CN116263583A

CN116263583A - Two-wire transmitter and application method thereof

Info

Publication number: CN116263583A
Application number: CN202111530885.2A
Authority: CN
Inventors: 刘思超; 水广伟; 杨振玲; 梁刚
Original assignee: Honeywell Environmental and Combustion Controls Tianjin Co Ltd
Current assignee: Honeywell Environmental and Combustion Controls Tianjin Co Ltd
Priority date: 2021-12-15
Filing date: 2021-12-15
Publication date: 2023-06-16

Abstract

The application provides a two-wire transmitter, including power supply circuit, two at least route operational amplifier analog output circuits of building, wherein two at least way analog output circuits share power supply circuit, and little control unit, this little control unit is configured to after power supply circuit begins work, control the output current of two at least way analog output circuits, make the electric current of two at least way analog output circuits is controlled by little control unit is complete. The two-wire transmitter further includes a start-up circuit, and the power circuit operates by turning on the start-up circuit. The micro control unit is also configured to control the starting circuit to be turned on and off, and when the two-wire transmitter is powered on by the micro control unit starting the starting circuit to enable the power circuit to work, the micro control unit turns off the starting circuit.

Description

Two-wire transmitter and application method thereof

Technical Field

The present invention relates generally to two-wire transmitters and methods of using the same. More particularly, the present invention relates to a two-wire temperature and humidity transmitter and method of use thereof.

Background

It is generally required in industry to measure various non-electrical physical quantities such as temperature, humidity, pressure, speed, angle, etc., and to convert the measured quantities into analog electrical signals for transmission to control rooms or display devices that are hundreds of meters away. Such devices that convert physical quantities into electrical signals are known as transmitters. The most widely used industry is to use 4-20 mA current to transmit analog quantity.

For analog sensor data transmission, a 4-20 mA current loop is a very common method for transmitting data acquired by a sensor. The sensor or transducer is typically designed to measure a series of values of the parameter being measured, which values are referred to as measured values. The measured value must be converted into a current in the measuring device so that the current in the loop is proportional to the measured value.

The principle of the two-wire transmitter is that 4-20 mA signals are utilized to provide electric energy for the transmitter. The two-wire system 4-20 mA current can be used for supplying power for the transmitter. The transducer is equivalent to a special load in a circuit, and is characterized in that the power consumption current of the transducer changes between 4mA and 20mA according to the output of the transducer. The display instrument only needs to be strung in the circuit. This type of transmitter requires only 2 wires to be externally connected and is therefore known as a two-wire transmitter.

In most analog control systems, the 4-20 mA current loop has become the standard for signal transmission and electronic control. FIG. 1 is a schematic diagram of a typical 4-20 mA current loop. In the current loop, current is drawn from the dc loop power supply and then connected through field wiring to a loop load resistor in the receiver or controller and back to the loop power supply, all of which are connected in a series circuit.

In some cases, in order to accelerate the power-on process of the transmitter, the transmitter comprises a constant current source bypass circuit, so that bypass constant current source current is arranged in 4-20 mA output of an analog output circuit built by an operational amplifier in the transmitter, a Micro Control Unit (MCU) can only control the current of the operational amplifier part, the two parts are added together to be 4-20 mA output, the bypass constant current source current can be output with uncontrollable extra current along with the change of environmental conditions, the output precision can not be completely guaranteed, and the cost is high.

In addition, when the temperature and the humidity are output to 4-20 mA in the same product, two sets of power supply systems are needed to isolate the ground plane, and the cost is high.

Disclosure of Invention

In one embodiment of the present invention, there is provided a two-wire transmitter including a power circuit, at least two analog output circuits built by an operational amplifier, wherein the at least two analog output circuits share the power circuit, and a micro control unit configured to control output currents of the at least two analog output circuits after the power circuit starts to operate, such that currents of the at least two analog output circuits are completely controlled by the micro control unit. The two-wire transmitter further includes a start-up circuit, and the power circuit operates by turning on the start-up circuit. The micro control unit is also configured to control the starting circuit to be turned on and off, and when the two-wire transmitter is powered on by the micro control unit starting the starting circuit to enable the power circuit to work, the micro control unit turns off the starting circuit. In another embodiment of the invention, the at least two analog output circuits each set the reference output plane as a ground plane.

Drawings

FIG. 1 is a schematic diagram of a typical 4-20 mA current loop;

FIG. 2 is a conventional two-wire transmitter with a constant current source bypass circuit;

FIG. 3 is a two-wire transmitter according to one embodiment of the invention;

FIG. 4 is a two-wire transmitter loop with two inputs in accordance with one embodiment of the invention; and

FIG. 5 is a flow chart of a method for a two-wire transmitter according to one embodiment of the invention.

