Development of sensing techniques and sensing strategies to realize electric and magnetic field based miniaturized angle sensors

Most of the sensor systems used in sectors such as robotics, automotive, process, and aerospace require high-performing, miniaturized angle sensors. Parameters such as accuracy, sensitivity, resolution, range, and low power consumption are important for these sensors, depending on the application. In addition, they prefer to have all or some of the additional specifications or features such as (a) compactness, (b) electrical stability against vibrations and mechanical misalignments, (c) tolerance to wear and tear, (d) simplicity in manufacturing, (e) multiple sensing functions, (f) ability to perform normally even if the environment is harsh and (g) less expensive. Numerous angle sensors based on various measurement mechanisms have been reported, but most of them have drawbacks such as bulkiness, sensitivity to misalignments, and high-power consumption. Therefore, it is necessary to develop sensing methods and measurement circuits that will enable the construction of sensors, accounting for the aforementioned parameters without any performance degradation. The development of new and/or improved magnetic and electric field-based miniaturized angular position sensing techniques, coupled with efficient measurement electronics, are the main focus of the work reported in this thesis. 

The sensors based on the magnetic/inductive principle provide superior performance under harsh environments. Although the highly sensitive variable reluctance type angle sensors are available, they are relatively bulky and have noticeable axial thickness. The magnetic parts employed are also not easy to manufacture. Planar coils, made on rigid or flexible printed circuit boards, are compact and offer a cost-effective manufacturing solution that is scalable. It will be advantageous if the variable reluctance and planar coil-based approaches can be combined, retaining the high sensitivity of the variable reluctance approach and compactness of the planar coil units. In the first work reported in the thesis, a thin variable reluctance angular position sensor, based on planar coils, whose output is insensitive to the axial misalignment of the shaft was realized. The functionality and performance of the proposed sensor were validated using simulation and by testing a prototype developed. The proposed sensor is thin compared with most of the other commercially available optical and magnetic angle sensors reported. The prototype sensor gave accurate results up to 50 rpm, and the accuracy is deteriorated for higher speeds due to the low update rate of the measurement system. Later, in order to extend the rotational speed sensing range, two inductance measurement circuits were developed; (1) based on differential measurement, and (2) ratiometric measurement. The proposed circuits measure the differential or ratiometric inductances of the sensing coils directly from the sensor rather than measuring the individual inductances of the sensing coils separately. As a result, the sensor performs better, and measurement speed is improved by ten times. 

Eddy current sensors are considered as a potential option due to their high resolution, reliability, and endurance in harsh and hostile environments. In this thesis, a novel eddy current-based angle sensor has been developed, and the details are presented. The compactness has been achieved by using the existing rotating shaft as the sensing element. The modification to the shaft is minimal; a small surface groove is introduced without affecting the mechanical strength.

A sensor prototype was built and tested. The proposed eddy current sensor is thin, easy to manufacture at low cost, tolerant to axial vibration by design, and has a 360o sensing range. In addition, a novel, thin, non-contact eddy current-based linear displacement sensor has been developed and presented. The moving part of the sensor is a conductive sheet with a simple surface groove. The displacement of the moving part is determined by the change in the inductance of four identical stationary planar coils kept underneath the moving part. This sensor is well suited for applications where the vertical space available for installation is limited. A sensor that can simultaneously measure both the linear and angular position of the shaft for a specified range is valuable in industrial machinery and robotics. The deployment of two different independent sensors in a system requires more space, in addition, in most of the cases, sensors available for linear displacement cannot accommodate the rotational movement of the object whose position is being sensed. The same is applicable to angle sensors when the object undergoes linear movement. A noncontact capacitive sensor suitable to measure both angular and linear displacements is proposed next. Since the measurement of angular displacement is entirely independent of that of linear displacement, there is less chance of error propagation, and computation is made simpler.

The angle sensors presented in this thesis lead to significant advancements in one or more features, such as good accuracy (< 0.9%), less axial or radial thickness (< 5 mm), insensitivity to axial or radial misalignments, ease of manufacture, low power consumption (25 mW), multiple parameters sensing capability, and affordable production. All the sensors and associated systems developed and reported in the thesis are experimentally tested and compared against the existing schemes showing quantitative and qualitative enhancement.