Capacitive sensor systems

Crankshaft bearings manufactured with a linear capacitive sensor system in the automotive industry (Kunzmann et al. 2005)... 

W. Bracke, P. Merken, R. Puers, C. Van Hoof, Ultra-low-power interface chip for autonomous capacitive sensor systems, IEEE Transactions on Circuits and Systems I Regular Papers 54 (2007) 130-140. 

The features of the monoHthic integrated sensor systems have not yet been fully exploited. The almost linear relationship between input reference voltage and microhotplate temperature renders the systems suitable for applying any temperature modulation protocol. Due their compatibility with other CMOS-based chemical sensors the microhotplates can be also combined with, e.g., polymer-based mass sensitive, calorimetric or capacitive sensors. The co-integration with such sensors can help to alleviate problems resulting from cross-sensitivities of tin-oxide based sensors to, e.g., volatile compounds such as hydrocarbons. A well-known problem is the crosssensitivity of tin oxide to humidity or ethanol. The co-integration of a capacitive sensor, which does not show any sensitivity to CO, could help to independently assess humidity changes. 

An ECT system is composed of three basic components (1) a capacitance sensor, (2) a data acquisition system, and (3) a computer system for reconstruction and viewing. Figure 1 is a sketch of the ECT system with all three components (Warsito and Fan, 2003). The capacitance sensor is made of nr capacitance electrodes distributed around the wall of the process vessel. The ne capacitance electrodes provide up to ne(ne—1)/2 combinations of independent capacitance measurements between the electrode pairs. The capacitance measurements are related to the local dielectric constant (permittivity) filling the process vessel between electrode pairs (Figure 2) (Warsito and Fan, 2001b). The relation between the electric potential and the permittivity distributions follows Poisson equation shown in Equation (1). 

Figure 6.178 Leakage detection system for flange connections, pump seals or container walls (max. four sensor heads). The capacitive sensor head (normally exposed to air) shows an increased capacitance when plunged in a liquid with a relative dielectric constant sr > 1. (For values of s see Table 6.43.) This causes a frequency shift in an oscillator circuit and hereby generates an alarm.

Pry, W.C. Stagner, W.C. Wichman, K.C. Computer-interfaced capacitive sensor for monitoring the granulation process 2 system response to process variables. Pharm. Tech. 1987,10, 30-41. 

Capacitive sensors are one option for detecting the electrolyte level in the system. They have the ability to detect nonmetallic materials such as electrolyte solution and are available in either normal-ly-open (NO) or normally-closed (NC) circuit configuration. 

The AFM scanners are calibrated by imaging a grating of defined spacings (Fig. 2.28a). Closed loop systems require only an occasional check of the calibration of the capacitive sensors open loop scanners, however, require, in particular for the x, y calibration, a regular (monthly) check and calibration. For a typical AFM system, the deviation should not exceed 5% otherwise a calibration is needed. Based on the images acquired on the gratings, automated calibration routines correct altered x, y, and z sensitivities, scanner bow effect (Fig. 2.28b), etc. 

In addition to reaction chambers and delivery systems, a number of supervising and sensor systems are of utmost importance for control and safety reasons. Sensors in automated workstations include measurement of temperature (thermocouple, thermistor, semiconductor), pressure, liquid flow and gas or liquid level. To monitor the presence or absence of vessels or devices, systems like capacitance, inductivity, ultrasonic monitors, magnetic sensors or optical sensors (reflective, beam interruption, color) can be integrated in automated workstations. 

Fig. 7.2.3 Block schematic of a capacitive yaw-rate sensor system 1, sensor element 2, drive control unit 3, capacitance to voltage converter 4, synchronus demodulator 5, amplifier 6, low pass filter 7, signal conditioner for the reference phase 8, control unit...