Detailed Description

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention. The invention is not intended to be limited to the specific illustrated embodiments.

The invention aims to solve the problem that the invention aims to solve the error introduced by the constant current source used in the power-on process in the prior art, and the transmission output of 4-20 mA is completely controlled by the MCU. And the product controllability and the product precision are improved. In addition, the design also needs to solve the problem that the temperature and humidity transmitter in the integrated chip scheme in the prior art needs to isolate the power ground plane and needs two sets of power systems.

Transmitters are devices that control the circulation of current in a loop. It converts the physical or electrical parameter into a 4-20 mA signal, where 4mA represents a zero value and 20mA is full scale. Devices having a 4-20 mA transmitter output can include current sensors, flow sensors, pressure sensors, temperature transmitters, humidity transmitters, and signal isolators.

One of the main reasons for using a 4-20 mA current loop is that the loop current does not change with long field wiring as long as the voltage developed in the loop (referred to as the constant current output voltage) can maintain a maximum loop current. Another advantage is that the current loop has a low impedance and is not particularly susceptible to noise or EMI. A third advantage is the live zero characteristic of the loop (4 mA lower limit), which can be self-diagnostic if there is a broken or bad connection in the loop or if the loop power fails. The 4-20 mA current loop allows other current operated devices, such as remote readers or recorders, to be connected in series with the loop under the constraints allowed by the constant current system output voltage of the loop. The low level of maximum loop current (20 mA) allows the use of a relatively simple safety barrier, limiting the loop current to intrinsically safe levels to prevent ignition at dangerous locations.

When current is transmitted in the loop, there is a voltage drop due to the field wiring and any connected devices. However, these voltage drops do not affect the current in the loop as long as the total loop voltage is sufficient to maintain maximum loop current. As shown in fig. 1, the element in the loop responsible for maintaining a steady current is a loop dc power supply. The loop will operate at a voltage in this range called its constant current system output voltage. The voltage chosen by the designer depends on the number of elements in series with the loop. The loop supply voltage must always be higher than the sum of all voltage drops in the circuit, including the field wiring voltage drop. The sum of all these voltage drops is referred to as the minimum constant current system output voltage of the loop. The constant current system output voltage must be able to meet some requirements, two of which are foremost, the supply voltage must be able to power all devices in the loop, including the field-line voltage drop, typically 20mA when the current is at its maximum; and the maximum voltage output of the loop power supply must be equal to or lower than the maximum rated voltage of any device in the loop.

The sensor or sensors that measure a physical parameter (e.g., temperature, humidity, pressure, position, or fluid flow) are coupled to a signal conditioning circuit that converts the measured parameter value into an electrical signal output signal such as a voltage or current proportional to the measured physical parameter. If the electrical signal is a 4-20 mA DC output connected to a current loop, the hardware and electronic system that feeds the current into the loop is called the transducer. A transducer may consist of a single device containing both sensing elements and internal electronics, or it may utilize a sensor or sensor connected to separate signal conditioning electronics configured as a 4-20 ma current transducer.

4-20 mA current is circulated in the loop. The distance between the sensor-transmitter combination and the process controller or reader may be hundreds of meters or more. A field wire is used in the loop to connect the transmitter to process monitoring or control hardware. It is important to consider them as one element of the loop because they have some resistance and produce a voltage drop just like any other element in the loop. If the sum of all the voltage drops is higher than the constant current system output voltage of the loop power supply, the current will be disproportionate to the measured parameter and the system will produce unusable data.

After the loop current is generated, it must typically be further processed in the system. For example, the current may be fed back to a temperature humidity controller to raise or lower the temperature in order to initiate or control the process. The control function is easier to implement with voltage than with current. The receiver is the part of the loop that converts the loop current into a voltage. The receiver is a device such as a meter, recorder, driver, control or SCADA module, read loop current. Devices with mA inputs include analog panel meters, digital panel meters, pressure gauges, process meters, meter relays, controllers, process recorders, and loop power indicator lights. Many applications do not measure milliamps directly, but instead insert a resistor in the loop to convert the current to voltage. The drop in resistance is then measured with a voltmeter. Typically the 1-5V signal is 250W and the 2-10V signal is 500W. In fig. 1, the receiver may be a simple resistor in series with the loop, so that the voltage produced by the receiver is proportional to the measured physical parameter according to ohm's law and is displayed by the meter.

