Constant temperature control circuit of calorimetric flow sensor based on PWM duty ratio control

Abstract

The utility model discloses a constant temperature control circuit for a calorimetric flow sensor based on PWM duty ratio control, and the calorimetric flow sensor comprises a heating element, and two temperature measurement elements which are disposed at two sides of the heating element and are used for obtaining the temperature change of a flow field. The constant-temperature control circuit comprises a PWM duty ratio constant-temperature control sub-circuit electrically connected with the heating element; the temperature feedback sub-circuit is arranged close to the heating element; the PWM duty ratio constant temperature control sub-circuit and the temperature feedback sub-circuit are both in communication connection with the single-chip microcomputer. The constant temperature control circuit of the calorimetric flow sensor based on PWM duty ratio control provided by the utility model improves the constant temperature control effect of the heating element in the calorimetric flow sensor, and improves the stability and accuracy of the measurement of the flow sensor.

Description

Technical Field

[0001] This utility model relates to the field of sensor design technology. More specifically, this utility model relates to a constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control. Background Technology

[0002] Calorimetric flow sensors, also known as thermal gas flow sensors, operate based on the principles of fluid heat transfer. They typically consist of a heating element (such as a heating resistor) and temperature sensing elements (such as thermistors or thermocouples) positioned on either side of the heating element and aligned with the fluid flow direction. When fluid flows through the sensor, it carries away some heat from the heating element; this heat loss is proportional to the fluid's mass flow rate. By measuring the temperature change of the heating element or the temperature difference between the two sensing elements and converting this temperature information into an electrical signal, the fluid's mass flow rate can be indirectly calculated.

[0003] The constant temperature control circuit of the heating element is one of the important parts of the calorimetric flow sensor. Whether the calorimetric flow sensor has high sensitivity, high accuracy and high flow range is determined by the heating element in the sensor. The stable and constant temperature generated by the heating element can make the temperature difference generated by the temperature measuring elements on both sides more accurate and can output a stable electrical signal.

[0004] In existing technologies, the temperature control circuit for heating elements generally adopts the following two methods:

[0005] Method 1, such as Figure 5 A constant temperature difference circuit was designed to maintain a constant temperature difference between the ambient temperature and the intermediate heat source during the measurement process. R7, R8, and R11 are externally soldered resistors, while R9 and R10 represent the ambient resistance and heating resistance of the flow sensor, respectively. The constant temperature difference is achieved through the feedback effect of the operational amplifier U2. Based on the virtual short and virtual open characteristics of the operational amplifier, a suitable external resistor was designed to achieve a constant temperature difference between the environment and the heat source during the measurement process.

[0006] Disadvantages: (1) When changing the temperature difference of the heating element, the external resistor needs to be re-welded, which reduces the flexibility of the flow sensor in adjusting the temperature of the heating element; (2) Changes in ambient temperature may cause changes in the element temperature, thereby affecting the accuracy and stability of the constant temperature difference circuit.

[0007] Method 2, such as Figure 6A constant temperature control circuit was designed to achieve constant temperature control of the heating element within a certain temperature range. R12, R13, and R15 are external resistors; R14 is a thermistor (placed near the heating element, its resistance changes with the element's temperature, decreasing as temperature increases); R18 is the heating resistor; U3 is an operational amplifier; and Q2 is an N-type MOSFET (when the operational amplifier outputs voltage, the MOSFET turns on, creating a circuit and heating the resistor; when the operational amplifier has no output voltage, the MOSFET turns off, and heating stops). When the predetermined heating temperature is reached, the thermistor's resistance causes the forward input voltage in the operational amplifier circuit to be greater than the reverse input voltage. At this point, the operational amplifier outputs a high-level voltage, causing the MOSFET to turn on and the heating resistor to begin heating. Once the predetermined temperature is reached, the thermistor's resistance causes the reverse input voltage to be greater than or equal to the forward input voltage. At this point, a low-level voltage is output, causing the MOSFET to turn off and heating stops.

[0008] Disadvantages: (1) When changing the temperature difference of the heating element, it is necessary to re-weld the external resistor, which reduces the flexibility of the flow sensor in adjusting the temperature of the heating element; (2) Since the heating element has two states, heating and not heating, the temperature of the heating element will fluctuate within the predetermined temperature range. Utility Model Content

[0009] One object of this invention is to solve the above-mentioned problems and / or defects, and to provide the advantages that will be described later.

[0010] To achieve these objectives and other advantages of this invention, a constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control is provided. The calorimetric flow sensor includes a heating element and two temperature sensing elements disposed on both sides of the heating element for acquiring changes in the flow field temperature. The constant temperature control circuit includes:

[0011] A PWM duty cycle constant temperature control sub-circuit electrically connected to the heating element;

[0012] A temperature feedback sub-circuit positioned near the heating element;

[0013] The PWM duty cycle constant temperature control sub-circuit and the temperature feedback sub-circuit are both connected to the microcontroller.

