The majority of sensors are capable of measuring the macrogeometrical features. Any kind of mechanical contact of a sensor with the rotating grinding wheel causes serious problems because the abrasives always tend to grind the material of the touching element. Only by realizing short touching pulses with small touching forces and by using a very hard tip material like tungsten carbide is it possible to achieve satisfactory results. Another group of sensors for the measurement of grinding wheels is based on pneumatic systems. Although this method is, in principle, unable to detect microgeometrical features of a grinding wheel due to the nozzle diameter of 1 mm or more, the method is able to determine macrogeometry. A distinction should be made between systems that employ a compressed air supply or those that do not. The latter responds to airflow around the rotating grinding wheel. The results obtained reveal a dependence of the airflow on the distance of the sensor to the surface, on the circumferential speed, and to some extent on the topography of the grinding wheel. The method with a compressed air supply is based on the nozzle — bounce plate principle, with the grinding wheel being the bounce plate. These systems are capable of measuring the distance changes related to radial wear with a resolution of 0.2 pm. This feature, comparatively easy setup, and moderate costs are the main reasons that pneumatic sensors have already found acceptance in industrial application.
Another possibility to register the macrogeometry of a grinding wheel was reported by Westkamper and Klyk [1993]. In high-speed ID grinding with CBN wheels, a spindle with active magnetic bearings (AMB) was used to achieve the necessary circumferential speed of 200 m/s with small-diameter wheels. These spindles have the opportunity to shift the rotor from rotation around the geometrical center axis to the main axis of inertia to compensate any unbalance. It is necessary to use balancing planes especially if electroplated CBN wheels are used without the possibility of dressing. To measure the runout of these, grinding wheels on the abrasive layer at the very high circumferential speed capacitive sensors have shown the best performance.
Also AE can be used to determine the macrogeometry of the grinding wheel. Oliveira, Dornfeld, and Winter [1994] proposed a system consisting of a single-point diamond dresser equipped with an AE sensor to detect the position of the grinding wheel surface. AE signals can be obtained without physical contact of the dresser and the wheel due to turbulence. In total, three different contact conditions can be distinguished including noncontact, elastic contact, and brittle contact.
Another principle used to determine radial wheel wear is based on a miniature radar sensor [Westkamper and Hoffmeister 1997]. The radar technique is well known from speed as well as traffic control with a maximum accuracy in the centimeter range. The sensor used for grinding works on an interferometric principle. With an emitting frequency of 94 GHz and a wavelength of X = 3.18 mm, this sensor has a measuring range of 1 mm and a resolution of 1 pm. Main advantages are the robustness against any dust, mist, or coolant particles and the possibility to measure on any solid surface. The sensor was used in surface grinding of turbine blades with continuous dressing (CD). A control loop was established to detect and control the radial wear of the grinding wheel, taking into account the infeed of the dressing wheel.