According to the actual needs of petrochemical plants, we have developed a smart temperature transmitter that supports the HART protocol. It uses a high-performance single chip computer C8051F410 and a HART modulation and demodulation chip as core components. The power supply circuit and signal conditioning circuit are uniquely designed. The overall circuit structure is simple, low-cost, and cost-effective.
1. Overall System Design
① Loop power supply and 4-20MA current loop output.
② Signal conditioning and microcontroller arithmetic output.
③ HART communication.
④ Display and parameter setting.
1.2 Power supply
The power supply circuit provides constant voltage and current with an operating current of 3.5 mA and an operating voltage of 3.3 V to provide power to all transmitter circuits.
1.3 RTD signal
The RTD signal conditioning section provides constant current excitation to the sensor. It introduces the sensor’s output voltage into the analog input interface of the A/D converter after appropriate filtering.
The A/D converter section amplifies the sensor output signal. Then, it converts it into a digital quantity for use by the MCU, which linearises and compensates the voltage signal to obtain the temperature value to be measured.
1.4 Module Signal
The HART communication module consists of a HART modem, a D/A converter, and peripheral circuitry. The D/A output part completes the conversion from voltage to loop current, converting the digital output from the MCU into the corresponding 4~20mA current signal.
1.5 Output signals
The HART modem receives the modulating signal superimposed on the 4~20mA current +-0.5mA, demodulates the detected FSK digital signal, and transmits it to the MCU via serial communication.
The digital signal of the answer frame generated by the MCU is modulated and demodulated by the HART to make an FSK frequency shift keying signal, which is superimposed on the 4~20mA current signal and sent out.
The parameter display and setting section consists of a segment LCD and micro-keys for transmitter parameters and display settings.
1.6 Main technical specifications
① Supply voltage: 9~32VDC.
② Operating ambient temperature -40~85 degrees.
③ Full range accuracy 0.5.
2. Hardware design
2.1 Current loop design
To complete the work of the smart transmitter, the supply voltage should reach 9~32VDC, and the operating current should reach 3.5-22Ma.
The power supply design uses the micro-power OPA244 operational amplifier. The amplifier allows an operating voltage up to 36V and consumes only 35 microamps op-amp. A design was developed with a parallel type reference source to provide an operating current of 3.5 room amps.
A constant voltage, a constant current circuit with an operating voltage of 3.3V, provides power for the transmitter’s circuitry. It ensures that variations in the transmitter’s operating current do not affect its current output.
2.2 Current output of the transmitter
As the op-amp has a static power consumption of 35UA, the feedback current consumes 33UA, and the reference source has a shallow quiescent current of 10UA. Therefore, the current that can be supplied to the rest of the transmitter is 3.4MA.
If the current used by the rest of the transmitter is less than 3.4MA, the excess current flows through the reference source. Conversely, the circuit fails if the current is more significant than 3.4 Ma. Therefore, all other transmitter circuits are designed to ensure that the total current is less than 3.4 mA.
The DA output of the microcontroller controls the transmitter output corresponding to the sensor signal. It is achieved by a constant current output circuit consisting of TLV2252, T31, and T32.
To ensure the output accuracy of the instrument, the current output driving part adopts the Darlington structure with a current amplification of more than 10,000 times, ignoring the influence of the instrument output accuracy due to the base current outflow.
2.3 Signal Conditioning
The low-supply rail-to-rail operational amplifier TLV2252 was used for the RTD signal conditioning section. The differential amplifier circuit addresses the effect of the resistance of the RTD lead wires on the RTD measurement temperature.
To maximise the efficiency of using the AD converter within the subsequent microcontroller, a de-zeroing resistor R19 is added to the circuit so that the R19 resistance value is equal to the RTD resistance value when the meter is minimal range. In this way, the amplifier voltage output is only related to the temperature change on the RTD resistance change.
Therefore, the efficiency of the AD converter use is improved, V = K1 (RT – R19), RTD excitation constant current source has C8051F410 to provide, C8051F410 can provide 0.25, 0.5, 1, 2 room amps multiple range constant current sources, to reduce power consumption, choose 0.5 room amps for RTD excitation constant current source.
2.4 Microcontroller circuit design
The C8051F410 microcontroller operates at the internal clock point 191406, which is also the point at which the C8051F410 is powered on to agree on a working frequency. At this point of operating frequency, no peripherals are opened, power consumption is 200 UA, AD, 0.5 and 0.25 room amps, two-way DA are opened, and the overall current supply consumption is less than two mA.
2.5 HART circuit design
The HART protocol communication circuit chip, which has a very low microamp and power consumption, makes it possible to ignore the impact of power consumption on the overall circuit design. The input signal is introduced directly into the chip via a capacitive isolation filter circuit. The output section is coupled to the current output circuit input via capacitive isolation and impedance matching to complete the HART communication circuit design.
2.6 Display and keypad control circuit design
The display circuit uses the HT1621 controller, which controls the segment LCD, to enable the transmitter to be set up on the transmitter when the HATR handheld programmer is unavailable. We have added three micro keys to its front panel to set the parameters. The LCD circuit consumes approximately 500 microamps of power. A backlight is used for the LCD to facilitate the use of the transmitter at night. Considering the energy consumption requirements, this design uses a 4~20 mA circuit ring to drive the LCD back.
3. Software design
The software mainly completes the following aspects of operation data acquisition, storage, signal self-calibration function, HART communication protocol, variable output, LCD, keyboard operation, etc.
The whole circuit design is simple and fully uses the resources carried by the C8051F410 at a meager cost. Furthermore, the use of large inductors and capacitors is controlled in the design to provide the basis for subsequent intrinsically safe methods.