[0014] Preferably, the constant temperature control sub-circuit includes:

[0015] The MOS transistor that communicates with the microcontroller;

[0016] A resistor connected in parallel between the MOSFET and the microcontroller;

[0017] The heating element is electrically connected to the MOS transistor.

[0018] Preferably, the temperature feedback sub-circuit includes:

[0019] A thermistor positioned close to the heating element;

[0020] An operational amplifier electrically connected to a thermistor;

[0021] The operational amplifier is connected to the microcontroller for communication.

[0022] This utility model has at least the following beneficial effects: This utility model uses a PWM duty cycle constant temperature control sub-circuit to control the working state of the heating element, and adjusts the PWM duty cycle by the voltage value fed back by the temperature feedback sub-circuit to achieve constant temperature heating of the heating element.

[0023] Furthermore, this invention adopts a "split-type" arrangement of two circuits, so that the ambient resistance and the heating resistance are in different circuits. That is, the voltage signal change caused by the resistance change of the ambient resistance can be used as feedback to determine whether it is necessary to adjust the PWM duty cycle to control the temperature of the heating element. This improves the constant temperature control effect of the heating element in the calorimetric flow sensor and enhances the stability and accuracy of the flow sensor.

[0024] In addition, this invention uses a gradually increasing or decreasing duty cycle control, which makes the temperature change of the heating element relatively smooth. Compared with the prior art, its temperature change fluctuation is small and its stability is better.

[0025] Other advantages, objectives and features of this invention will be partly apparent from the following description, and partly understood by those skilled in the art through study and practice of this invention. Attached Figure Description

[0026] Figure 1 This is a schematic diagram illustrating the principle of the calorimetric flow sensor of this utility model.

[0027] Figure 2 This is the circuit diagram of the PWM duty cycle constant temperature control sub-circuit in this utility model;

[0028] Figure 3 This is a circuit diagram of a thermistor-type temperature feedback sub-circuit in this utility model;

[0029] Figure 4 This is a schematic diagram of the working principle of the constant temperature control circuit of the calorimetric flow sensor based on PWM duty cycle control of the present invention.

[0030] Figure 5 This is a circuit diagram of a constant temperature difference circuit in one of the existing technologies;

[0031] Figure 6 This is a schematic diagram of a constant temperature control circuit in another existing technology. Detailed Implementation

[0032] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0033] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0034] It should be noted that in the description of this utility model, the orientation or positional relationship indicated by the terms is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this utility model and simplifying the description. It does not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model.

[0035] In the description of this utility model, unless otherwise explicitly specified and limited, the terms "installed", "equipped with", "sleeved / connected", "connected", etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. For those skilled in the art, the specific meaning of the above terms in this utility model can be understood according to the specific circumstances.

[0036] Example 1

[0037] A constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control, the structure of which is as follows: Figure 1-3 As shown, the calorimetric flow sensor includes a heating element 1 and two temperature sensing elements 2 disposed on both sides of the heating element 1 for acquiring changes in the flow field temperature. The constant temperature control circuit includes:

[0038] PWM duty cycle constant temperature control sub-circuit 3 is electrically connected to heating element 1;

[0039] Temperature feedback sub-circuit 4 is located near the heating element;

[0040] The PWM duty cycle constant temperature control sub-circuit 3 and the temperature feedback sub-circuit 4 are both connected to the microcontroller 5. In practical applications, the microcontroller is needed to adjust the PWM duty cycle in order to control the duty cycle of the constant temperature heating circuit. However, adjusting the PWM duty cycle through the microcontroller is an existing technology and will not be described here.

[0041] Working principle: In practical applications, the PWM duty cycle constant temperature control sub-circuit is used to control the working state of the heating element. The voltage value fed back by the temperature feedback sub-circuit is used to adjust the PWM duty cycle to achieve constant temperature heating of the heating element.

[0042] Compared with the prior art, this utility model adopts a "split" arrangement of two circuits, so that the ambient resistance and the heating resistance are in different circuits. Compared with the traditional constant temperature control circuit, the constant temperature control effect is better. At the same time, it does not require re-soldering external resistors to change the temperature of the heating element, making it more adaptable.

[0043] Example 2

[0044] This second embodiment is a preferred embodiment of the present invention, such as... Figure 2 As shown, it discloses the following improvements based on implementation method 1:

[0045] The constant temperature control sub-circuit includes:

[0046] MOS transistor 30 is connected to the microcontroller for communication.

[0047] A resistor 31 is placed between the MOSFET 30 and the microcontroller 5 in parallel.

[0048] The heating element 1 is electrically connected to the MOS transistor 30.

[0049] Working principle: such as Figure 2 R1 and R2 are external resistors, R3 is the heating element, and Q1 is a MOSFET. By inputting voltage signals with different duty cycles, the switching on and off of the MOSFET is controlled, thereby controlling the heating time of the heating element (i.e., when the voltage is at its peak and at a high level, the MOSFET will turn on and the heating element will start heating; when the voltage is at its trough and at a low level, the MOSFET will turn off and the heating element will stop heating).

