Electrostatic Sensors – Their Principles and Applications

Over the past three decades electrostatic sensors have been proposed, developed and utilised for the continuous monitoring and measurement of a range of industrial processes, mechanical systems and clinical environments. Electrostatic sensors enjoy simplicity in structure, cost-effectiveness and suitability for a wide range of installation conditions. They either provide unique solutions to some measurement challenges or offer more cost-effective options to the more established sensors such as those based on acoustic, capacitive, optical and electromagnetic principles. The established or potential applications of electrostatic sensors appear wide ranging, but the underlining sensing principle and resultant system characteristics are very similar. This paper presents a comprehensive review of the electrostatic sensors and sensing systems that have been developed for the measurement and monitoring of a range of process variables and conditions. These include the flow measurement of pneumatically conveyed solids, measurement of particulate emissions, monitoring of fluidised beds, on-line particle sizing, burner flame monitoring, speed and radial vibration measurement of mechanical systems, and condition monitoring of power transmission belts, mechanical wear, and human activities. The fundamental sensing principles together with the advantages and limitations of electrostatic sensors for a given area of applications are also introduced. The technology readiness level for each area of applications is identified and commented. Trends and future development of electrostatic sensors, their signal conditioning electronics, signal processing methods as well as possible new applications are also discussed

In-line powder flow behaviour measured using electrostatic technology

Within solid-dose manufacturing processes, powder flow and powder triboelectrification are critical to the quality of the final product. Off-line testers do not simulate the shear and packing conditions that a powder would experience in-process and may be unreliable in predicting in-line flow and charging properties, which are key components to successful formulation and process design. In this work, a dual-electrode, electrostatic powder flow sensor (EPFS) was used to obtain electrostatic signals that were generated in response to the pattern of flow of pharmaceutical powders in two density modes: The first being powders in lean phase flow, generated by free-fall of the powder from the outlet of a screw-feeder. The second being dense phase flow, through either 19.1 mm Ii.Dd. stainless-steel pipe or at the outlet of a tablet-press hopper. Powders were selected from a range of low to high cohesivity so as to study the effect of powder cohesion on the flow pattern. Electrostatic signals were then analysed by three distinct signal processing methods (RMS signal averaging, cross correlation, and Fast-Fourier-Transform) with a view to determining certain characteristics of powder flow, i.e. mass flow rate; cohesivity; and triboelectrification. In the first application a calibration was attempted to establish the link between the root-mean-square (RMS) of the electrostatic signal and the mass flow, as determined by the accumulation of mass on a balance placed below the screw-feeder (in the case of lean phase application) and the 19.1 mm i.d. pipe (in the case of dense phase application). In both cases it proved unsuccessful, owing to the instability in the electrostatic signal (i.e. its dependence on factors other than mass flow, for example inherent and induced charge fluctuations and moisture content). An alternative method for determining mass flow rate was proposed based on the second signal processing method, which involved the cross-correlation of signal from both sensors to determine the free-fall velocity. This method might work in future applications if combined with a suitable technique for determining the powder density. In the second application, a Fast-Fourier-Transform (FFT) of the electrostatic signal to yield an FFT spectrum was used to establish whether this technique could determine aspects of powder cohesivity. A correlation in rank order of cohesivity was observed between the ratio of the summed or averaged amplitudes at the three principle frequencies to the summed or averaged of the baseline components respectively, and the cohesivity of the powders, as determined by off-line powder rheometry assessments of dynamic flow and bulk properties. In the third application, the RMS signal normalised to the powder mass flow rate was used to study the time-dependent powder charging behaviour, which is induced by the transportation of the powder within the screw feeder. Characteristic relative charging profiles were obtained for each powder, which in some cases were coupled to charge-induced adhesion of the powder to the equipment. In the last application, the RMS signal generated from the EPFS sensor located at the outlet of the hopper on a rotary tablet press was used to interrogate the dense-phase intermittent-flow resulting from the dosing of the tablet die. Those more cohesive powders gave a larger RMS signal at the lower electrode (relative to the upper electrode) whereas less cohesive powders had similar RMS signals at each electrode. While the exact explanation of this effect is currently unknown these results suggest that the technique might be useful in the determination of die filling as a function of the input material characteristics. In summary, this work has provided some insight into the potential applications of EPFS for in-line measurement of powder flow and charging characteristics. Future work should focus on (i) developing an integrated sensor with an independent measurement of density to yield the powder mass flow using an inferential approach, (ii) co-use of techniques (such as Faraday-cup and charge decay analysers) to validate the in-line charging behaviour, (iii) further exploration of the significance of the signal amplitude difference at the tablet press hopper outlet in on the characteristics of the tablet compact

A Model-Based Analysis of Capacitive Flow Metering for Pneumatic Conveying Systems: A Comparison between Calibration-Based and Tomographic Approaches

