Encoder gives absolute accuracy

Developed for machine designers, machine tool builders and system integrators, a new absolute linear encoder has been launched that incorporates an AC inductive technology measuring system that relies on a ball-based design. The Spherosyn Absolute linear encoder, developed by Leicester-based firm Newall Measurement Systems, delivers fast and accurate measurements for CNC systems, pick-and-place machines and automotive transfer lines by reacquiring its true position without having to return to a datum point. The encoder is constructed from a stainless steel linear scale which contains a column of steel balls separated by scaled inserts. An aluminium-cast reader head, containing a coil assembly, supporting electronics (which relies on DSP technology to convert analogue signals to digital signals similar to telecommunications devices) and 16 sensors complete the new design. The linear encoder determines its absolute position by registering coded markers that are sandwiched between nickel-chrome steel balls (custom made in the US) housed in its stainless steel tube. This instantaneous measurement can be made after an emergency stop, a shift changeover or an overnight break, thus quickening the restart, reducing machine set-up times and improving the efficiency of the manufacturing process. Machines in production environments are also susceptible to small movements, especially when they are left overnight and cool down. As Mark Hudman, technical director at Newall, explains: “Machine operators don’t have to spend time referencing and setting up their machine at the start of their shift. The Spherosyn finds its true position immediately after power-up.” Based on Newall’s digital technology, the encoder is a practical and convenient measurement platform that offers resolutions to 0.1 microns and an accuracy range from +/- 10microns down to +/-3 microns. The encoder has a top traverse rate of 60m/s and is suitable for high-speed, repetitive movement applications or where machine datum operations are overly time consuming. Unlike most glass-scale encoders, the reader head of the Spherosyn Absolute is sealed to an IP67 rating, making it fully sealed against oil, dust, coolant and other workshop contaminants. The encoder will even continue to function when it is submerged in coolant and covered in swarf. There’s also no need for air purging and cleaning. Hudman continues: “The robust construction of the Spherosyn, with no fragile or easily damaged parts, means it can be mounted without fear of damage. Its high resistance to shock and vibration also means it can be fitted in almost any environment, no matter how demanding.” Hudman says the balls used in the encoder are not only machined to very precise dimensions, but also tested individually before being used. “These days, machine designers are asking for very accurate measurements to support their production processes. Some even require 1 micron accuracies – that’s 10 times finer than a flu virus!” According to Hudman, the Spherosyn has several advantages over glass-scale encoder designs. First, it has a non-contact design as the reader head and scale are separate parts. With glass, the reader head rides on a leaf spring carriage. Second, the Spherosyn’s component parts are all restrained by a potting compound restricting errors due to vibrations. Normally, glass designs have non-sealed electronics and are only sealed to IP53 (ie dust ingress and protected from sprays to 60 deg from the vertical). “We can also measure up to 12m continuous lengths, compared to glass designs that are typically limited to just 3m,” claims Hudman. According to Hudman, 90% of the UK market for encoders is currently dominated by standard incremental encoders, with linear versions down at just 5%.