CN111765931A

CN111765931A - Electromagnetic flowmeter excitation control system based on differential compensation PFM modulation

Info

Publication number: CN111765931A
Application number: CN202010636380.3A
Authority: CN
Inventors: 黄云志; 赵亚伪; 华启国; 杨双龙; 梁利平
Original assignee: Hefei University of Technology; Anhui Tiankang Group Co Ltd
Current assignee: Hefei University of Technology; Anhui Tiankang Group Co Ltd
Priority date: 2020-07-03
Filing date: 2020-07-03
Publication date: 2020-10-13
Anticipated expiration: 2040-07-03
Also published as: CN111765931B

Abstract

The invention discloses an electromagnetic flowmeter excitation control system based on differential compensation PFM modulation, which consists of a direct-current power supply, a PFM controller, a filter circuit, a ripple compensation circuit, a differential compensation circuit, an amplitude limiting feedback circuit, an excitation time sequence circuit, an excitation coil driving circuit and a current sampling circuit. And the PFM modulation technology is adopted to realize constant current control, improve the system efficiency and reduce the system power consumption and the system temperature rise. And the differential compensation circuit is used for inhibiting the overshoot of the exciting current, reducing the overshoot and dynamic adjustment time of the exciting current and improving the exciting frequency. The ripple compensation circuit is used for reducing the exciting current ripple and improving the precision of the exciting current. The excitation time sequence circuit generates two-way complementary rectangular wave excitation signals with dead zones, the excitation coil driving circuit is controlled, and the output end of the excitation coil driving circuit is connected with the excitation coil to realize excitation current commutation. The excitation control system can improve the excitation frequency, reduce the power consumption of an excitation system and the temperature rise of the system, and reduce the excitation current ripple.

Description

Electromagnetic flowmeter excitation control system based on differential compensation PFM modulation

Technical Field

The invention relates to the field of flow measurement, in particular to an electromagnetic flowmeter excitation control system based on differential compensation PFM modulation, which realizes high-frequency rectangular wave excitation, reduces the power consumption and temperature rise of an excitation system, reduces excitation current ripples, and is suitable for high-speed measurement of slurry fluid.

Background

The electromagnetic flowmeter is an instrument for measuring the flow of conductive liquid, and the working principle of the electromagnetic flowmeter is Faraday's law of electromagnetic induction. The current is introduced into the magnet exciting coil to establish a magnetic field, the conductive liquid cuts a magnetic induction line in the magnetic field, electromotive force can be induced at two ends of the conductive liquid, and the electromotive force is in direct proportion to the flow rate of the liquid, so that the flow rate of the conductive liquid can be measured by detecting the induced electromotive force, and the volume and the quality of the liquid can be measured. The electromagnetic flowmeter consists of a primary instrument sensor and a secondary instrument transmitter, and the transmitter consists of a signal conditioning system and an excitation control system. In the current application, low-frequency rectangular wave excitation is the main excitation mode of the electromagnetic flowmeter.

In order to measure a slurry fluid at a high speed and suppress low-frequency noise such as slurry, it is necessary to increase the excitation frequency of the electromagnetic flowmeter to obtain a quick response performance of the electromagnetic flowmeter. The excitation period is shortened due to the increase of the excitation frequency, the steady-state time of the excitation current is short or the excitation current cannot enter the steady state, the acquisition of the output signal of the sensor of the electromagnetic flowmeter is not facilitated, and the zero drift phenomenon exists in the output signal of the sensor of the electromagnetic flowmeter; meanwhile, the power consumption of an excitation system is large, the temperature is increased, and the reliability of the long-term work of the electromagnetic flowmeter is influenced.

In order to realize high-frequency excitation and ensure a steady-state section of the excitation current for a certain time so as to collect an output signal of the sensor, the rise time and the dynamic adjustment time of the excitation current need to be reduced. The high-voltage direct-current power supply is applied, so that the rising speed of the exciting current can be accelerated, the rising time of the exciting current is reduced, the high-voltage direct-current power supply causes the overshoot of the exciting current, and the dynamic adjustment time is increased; meanwhile, the ratio of the product of the exciting current and the direct-current resistance of the exciting coil to the high-voltage direct-current power supply is small, and the efficiency of an excitation control system is low.

At present, an excitation control system of an electromagnetic flowmeter is mainly based on two design principles, one is based on a linear power supply principle, and the other is based on a switching power supply principle.

Based on the linear power supply principle, a constant current source circuit is built in a linear region by utilizing a linear power supply chip or controlling the work of a transistor. For example, chinese patent publication No. CN105841761A discloses a constant current control system for a field coil of a variable frequency electromagnetic flowmeter, and chinese patent publication No. CN103759773A discloses a field voltage adjusting method, a control circuit, and a field circuit of an electromagnetic flowmeter. When high-frequency excitation is realized based on the linear power supply principle, the voltage drop at two ends of a linear power supply chip or a transistor is overlarge, so that the power consumption is larger, and a larger radiating fin is required to be added to reduce the temperature rise of a constant current control device. The patent disclosed in chinese patent publication No. CN105841761A adopts high-low voltage power supply switching to realize high-frequency excitation in frequency conversion, and has the problems of complicated control circuit for switching high-voltage and low-voltage power supplies, long switching time in working state, large excitation power consumption due to the linear power supply principle adopted for constant current control, and the like.

