Blum-Novotest introduces 25 mm digital/analogue touch-probe

Previously, touch-probes in general have relayed a digital signal to the machine control, which by 'reading' the position of the axes, can detect the exact position of the measured point in the machine tool. The design of Blum TC76-Digilog is based on the principle that, if it is possible to measure the start of the shadowing of the light barrier, it must also be possible to analyse the percentage increase in the shadowing that occurs when the stylus is deflected ever further. This enables an analogue signal to be generated that greatly extends the range of application of measurement in the machining centre and can, as Blum points out, also save a great deal of time. Digital measurements naturally continue to be possible. Due to the leverage ratios between the inside of the touch-probe and each stylus used, usable analogue measurement ranges of 475 µ for a 30 mm long stylus and up to 1,175 µ for a 100 mm long stylus are possible. The signal is routed via either a built-in or mounted base into which the touch-probe is screwed, and a cable up to 0.5 m long to an A/D converter. From this converter even relatively long distances can be bridged to the Blum D/A converter. Its output signal is a typical 0-10 volt signal that must continue to be processed by many controls. There are two reasons for this somewhat unusual signal path: analogue transmissions are relatively susceptible to cross feeds and interference caused by electrical emissions, which is why the first part of the cable is kept as short as possible. The following digital section is less susceptible. However, no standard transmission protocol transmits the 50,000 readings per second generated by the TC76-Digilog, so on the transmission section to the switch cabinet a special Blum protocol is used. Analogue measurement always has advantages when surfaces or lines are being assessed – for example, when a workpiece surface has to be checked for machining errors. When a switching digital probe is used in such cases a very large number of points has to be measured to achieve an adequate resolution. An analogue probe, on the other hand, can be moved in 'scanning' mode over the surface and record more points in a fraction of the time. Speeds of up to 2 m/min are possible with outstanding accuracy, says Blum, adding that under test, probes were even moved at 5 m/min, with measurement accuracy remaining very good. However, for the most part, such extreme speeds are not necessary. Blum points out that previously in one application it took 30 sec to measure 40 points; with the TC76-Digilog the measurement time on the same surface was reduced to 2 to 3 sec, with 100,000 recorded readings with higher precision. When mechanically connected, the Blum TC76-Digilog touch-probe is compatible with typical 25 mm touch-probes with a M16 fastening screw thread, enabling digital measurement to continue without restriction.