Accuracy and Resolution Analysis of a Direct Resistive Sensor Array to FPGA Interface

Abstract

Resistive sensor arrays are formed by a large number of individual sensors which are distributed in different ways. This paper proposes a direct connection between an FPGA and a resistive array distributed in M rows and N columns, without the need of analog-to-digital converters to obtain resistance values in the sensor and where the conditioning circuit is reduced to the use of a capacitor in each of the columns of the matrix. The circuit allows parallel measurements of the N resistors which form each of the rows of the array, eliminating the resistive crosstalk which is typical of these circuits. This is achieved by an addressing technique which does not require external elements to the FPGA. Although the typical resistive crosstalk between resistors which are measured simultaneously is eliminated, other elements that have an impact on the measurement of discharge times appear in the proposed architecture and, therefore, affect the uncertainty in resistance value measurements; these elements need to be studied. Finally, the performance of different calibration techniques is assessed experimentally on a discrete resistor array, obtaining for a new model of calibration, a maximum relative error of 0.066% in a range of resistor values which correspond to a tactile sensor.

Reducing Measurement Time in Direct Interface Circuits for Resistive Sensor Readout

“…The charge process (and the counter advance) ends at the moment b 11 becomes 1, with the discharge process then starting. Therefore, the maximum charge time is 2 11 •T CK , where T CK = 20 ns is the period of the clock signal used in the counter. In our case, this maximum charge time is therefore 40.96 µs.…”

“…If the results of Equations (8), (11), and (12) are substituted in this expression, R S can be estimated by the equation…”

“…Such simplicity means that this type of circuit is particularly suited for portable applications, where both the number of components and their consumption are very important. DICs are also highly versatile in terms of the PDDs used, allowing the use of both microcontrollers [4][5][6][7][8][9][10] and field-programmable gate arrays (FPGAs) [11][12][13][14][15][16]. The calculation capabilities of a PDD connected to a DIC allow them to function as smart sensors, pre-processing information from the sensor and thus reducing the workload in subsequent high-level processing and decision stages [17].…”

“…Although there are DICs for reading both capacitive and inductive sensors, those for resistive sensors have probably been the most widely used and analyzed in the literature, whether individually [3,7,18,19] or grouped in arrays [11,14]. Several issues need to be considered when designing a resistive DIC, such as accuracy, uncertainty, resolution [6,8,16,[20][21][22], and calibration points [23].…”

Seat Occupancy Detection Based on a Low-Power Microcontroller and a Single FSR

“…As for the read-out electronic circuit, it is proposed to read both large- and small-signal variations of the FSR applying the concept of direct interface circuit (DIC) [12], where the sensor is directly connected to a low-cost microcontroller unit (MCU) without using intermediate analogue electronics either an analogue-to-digital converter (ADC). These MCU-based circuits have been extensively analyzed and proved for resistive [13,14,15], capacitive [16,17,18] and inductive [19,20] sensors with different topologies, but not for resistive sensors undergoing both large- and small-signal variations, as we have in the application of interest here. DICs have also been suggested for sensors providing a quasi-static analogue output voltage [21,22] and as a versatile interface circuit for the measurement of different types of sensor [23].…”