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Velocity Sensor The Output Speed Signal

Sinocera Piezotronics, INC | Updated: Oct 17, 2017

Velocity Sensor The output speed signal

Basic speed sensor type

In general there are two types of speed sensors: AC response speed sensor DC response speed sensor

As the AC response of the speed sensor, as its name, its output is AC-coupled. Such sensors can not be used to test static speeds, such as gravity speed and centrifugal speed. They are only suitable for measuring dynamic events. The DC response of the speed sensor, with DC coupling output, can respond to the speed signal as low as 0 Hz. So the DC response of the speed sensor is suitable for simultaneous testing of static and dynamic speeds. It is not only the speed sensor that only needs to test the static speed to select the DC response.

Speed, speed, displacement

Many studies of vibration require information on speed, velocity and displacement, which are important information that engineers need to design and validate the structure. In general, the speed provides a good reference, while the speed and displacement are the variables required for the calculation. In order to calculate the speed and displacement from the speed, the speed signal output from the sensor will be done by digital or analog, respectively, and quadratic integration. This may cause the AC-coupled sensor to cause problems. To demonstrate this problem, it is assumed that an AC sensor is used to measure a wide pulse half sine wave signal. Due to the inherent AC RC time constant, the sensor output can not be well matched with the input pulse. For the same reason, at the end of the pulse, the sensor output will produce a negative zero offset. The following figure shows the relationship between the output of the sensor (red curve) and wide pulse half sine speed input (blue curve).

This seemingly slight difference in magnitude will produce a significant error after the integration. DC response of the speed sensor is not such a problem, because its output can accurately follow the slow changes in the input. In actual daily applications, the input signal may not be a simple half-sine pulse, but with AC-coupled sensors to test any slow-changing signal when such a problem will always exist.

Now we look at a variety of commonly used speed sensor technology.

AC response speed sensor

The most commonly used AC response speed sensor is the use of piezoelectric components as its sensitive unit. When the speed is input, the detection mass in the sensor "moves" causes the piezoelectric element to generate a charge signal proportional to the input speed. From the electrical point of view, the piezoelectric components as an active capacitor, the internal resistance in the 10x9 ohm level. The RC time constant is determined by the internal resistance and capacitance, which also determines the high frequency pass characteristics of the sensor. For this reason, the piezoelectric speed sensor can not be used to measure static events. Piezoelectric elements can come from nature or man-made. They have different signal conversion efficiency and linearity. There are two kinds of piezoelectric speed sensor on the market - charge output type, voltage output type.