1. Ferraris sensors for analyzing systems

Speed-controlled drives are subject to ever increasing demands with regard to dynamics, smooth running and disturbance resistance. To achieve this, vibration must be reduced as far as possible - also to prevent wear or undesirable side-effects such as the generation of noise and heat.

The acceleration is indispensable as a state-variable for precise analysis of the dynamic response of a drive system. This is because it represents the direct, undelayed response of a mass being moved in reaction to all the forces acting on it. If one assumes that in the typical drive control loop there is usually a position sensor implemented to detect the actual value, then it would – theoretically – be possible to calculate the acceleration by double-differentiation of the position signal. In practice, the signal derived in this way would be useless, since each differentiation exaggerates any errors present, and so a double differentiation would inevitably produce a very noisy signal. The situation is even more critical for highly dynamic systems. In this case, even a directly produced velocity signal, such as the output signal from a tachogenerator, is not suitable for the generation of a good acceleration signal – even the short sampling time that is required by the control system for a single differentiation will lead to sizeable quantization errors – quite apart from the amplification of any errors. In other words, for the analysis of highly dynamic systems, the acceleration must be measured directly.

Classic acceleration sensors on the spring-mass principle have inherent disadvantages. They measure the absolute acceleration not the relative acceleration, which may be relevant. For example, consider a handling robot, with the hand axis mounted on a rotary axis, and where it is necessary to sense the dynamics of the hand movement relative to the higher-level rotary axis. Furthermore, a spring-mass system is frequently sensitive to motion orthogonal to the measurement axis, so that the required measurement may be falsified. This effect can arise, for instance, on a machine-tool compound slide, where the top slide is moving in the X direction, while the cross slide is simultaneously moving in the Y direction. And where rotary movements are concerned, the application of absolute acceleration sensors is extremely complicated. The energy supply and signal transmission requires the use of slip-rings or contactless forms of transmission, such as rotary transformers or telemetry systems.

Considerable improvements in the analysis of drive systems can be achieved by using relative acceleration sensors based on the Ferraris principle, named after the Italian Galileo Ferraris. The principle is that permanent magnets mounted in a fixed detector unit induce eddy currents in a moving, conductive, but non-magnetic material. For measuring rotary acceleration this material can be in the form of a disk, for linear acceleration it is formed as a strip of metal (see picture on top). The eddy currents and the magnetic fields that they generate are proportional to the radial velocity of the disk (or the linear velocity of the strip). A change in the eddy current produces a voltage in the coils mounted in the detector unit that is proportional to the rate of change of the velocity, i.e. proportional to the acceleration. The reverse application of this principle has, incidentally, been used for a very long time in electricity consumption meters. The decisive factor is that the differentiation is not based on a sample over a discrete time period, but is a physical effect, so that the user sees a dynamic, low-noise acceleration signal.