PL194588B1 - Linearising circuit for a platinum temperature sensing element - Google Patents

Abstract

Platinum sensor linearization system temperature, characterized in that it contains a key switching (KL) connected to the converter analog to digital (AC) that is connected with a memory chip (ROM) connected with digital-to-analog converter (CA) and includes a control circuit (US) connected with switch key (KL) and element memory (ROM).

Description

Description of the invention

The subject of the invention is a linearization system for a platinum temperature sensor, especially for measuring devices.

Known solutions for linearization of a platinum sensor consist of analog amplifiers, reference sources and resistor networks, in which the linearization is the result of an appropriately selected amplifier feedback. This solution is thermally unstable and requires troublesome and precise selection of components. Other known linearization systems include an analog-to-digital converter and a permanent memory element in which the temperature-dependent linearly dependent values are contained in the form of an array. The disadvantage of this solution is the costly precise analog-to-digital converter with a resolution corresponding to the full non-linear measurement range of the platinum sensor. Still other circuits include microprocessor systems that convert the input data to a linear form according to mathematical formulas. These formulas are empirical and not very precise, and the systems are complex and expensive.

The linearization circuit according to the invention consists of a control circuit connected to a switching key and a memory circuit, and an analog-to-digital converter connected to a memory circuit connected to a digital-to-analog converter. The system according to the invention makes it possible to build a precise temperature transmitter using cheap, easily available components.

The subject of the invention is shown in the drawing in the form of a diagram.

The linearization system works in two phases. In the first phase, a rough measurement of the sensor resistance is performed, the temperature range is determined and the numerical value of the correction compensating for non-linearity is determined. In the second phase, the voltage across the sensor is linearly dependent on the temperature and is fed to the output of the system. Initially, the known current I0 flows through the platinum PT sensor. Voltage U0 appears on the sensor and through the KL key is applied to the AC analog-to-digital converter. This converter measures the voltage U0 with a low accuracy and converts it into a digital form by controlling it by the ROM memory chip. A table of corrections is included in the ROM memory chip, arranged in such a way as to assign a correction to compensate for the non-linearity of the sensor to each temperature range. In the second phase, this amendment controls the CA digital to analog converter which has a current output. The Ix current with a value corrected by means of a correction depending on the predetermined temperature range causes the appearance of the voltage Ux linearly dependent on the temperature on the sensor. The KL key gives this voltage to the output of the system. The control circuit US changes the position of the key KL and controls the ROM memory chip. The slope of the characteristics of the platinum PT sensor made as a 100 ohm resistor varies from 0.431 ohma / K for -200 ° C to 0.296 ohma / K for + 800 ° C. The 8-bit analog-to-digital CA converter with a current output of up to 2 mA allows to obtain a voltage on a PT platinum sensor of 0.5 mV / K with changes in the Ix current from 1.16 mA to 1.69 mA. Linearization will be performed in more than 70 steps, which is approx. 14 K per step. The estimated accuracy for the extreme ranges is about 0.047%. Moreover, the system according to the invention makes it possible to reduce the effect of heating the platinum sensor PT with the measuring currents I0 and Ix. The current flow is only needed when making a measurement. Between the individual measurement phases there may be periods during which no current flows. Due to the short measurement time with an analog-to-digital AD converter of a few microseconds, at a measurement frequency of 1000 per second, the heating effect is reduced by approx. 100 times.

Claims (1)

Patent claim Linearization circuit of a platinum temperature sensor, characterized in that it comprises a switching key (KL) connected to an analog-to-digital converter (AC) which is connected to a memory chip (ROM) connected to a digital-to-analog converter (CA) and comprises a control circuit (US) connected to with switch key (KL) and memory element (ROM).