The frequency response of the system is a high-pass filter, since, for tor 1, Vo(ift))/X] (io)) = E/xo, which is a constant. However, the response drops off for low frequencies, and it is zero when at = 0. This frequency response is sufficient for a microphone that does not measure sound pressures at frequencies below 20 Hz. The input impedance of the read-out device must be high (10 MS2 or higher) in order to achieve a required low-frequency bandwidth. Capacitance sensors are not suitable for measuring most physiological signals because the frequency spectra of these signals have dominant low-frequency components. 

In the example just cited, the photoelectric signal is capacitance coupled (AC coupled) to the metal electrode. The system possesses no interfacing problem. However, a number of investigators have observed that no DC photoelectric current can be observed from BR-coated metal electrode sensor systems. In... 

Endres H. E., Hartinger R., Schwaiger M., Gmelch G., and Roth M., A capacitive COj sensor system with suppression of the humidity interference, Sens. Actuators B, 57, 83-87, 1999. 

It is a steel carriage which holds the test gear. It will integrate several parts as the fixing system for the micro gear, the pressure system which will assure the tooth contact without backlash among both gears teeth once the nominal centres position is reached, and a capacitive sensor. 

Fixed carriage Test gear Fixing system Capacitive sensor Fiexibie fixture... 

There are different technologies available for the development of tactile sensors such as optical [1], capacitive [2], resistive [3] and micro electro mechanical systems (MEMS) [4] technologies. Each suffers from a limitation, for example capacitive sensors are... 

Polliack et al. recently compared the accuracy, hysteresis, drift, and effect of curvature on sensor performance for these commercial prosthetic systems (force sensitive resistors F-Socket Measurement System, Tekscan, Inc., Boston, Mass. and Socket Fitting System, Rincoe, Golden, Colo prototype capacitive sensor. Novel Electronic, Minneapolis, Minn.). These authors concluded that the current systems were more appropriate for static use because the hysteresis, drift, and large standard deviations become more problematic during dynamic and long-term loading. 

Impedimetric measurements are inherently nonselective, but the modification of electrode surfaces with selective recognition agents has resulted in selective sensing devices based on impedimetric transducers. Reviews of immunosensors and cell-based biosensors include sections on impedance transduction [177, 250, and references therein]. A review of bioelec-tronic noses, devices intended to mimic the human olfactory system in detecting and identifying odors, includes interdigital structures for complex impedance measurements and arrays of capacitive sensor elements [251]. 

Kummer AM, Hierlemann A, Baltes H (2004) Tuning sensitivity and selectivity of complementary metal oxide semiconductor based capacitive chemical microsensors. Anal Chem 76 2470-2477 Kiunmer AM, Burg TP, Hierlemann A (2006) Transient signal analysis using complementary metal oxide semiconductor capacitive chemical microsensors. Anal Chem 78 279-290 Kurzawski P, Hagleitner C, Hierlemann A (2006) Detection and discrimination capabilities of a multitransducer single-chip gas sensor system. Anal Chem 78 6910-6920... 

Endres HE, Hartinger R, Schwaiger M, Gmelch G, Roth M (1999) A capacitive CO sensor system with suppression of the hunudity interference. Sens Actuators B 57(l-3) 83-87 Erdamar O, BUen B, Skarlatos Y, Aktas G, Ind MN (2007) Effects of humidity and acetone on the optical and electrical properties of porous silicon nanostructures. Physica Status Sohdi C 4 601-603 Fiiijes P, KovScs A, Diicso Cs, Adam M, Muller B, Mescheder U (2003) Porous sihcon-based humidity sensor with interdigital electrodes and internal heaters. Sens Actuators B 95 140-144 Goeders KM, Colton JS, Bottomley LA (2008) Microcantilevers sensing chemical interactions via mechanical motion. Chem Rev 108 522-542... 

Prakash, S. B., Abshire, P., Urdaneta, M. and Smela, E. (2005) A CMOS capacitance sensor for cell adhesion characterization, IEEE International Symposium Circuits and Systems (ISCAS05), 23-26 May 2005, Kobe, Japan, 4, 3495-8. 

---