[0050] Existing temperature control circuits control temperature by varying the resistance between circuits, such as... Figure 4 As shown, this utility model uses an STM32 microcontroller to control the MOSFET through code by controlling the PWM duty cycle, thereby achieving constant temperature heating control of the heating element.

[0051] Example 3

[0052] This third embodiment is a preferred embodiment of the present invention, such as... Figure 3 As shown, it discloses the following improvements based on implementation method 1:

[0053] The temperature feedback sub-circuit includes:

[0054] A thermistor 40 is positioned near the heating element;

[0055] Operational amplifier 41 is electrically connected to the thermistor 40;

[0056] The operational amplifier 40 is communicatively connected to the microcontroller 5.

[0057] Working principle: In Figure 3 In this circuit, R5 and R6 are external resistors, R4 is the thermistor 40 (located near the heating element, its resistance changes with the temperature of the heating element; the higher the temperature, the lower the resistance), and U1 is operational amplifier 41 (which forms a voltage follower; the positive input voltage of the voltage follower equals the output voltage). When the heating element 1 heats up, R4 continuously changes, and the positive input voltage of the operational amplifier circuit (which equals the output voltage) continuously changes (the smaller R4 is, the smaller the output voltage). The voltage value corresponding to the predetermined temperature can be used as a reference. When the real-time output voltage is greater than the reference value, the duty cycle continuously increases; when the output voltage is less than the reference value, the duty cycle continuously decreases, thus adaptively adjusting the heating element to handle the rated constant temperature operating state.

[0058] In existing temperature control circuits, the ambient resistance and the heating resistance are in the same circuit, such as... Figure 4 This invention uses a microcontroller to control the PWM duty cycle, so the ambient resistance and the heating resistance are in different circuits. The resistance change of the ambient resistance is fed back through a voltage signal, and the microcontroller adjusts the PWM duty cycle based on the feedback received.

[0059] It should be noted that, because the duty cycle control circuit gradually increases and decreases, the temperature change is relatively smooth and stable, and the response is fast. To fully illustrate the issue, the following application examples 1-3 are used for verification:

[0060] Application Example 1:

[0061] The results in Table 1 are obtained by detecting temperature changes over 1 to 5 seconds at a constant temperature of 50℃.

[0062] Table 1

[0063] Detection time 1s 2s 3s 4s 5s Real-time temperature monitoring 49.7℃ 54.6℃ 53.5℃ 49.8℃ 50.8℃

[0064] Application Example 2:

[0065] The results in Table 2 are obtained by detecting temperature changes over 1 to 5 seconds at a constant temperature of 100℃.

[0066] Table 2

[0067] Detection time 1s 2s 3s 4s 5s Real-time temperature monitoring 103.8℃ 104.8℃ 105.9℃ 103.2℃ 102.8℃

[0068] Application Example 3:

[0069] The results in Table 3 are obtained by detecting temperature changes over 1 to 5 seconds at a constant temperature of 150℃.

[0070] Table 3

[0071] Detection time 1s 2s 3s 4s 5s Real-time temperature monitoring 151.9℃ 152.5℃ 153.3℃ 155.0℃ 153.1℃

[0072] The above three application examples show that the temperature change using the duty cycle control circuit is relatively smooth, with little temperature fluctuation and good stability.

[0073] The above solutions are merely illustrative examples of preferred embodiments, but are not limited thereto. When implementing this invention, appropriate substitutions and / or modifications can be made according to the user's needs.

[0074] The number of devices and processing scale described herein are for the purpose of simplifying the description of this utility model. Applications, modifications, and variations of this utility model will be readily apparent to those skilled in the art.

[0075] Although embodiments of this utility model have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for this utility model. Other modifications can be readily implemented by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, this utility model is not limited to the specific details and examples shown and described herein.

Claims

1. A constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control, wherein the calorimetric flow sensor includes a heating element and two temperature sensing elements disposed on both sides of the heating element for acquiring changes in the flow field temperature, characterized in that, The constant temperature control circuit includes: A PWM duty cycle constant temperature control sub-circuit electrically connected to the heating element; A temperature feedback sub-circuit positioned near the heating element; The PWM duty cycle constant temperature control sub-circuit and the temperature feedback sub-circuit are both connected to the microcontroller.

2. The constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control as described in claim 1, characterized in that, The constant temperature control sub-circuit includes: The MOS transistor that communicates with the microcontroller; A resistor connected in parallel between the MOSFET and the microcontroller; The heating element is electrically connected to the MOS transistor.

3. The constant temperature control circuit for a calorimetric flow sensor based on PWM duty cycle control as described in claim 1, characterized in that, The temperature feedback sub-circuit includes: A thermistor positioned close to the heating element; An operational amplifier electrically connected to a thermistor; The operational amplifier is connected to the microcontroller for communication.

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