Basic Principle
The transduction principle is resistance variation with the position. A resistive film is applied to the exterior of a cylinder made of a dielectric material. Initially, the piston itself provides a capacitive coupling to the variable resistance, hence eliminating the direct sliding contact that makes potentiometers so unreliable. In reality, there will be a small signal pickup head attached to the piston, to avoid modifying the piston and simply the assemble process. Following figure shows the mechanical nature of the construction and equivalent electrical circuit. As a consequence of the construction principle this sensor has been dubbed the “Capacitively Coupled Position Transducer” or CCPT. The pneumatic applications are particularly appealing. Figure above shows a pneumatic actuator that has been “sensorized” with the concept which would lead to an increase in cost of only pennies per unit. In tests conducted at the Fluid Power and Motion Control Center at Georgia Tech, such units have been cycled over a million times without loss of accuracy which can be less that 1% . In addition to the excellent accuracy and longevity, the measurement range is upwards of 80% of the length of the device, a distinct advantage over the LVDT. Temperature variations are also of minimal consequence.
Researcher H. Zhu produced this sensor in his Ph.D. thesis research advised by Wayne Book, HUSCO/Ramirez Professor of Fluid Power and Motion Control, out of the necessity of position sensing in a dense array of actuators such as shown in the figure at the right side of this page. The project, supported by the National Science Foundation called “Digital Clay”,* has produced a new human/computer interface for shape input and display. The prototypes that have been built required a high density miniature actuator array operating at low pressure with feedback control. The number of actuators in this application could be in the thousands, but currently a 5x5 array has been constructed to demonstrate the feasibility of the overall operation and includes sensing and feedback control of the actuator position. In this application the cylinders are glass with chemical vapor deposition of the resistive film and the pistons are graphite.
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CCPT Sensor Embedded Actuator |
Signal Processing
Signal processing can be added later if position sensing is initially needed for a given application. Both sinusoidal excitation (e.g., Linear Variable Deferential Transformer (LVDT) signal conditioner) and Square-wave excitation can be used to energize the sensor. Related signal condition technologies can be found in the section of "CCPT sensor"
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| LVDT Type Signal Conditioner [Enlarge] |
Square-wave Signal Conditioner [Enlarge] |
This Square-wave excited signal condition technique delivers the following advantages when compared to traditional LVDT signal conditioning:
- Significant reduction in the costs of the processing unit
- Simplified signal conditioning circuit
- Increased range of working bandwidth
- Reduction in the susceptibility of signal to noise
While there are numerous research issues that must be addressed to make this sensing strategy applicable to the common applications of fluid power, the current stage of the technology is directly applicable to pneumatic and low pressure hydraulic applications. Students at Georgia Tech have created an award winning startup company, Crossing Point Technologies, to develop the idea commercially. Meanwhile, researchers at the Fluid Power and Motion Control Center envision ubiquitous sensors of this type in the factory and plan to study the massive deployment using energy harvesting and wireless communication.
For more details, please refer to the Related Publications listed below.
5x5 Cell Array prototype for 
![[Enlarge]](../Sensor/LVDT_SC.jpg){kind=link}
![[Enlarge]](../Sensor/SQSC_SC.jpg){kind=link}