Technical sciences/11. Robotics

Bayasilova Z.A.¹, c.ph.-m.s. Baktybayev M.K.¹, d.t.s. Mikhaylov P.G.²

Kazakh National Research Technical University after K.I. Satpayev, Kazakhstan¹

Penza branch of the Moscow State University of Technology and Management after K.G. Razumovsky, Russia²

Combined temperature and pressure sensors

Currently combined temperature and pressure sensors can be used in various fields of contemporary life: automotive industry, aircraft building, housing and communal services, oil refining and chemical industry, medicine, etc.

Measurements of temperature and pressure are required much more often than measurements of other non-physical quantities. Therefore, temperature and pressure sensors are extremely in demand. Moreover, all of them possess similar, well-examined mechanisms. Nowadays hundreds of foreign and domestic companies actively produce pressure and temperature sensors.

Due to the evident cheapness, semiconductor sensors based on the strain-resistance effect are of the higher demand rate. Under the influence of pressure, the resistance of those sensors is inclined to change. The main disadvantage of these sensors is that the resistance varies not only under the influence of pressure, but also temperature. This shortage can be eliminated through installing a thermal compensation scheme. This solution is both simple and reliable, but still has several disadvantages.

Typically, the thermal compensation scheme is a link located behind the temperature measurement link. Therefore, the overall output characteristic of the sensor is considered to be the multiplication product of the two following link characteristics: temperature compensation and pressure measurement. The problem is in the fact it is difficult to select a thermal compensation scheme, so that the temperature affects it in an opposite way throughout the entire range of both temperature and measured pressures.

The task simplifies in the case we use a combined temperature and pressure sensor, connected to a microcontroller. In this case, it is enough to record the pressure-temperature dependence into the memory of the microcontroller in order to solve the problem. At the same time, you can record its own dependence for each specific sensor, measured during the way-out from production. This allows us to increase the lifetime of the sensor, since after the sensor examination procedure it is possible to record a newly corrected characteristic of the sensor. However, in the case of the thermo-compensation circuit, this procedure is so laborious that it is easier to remove this sensor.

The usage of the thermal compensation scheme in a combined temperature and pressure sensor gives an opportunity to a further increase in accuracy of the sensor as well as a decrease in requirements toward the resolution, accuracy, and length of the measured voltage range in the microcontroller analog-to-digital converter.

In addition to these advantages, this sensor finds new spheres of usage where both temperature and pressure measurements are required simultaneously. Such spheres include rocket engineering, mechanical engineering and heat meters.

The use of combined temperature and pressure measurement reduces the cost of the final product, simplifies the process of construction, since mounting one sensor is simpler than two. For the matter further, it is able to reduce the distance between the pressure and temperature measurement point up to zero, which may be necessary for some scientific studies where both temperature and pressure distribution in space through using a large number of sensors is especially required.

In order to unify such a sensor, a microcontroller board with an output interface chip, such as RS-232, RS-485 with MODBUS or PROFIBUS top-level protocol or short-range I2C, SPI, and Usart interface can be used. This will entail an increase in the price of the sensor, but heighten the accuracy of the system and allow to maintain the corrective characteristics in the memory of the microcontroller. Such a solution will automate the measurement process, the data will be stored in the memory of the microcontroller every certain period of time, lead an archive of readings, error reports, create triggers for giving an alarm when the preset pressure and temperature parameters are exceeded, and optimize the energy consumption of the sensor. This will ultimately simplify the task for the developer of the company, acquiring the sensor, which can compensate for the price increase.

There are 3 types of pressure sensors, designed for measuring absolute, excessive and differential types of pressure. Absolute pressure sensors measure pressures from 0. Differential pressure sensors measure the pressure difference between 2 points. Excessive pressure sensors are considered to be a special case of differential sensors and measure pressure relatively to atmospheric pressure. The latter are more widely used in housing and communal services [1].

Picture 1 – Pressure types

The pressure sensor is a sensory element placed between two chambers - one from which contains measured pressure, whereas the other - bearing pressure. In absolute sensors, the effect on the crystal exists only on one side. One of the world leaders in the production of semiconductor pressure sensors is NXP Company. In 2016, it became the fourth in the world in terms of the volume of semiconductor products. This company produces various pressure sensors in the range from 0 to 10,000 kPa. Let us consider the sensor scheme MPX2100, constructed by NXP in a section-form, demonstrated on the picture 2 below [2]. This sensor has 7 modifications of different designs, allowing it to be used for measuring all three types of pressure.

Picture 2 – Absolute pressure sensor [2]

Picture 3 – Excessive or differential pressure sensor [2]

The absolute pressure sensor is shown on the picture 3, the excessive or differential - on the picture 2. Apparently, the main difference between the sensors lies in construction of the matrix. The excessive or differential pressure sensor suppose that the pressure P1 will always be higher than the pressure P2, a vacuum is applied in the absolute pressure sensor toward P1. The silicon gel isolates the silicon matrix and the supply wires from the environment influence, without interfering with the influence of pressure on the silicon matrix. The manufacturer indicates that the sensor suppose the exposure to dry air. Exposure to other gases or mediums may result in inoperability or shortening the lifetime of the product [2].

The MPX2100 sensor uses the piezoresistive principle of operation. The point is that under the influence of pressure differences, the resistance of tensoresistors changes. The disadvantage of this method is the influence of temperature, for the elimination of which thermal compensation schemes are applied. In addition, the temperature can be measured and compensated through a microcontroller [2].

Summarizing, the addition of a temperature measurement channel and microcontroller to the sensor allows not only to combine the temperature and pressure measurement in one sensor, but also to increase the accuracy of the pressure measurement due to digital thermal compensation.

References

1. Vigleb G. Datchiki. Ustroystvo i primeneniye. – M.: Mir.-1989. – 196 s.

2. Datasheet «100 kPa On-Chip Temperature Compensated and Calibrated Silicon Pressure Sensors». - Freescale Semiconductor. - 2008. – 10 s.

3. Mikhaylov P.G., Petrunin G.V. Sensornyye elementy robotov i robototekhnicheskikh kompleksov. – Penza: PGU. – 1998.

4. Mikhaylov P.G., Belousov Ye.F. Mikroelektronnyye datchiki. Proyektirovaniye, izgotovleniye, diagnostika. - Penza: PGU. – 2001 – 87 s.