Hall Sensors

A Hall sensor uses the Hall effect to measurement the distance between the sensor and a magnetic object. When the position changes, the magnetic flux picked up by the sensor changes, which in turn modifies the output voltage of the sensor.

In the first stage experiment, we have used the sensors produced by Allegro http://www.allegromicro.com/en/index.asp. In particular, we tested the A1322 model. It has a good linearity and wide detection band. The problem we find with such sensor is that the undesired coupling between the coil and the sensor. Basically, the sensor not only picks up the field changes induced by the magnet and also the one created by the coil. In certain frequency, those two signals destructively interfere with each other and create a dip in the open-loop transfer function of the feedback system, as shown by the figure below. We have added more details in the 40m elog.

attachment:contribution_plot.png

In this figure, the green dots denote the measurement data of the open-loop transfer function of the feedback control loop. The Black curve is the simulink model of the system with magenta and blue curves denoting the contribution from mechanical and hall sensor respectively.

Inductive sensor

A typical inductive sensor is the eddy-current sensor. The sensor creates alternating magnetic field and it induces eddy currents inside the conductive target, which in turn modulates the magnetic field and it gets detected by the sensor.

attachment:inductive_sensor.PNG

This figure is from the webpage of lion precision http://www.lionprecision.com/inductive-sensors/index.html, which produces such kink of sensor.

The limitation of such sensor is that it requires the sensing objects to be conductive. In addition, since it is sensitive to the change in the ambient magnetic field, such device would has a similar problem as the hall sensor, due to the coupling between the sensor and the control coil.

Capacitive Displacement Sensors

OSEMs

Michelson