficult. The REVO-2 has integrated electronics for direct interfacing with the CAL line and signal output channels and effectively perform the same function as the ATOM diagnostic kit, via a LabVIEW program.

Richard Toller, Technical Manager for Renishaw's CMM Products Division, explains: "The ATOM encoder provides a level of plug-and-play convenience that simply did not exist before. The ease of installation and alignment, coupled with excellent technical support, allowed the REVO-2 design team to fully meet the design specification while reducing overall production cycle time."

Streamline your manufacturing process

Process optimisation may be the most important step in improving overall efficiency. The process engineer must ask: 'How does each part, each machine and each production task affect the entire process?'

ATOM is designed to support manufacturing and servicing operations with unrivalled technical support, streamlined installation and robust calibration procedures. The end result is reduced process cycle times, higher unit yields, greater efficiency and lower production costs. REVO and ATOM are leading products that are now combined in the powerful REVO‑2.

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