In a loop including a conventional two-wire transmitter, as shown in FIG. 2, the two-wire transmitter includes a micro control unit (Microcontroller Unit; MCU), also known as a single-chip microcomputer (Single Chip Microcomputer) or a single-chip microcomputer. Such two-wire transmitters also include a constant current source bypass circuit. The output of the analog output circuit built by the operational amplifier in the MCU and the current in the bypass constant current source circuit jointly output 4-20 mA, and the MCU can only control the current of the operational amplifier part.

In one embodiment of the present invention, a dedicated start-up circuit is added to a two-wire transmitter to replace the constant current source bypass circuit in the prior art scheme, such as the two-wire transmitter according to one embodiment of the present invention shown in FIG. 3. After the two-wire transmitter can normally work through the starting circuit, the MCU closes the starting circuit to realize the power-on process of the circuit, and uncontrollable current introduced by the bypass constant current source circuit is avoided. In one embodiment of the invention, the two-wire transmitter may include an analog output circuit, a power circuit, and a micro control unit MCU built from an operational amplifier in the MCU. The two-wire transmitter can also include a start circuit. The power supply circuit works after the starting circuit is started, and the micro control unit MCU controls the output current of the analog output circuit and controls the starting circuit to be turned on and turned off. When the two-wire transmitter is powered on by the micro control unit MCU starting the starting circuit to enable the power circuit to work, the micro control unit closes the starting circuit, so that the current of the analog output circuit is completely controlled by the micro control unit MCU.

The DC power supply provides loop current and may also power the transmitter and receiver. 24V is a typical value, although 10, 15, 28 and 36V are also used in some installations. For the loop of a two-wire device, the power supply only provides loop current. The multiple loops may operate from one power supply provided that it is rated for maximum drive current sharing a low-level connection between all loops and the receiver.

The loop powered receiver obtains its operating power from the 4-20 ma loop without requiring additional system connections. The input low connection of the receiver may be tied internally to system ground. This can be used in the case of instruments with multiple inputs, such as recorders or PLCs. The receiver input configuration is another factor to consider in analyzing the system basis, commonality, and signal return. If the loop supply voltage is sufficient and the input of the additional receiver can float off the ground, multiple receivers can be connected in series in one loop. An externally powered receiver may provide relays or control outputs, digital communications and other extended functions. Some meters and controllers also provide 4-20 mA outputs to retransmit the measurement signals.

FIG. 4 illustrates a two-wire transmitter loop with a transmitter and an n-channel receiver in accordance with one embodiment of the invention. The transmitter can transmit n non-electrical physical quantities, such as temperature, humidity, pressure, speed, angle, etc., such as temperature, humidity, or a combination thereof, and has a corresponding n-way output circuit. The n-way output of the two-wire transmitter uses the same power circuit, rather than using separate ground or separate power supplies. After the power supply circuit starts to work, the MCU in the two-wire transmitter controls the output current of the n-path analog output circuit, so that the current of the n-path analog output circuit is completely controlled by the MCU. In a particular embodiment of the invention, a start-up circuit is introduced, the power supply circuit being operated by turning on the start-up circuit.

When the two-wire transmitter is electrified by starting the starting circuit through the micro control unit so that the power circuit works, the MCU closes the starting circuit.

In a particular embodiment of the invention, the two-wire transmitter is a temperature and humidity transmitter, the non-electrical physical quantity to be measured is temperature, humidity or a combination thereof, and has a first output and a second output, the first output being a first analog electrical signal representative of temperature and the second output being a second analog electrical signal representative of humidity. The first output of the transmitter is connected to the first input of the dual channel receiver and the second output of the transmitter is connected to the second input of the dual channel receiver. The input of the transducer is connected to the positive pole of the DC power supply, while the output of the dual channel receiver is connected to the negative pole of the DC power supply. In a particular embodiment of the invention, the transmitters all have a two-wire transmitter configuration as shown in FIG. 3, wherein the first temperature output and the second humidity output use the same power circuit. In order to avoid the problems of isolating a power ground plane and needing two sets of power supply systems, a reference output plane is set to be the ground plane in an analog output circuit built by an operational amplifier, so that two paths of analog quantities of 4-20 mA can be output through the same reference ground. The temperature and the humidity can be output in the unified hardware platform by referring to the power supply system through the unified reference ground plane, so that the cost is saved.