Pneumatic conveying is a standard transportation technique for bulk materials in various industrial fields. Flow metering is crucial for the efficient and reliable operation of such systems and for process control. Capacitive measurement systems are often proposed for this application. In this method, electrodes are placed on the conveyor systems transport line and capacitive signals are sensed. The design of the sensor with regard to the arrangement and the number of electrodes as well as the evaluation of the capacitive sensor signals can be divided into two categories. Calibration-based flow meters use regression methods for signal processing, which are parametrized from calibration measurements on test rigs. Their performance is limited by the extend of the calibration measurements. Electrical capacitance tomography based flow meters use model-based signal processing techniques to obtain estimates about the spatial material distribution within the sensor. In contrast to their calibration-based counterparts, this approach requires more effort with respect to modeling and instrumentation, as typically a larger number of measurement signals has to be acquired. In this work we present a comparative analysis of the two approaches, which is based on measurement experiments and a holistic system model for flow metering. For the model-based analysis Monte Carlo simulations are conducted, where randomly generated pneumatic conveying flow patterns are simulated to analyze the sensor and algorithm behavior. The results demonstrate the potential benefit of electrical capacitance tomography based flow meters over a calibration-based instrument design

Development of a Fast Gas-Solid Flow Simulation for Control of the Pneumatic Conveying System on Air Seeders

Limitations of the pneumatic conveying system are an obstacle to the improvement of air seeding technology. Operators often run conveying velocities far above the minimum requirement. This is common because lower conveying velocities - which could reduce waste, energy consumption and hydraulic requirements - put the system at risk of blockages and non-uniform distribution. Furthermore, new precision technologies such as variable rate application and sectional control introduce imbalances to the highly coupled and distributed conveying system. Incorporating adaptive control mechanisms has been theorized as a potential means of improving conveying system performance. Real-time prediction of conveying system flow conditions is a prerequisite for the proposed control strategies. There is limited existing research regarding control and modeling for air seeders or similar pneumatic conveying systems. While there is extensive research for multiphase flow modeling, few examples prioritize computational efficiency to the extent that real-time simulation is feasible. Application to control dictates that computational speed, in addition to accuracy, is essential. The purpose of this research was to identify, develop and validate a method for predicting flow conditions within a pneumatic conveying system that is suitable for control applications. A low-computational cost, one-dimensional model and simulation have been developed for fast prediction of bulk multiphase flow conditions within the pneumatic conveying system. The model is a simplified form of the Eulerian-Eulerian (two-fluid) equations for fluid-particle flows. The differential model equations were discretized via the finite volume method and solved using computational fluid dynamics techniques. Specifically, the SIMPLER algorithm for the solution of coupled equations was used. The simulation program, which employs the numerical methods to obtain solutions to the discrete equations, was implemented in MATLAB®. Experimental data were collected using a laboratory apparatus which approximated a straight horizontal pneumatic conveying line. The inner diameter of the experimental conveying line was 57.4 mm. Spherical plastic particles with a mean diameter of 3.56 mm were conveyed. Testing consisted of dilute flows only that were relevant for air seeding conditions. Experiments covered air velocities of 20 to 30 m/s and mass loadings of 0.84 to 4.68. Recorded data included steady-state and transient measurements for fluid pressure and bulk particle velocity. The experimental data were used to validate simulation results. The accuracy of the model for steady-state conditions was acceptable for sufficiently dilute and well-developed flow. The simulation predicted experimental fluid pressure within 6% in all tests. For moderate mass loadings, simulation error for particle velocity was below 10%. At higher mass loadings, accuracy for particle velocity began to deteriorate and an error of > 25% was observed. Analysis of the model’s accuracy for transient conditions was inconclusive. Evidence suggested that transient simulation results may be quite good. However, limitations of the continuous equations and experimental factors complicated the analysis, preventing a definitive verdict regarding transient accuracy. Simulation performance with respect to computing time was excellent. Simulation results were found to be relatively insensitive to the size of time and spatial step used, allowing for the program to execute in less time than was being simulated. The fastest execution recorded required 5.0 sec to simulate 60 sec of transient flow, and results deviated minimally from higher resolution simulations. Results indicated the potential for optimization between speed and accuracy. While the simplified model only calculates a limited number of bulk flow properties, it delivered timely results with reasonable accuracy and with relatively low computational effort. Assessment of the developed model and simulation has concluded a suitable potential for control application. Acceptable accuracy and computing speed were obtained to justify further development efforts. The prescribed methodology provides a foundation for future expansion and improvement. There is potential to incorporate fast multiphase flow simulation into control infrastructure to improve the performance of the air seeder conveying system

Electrostatics of granular flow in Pneumatic conveying systems

Ph.DDOCTOR OF PHILOSOPH

Center Director's Discretionary Fund 2005 Annual Report

The FY 2005 CDDF projects were selected from the following spaceport and range technology and science areas: fluid system technologies; spaceport structures and materials; command, control, and monitoring technologies; and biological sciences (including support for environmental stewardship). The FY 2005 CDDF research projects involved development of the following: a) Capacitance-based moisture sensors to optimize plant growth in reduced gravity; b) Commodity-free calibration methods; c) Application of atmospheric plasma glow discharge to alter the surface properties of polymers for improved electrostatic dissipation characteristics; d) A wipe-on, wipe-off chemical process to remove lead oxides found in paint; e) A robust metabolite profiling platform for better understanding the "law" of biological regulation; f) An explanation of the excavation processes that occur when a jet of gas impinges on a bed of sand; g) "Smart coatings" to detect and control corrosion at an early stage to prevent further corrosion h) A model that can produce a reliable diagnosis of the quality of a software product; i) The formulation of advanced materials to meet system safety needs to minimize electrostatic charges, flammability, and radiation exposure; j) A lab-based instrument that uses the electro-optic Pockels effect to make static electric fields visible; k) A passive volatile organic compound (VOC) cartridge to filter, identify, and quantify VOCs flowing into or emanating from plant flight experiments