文档介绍：A Low-Cost, Smart Capacitive Position Sensor
Abstract
A new high-performance, low-cost, capacitive position-measuring system is described. By using a highly linear oscillator, shielding and a three-signal approach, most of the errors are eliminated. The accuracy amounts to 1 μm over a 1 mm range. Since the output of the oscillator can directly be connected to a microcontroller, an A/D converter is not needed.
I. INTRODUCTION
This paper describes a novel high-performance, low-cost, capacitive displacement measuring system featuring:
1 mm measuring range,
1 μm accuracy,
s total measuring time.
Translated to the capacitive domain, the specifications correspond to:
a possible range of 1 pF;
only 50 fF of this range is used for the displacement transducer;
50 aF absolute capacitance-measuring uracy.
Meijer and Schrier [l] and more recently Van Drecht,Meijer, and De Jong [2] have proposed a displacement-measuring system, using a PSD (Position Sensitive Detector) as sensing element. Some disadvantages of using a PSD are the higher costs and the higher power consumption of the PSD and LED (Light-Emitting Diode) pared to the capacitive sensor elements described in this paper.
The signal processor uses the concepts presented in [2],but is adopted for the use of capacitive elements. By the extensive use of shielding, guarding and smart A/D conversion,the system is able bine a high accuracy with a very low cost-price. The transducer produces three-period-modulated signals which can be selected and directly read out by a microcontroller. The microcontroller,in return, calculates the displacement and can send this value to a puter (Fig. 1) or a display or drive an actuator.
Electronic Circuit
puter
Actuator
Display
Fig. 1. Block diagram of the system
Fig. 2. Perspective and dimensions of the electrode structure
Ⅱ. THE ELECTRODE STRUCTURE
The basic sensing element consists of two simple electrodes with capacitance Cx, (Fig. 2). The smaller one (E2) is s