FIG. 5 is a flow chart of a method of using a two-wire transmitter loop in accordance with one embodiment of the invention. In step 501, a transmitter product including, for example, a two-wire temperature and humidity transmitter is powered up. The temperature and humidity transmitter is provided with an MCU (micro control unit) for controlling the output current of an analog output circuit of the two-wire temperature and humidity transmitter. The MCU has a dedicated start-up circuit to replace the constant current source bypass circuit in the existing scheme. The MCU controls the starting circuit to be turned on and turned off. In step 502, the start-up circuit operates. In step 503, the power circuit of the two-wire temperature and humidity transmitter operates after the start-up circuit is started. In step 504, after each component (such as MCU) of the temperature and humidity two-wire transmitter loop works normally, the MCU turns off the start circuit to realize the power-up process of the temperature and humidity transmitter, avoiding the presence of uncontrollable current introduced by the bypass constant current source circuit. The temperature and humidity transmitter is started by a special starting circuit, and 4-20 mA current output of the temperature and humidity transmitter is completely controlled by the MCU, so that no large extra current component participates, the output is more completely controllable, the stability is higher, and the precision is higher. In step 505, the reference output plane is set to be a ground plane in each path of analog output circuit built by the operational amplifier of the MCU, so that the temperature transmitter output circuit outputs 4-20 ma current and the temperature transmitter output circuit also outputs 4-20 ma current. The analog output of the operational amplifier can transmit and output 4-20 mA of temperature and humidity in the same power supply system, so that the cost is saved.

Although a few embodiments have been described in detail above, other modifications are possible. For example, the logic flows described above do not require the particular order or sequential order described to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Other embodiments may be within the scope of the present invention.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific systems or methods described herein is intended or should be inferred. It is, of course, intended to cover all such modifications as fall within the spirit and scope of the invention.

Claims

1. A two-wire transmitter, comprising:

the power supply circuit is provided with a power supply circuit,

at least two analog output circuits built by the operational amplifier, wherein the at least two analog output circuits share the power supply circuit; and

and the micro control unit is configured to control the output currents of the at least two paths of analog output circuits after the power supply circuit starts to work, so that the currents of the at least two paths of analog output circuits are completely controlled by the micro control unit.

2. The two-wire transmitter of claim 1 further comprising a start-up circuit, the power circuit being operated by turning on the start-up circuit.

3. The two-wire transmitter of claim 2 wherein the micro-control unit is further configured to control the start-up circuit to turn on and off, the micro-control unit turning off the start-up circuit when the two-wire transmitter is powered up by the micro-control unit turning on the start-up circuit to operate the power circuit.

4. The two-wire transmitter of claim 1 wherein the output currents of the at least two analog output circuits are each 4-20 ma.

5. The two-wire transmitter of one of claims 1-4, wherein the two-wire transmitter is powered by a DC current source and its output current is received by a receiver.

6. The two-wire transmitter of claim 5 wherein the receiver is a multi-channel receiver that receives outputs from the at least two analog output circuits, wherein the at least two analog output circuits both set a reference output plane to a ground plane.

7. The two-wire transmitter of claim 6 wherein the at least two analog output circuits output 4-20 ma of current representing temperature, 4-20 ma of current representing humidity, or a combination thereof.

8. A method for a two-wire transmitter, wherein the two-wire transmitter comprises:

a power supply circuit;

a micro control unit;

the method comprises the following steps:

powering the two-wire transmitter;

the power supply circuit starts to work;

each part of the two-wire transmitter works normally; and

and controlling the output currents of the at least two paths of analog output circuits so that the currents of the at least two paths of analog output circuits are completely controlled by the micro control unit.

9. The method of claim 8, the two-wire transmitter further comprising a start-up circuit, the method further comprising: the power supply circuit is operated by turning on the start-up circuit.

10. The method of claim 9, wherein the micro-control unit is further configured to control the turning on and off of the start-up circuit, the method further comprising: when the two-wire transmitter is electrified by starting the starting circuit through the micro control unit so that the power circuit works, the micro control unit closes the starting circuit.

11. The method of claim 8, wherein the output currents of the at least two analog output circuits are each 4-20 ma.

12. The method of one of claims 8-11, wherein the two-wire transmitter is powered by a DC current source and its output current is received by a receiver.

13. The method of claim 12, wherein the receiver is a multichannel receiver that receives outputs from the at least two analog output circuits, the method further comprising:

the reference output planes of the at least two analog output circuits are each set as a ground plane.

14. The method of claim 13, wherein the at least two analog output circuits output 4-20 ma of current representing temperature, 4-20 ma of current representing humidity, or a combination thereof.