Based on the principle of a switching power supply, a constant current source circuit is constructed by utilizing a switching power supply chip or controlling the high-frequency on-off of a switching tube to perform chopping control on a direct current power supply. For example, chinese patent publication No. CN105021241 discloses an excitation control system of an electromagnetic flowmeter based on PWM modulation of current error control, which utilizes the characteristic of inductive load that the current in an excitation coil cannot change suddenly and the current of the excitation coil changes little within a minute time to control the on/off of an H-bridge high-end switch through a feedback current to perform constant current control, when an H-bridge high-end switch device is turned on, the excitation current rises, and when turned off, the excitation current falls, so that the excitation current fluctuates within a minute range, the excitation frequency is increased, and there are problems of large excitation current ripple, low PWM modulation light load efficiency, and the like.

In summary, when the inductance value of the excitation coil and the excitation current are large, the high-frequency rectangular wave excitation has the problems of large system power consumption, low light-load efficiency, short steady-state time of the excitation current, large current ripple and the like.

Disclosure of Invention

In order to overcome the defects in the prior art, the invention provides the excitation control system of the electromagnetic flowmeter based on the differential compensation PFM modulation, which can realize high-frequency rectangular wave excitation and has the characteristics of high excitation frequency, high response speed, low system power consumption, low temperature rise and small excitation current ripple.

In order to achieve the purpose, the invention adopts the following technical scheme:

the system comprises a direct current power supply, a PFM controller, a filter circuit, a ripple compensation circuit, a differential compensation circuit, an amplitude limiting feedback circuit, an excitation time sequence circuit, an excitation coil driving circuit and a current sampling circuit;

the direct-current power supply is a power supply of an excitation control system, the PFM controller is a constant-current control core of the system, and the output voltage of the direct-current power supply is chopped by adopting a PFM modulation technology; the filter circuit is connected to the rear stage of the PFM controller, filters high-frequency noise generated by high-frequency on-off of the power switch tube, stores energy when the power switch tube is switched on, and provides energy for the excitation coil connected with the excitation coil driving circuit when the power switch tube is switched off; the output signal of the filter circuit is sent to the excitation coil drive circuit; the excitation coil driving circuit is connected with an excitation coil, and realizes excitation current commutation under the drive of two-way complementary rectangular wave signals with dead zones generated by the excitation sequential circuit; the ripple compensation circuit acquires ripple signals at two ends of an inductor in the filter circuit, and the ripple signals at two ends of the inductor are sent to the PFM controller through a coupling capacitor; the amplitude limiting feedback circuit is positioned between the filter circuit and the reference ground, stores electric energy released by the exciting coil when the exciting current commutates, and limits the amplitude of the back electromotive force of the exciting coil; the current sampling circuit converts an exciting current signal into a voltage signal and sends the voltage signal to the differential compensation circuit; and the differential compensation circuit carries out differential compensation on the excitation current signal and then sends the excitation current signal to the PFM controller.

The PFM controller consists of a switching power supply chip U1, a turn-on timing resistor R1 and a bootstrap capacitor C3; the switching power supply chip U1 is a synchronous rectification step-down DC/DC converter to realize pulse frequency control; the on-timing resistor R1 sets the constant on-time of the high-voltage side power switch tube; the bootstrap capacitor C3 cooperates with the switching power supply chip U1 to drive the high-side power switch tube.

The ripple compensation circuit consists of a resistor R2, a capacitor C5 and a capacitor C6; a resistor R2 and a capacitor C5 in the circuit are used for generating ripples which are in phase with inductive current in a filter circuit, and the ripples are reduced by alternating current coupling of the capacitor C6 to a feedback pin of a switching power supply chip U1.

The differential compensation circuit consists of an integrated operational amplifier U2, resistors R3, R4, R5, R6 and a capacitor C7; the exciting current signal is amplified and subjected to differential compensation and then sent to a feedback pin FB of a switch power supply chip U1, so that the overshoot and dynamic adjustment time of the exciting current is reduced, and the exciting frequency of the system is improved.

The excitation sequential circuit consists of a digital signal processor chip U4 and an optical coupling isolation circuit; the optical coupling isolation circuit consists of an optical coupling isolation chip U3, a resistor R7, R8, R9, R10, R11, R12, R13, R14, a triode T1 and a triode T2, and is used for isolating a digital signal processor and a magnet exciting coil driving circuit; an ePWM module arranged on an on-chip peripheral of a digital signal processor chip U4 generates two-way complementary rectangular wave excitation timing signals PWM1 and PWM2 with dead zones, and excitation timing signals CON1 and CON2 are generated after optical coupling isolation.

The invention adopts PFM modulation technology to realize constant current control, utilizes a direct current power supply to accelerate the rising speed of exciting current, and utilizes the characteristic of fast transient response speed of PFM control technology to reduce the dynamic adjusting time of exciting current; the efficiency of the constant current source is improved by utilizing the characteristic of high light load efficiency of the PFM, the power consumption of an excitation system is reduced, the temperature rise of the system is reduced, and the reliability of long-term work of the electromagnetic flowmeter is ensured; differential compensation is adopted to reduce the overshoot of the system, so that the exciting current reaches a steady state as soon as possible, and the exciting frequency of the system is improved; the ripple compensation circuit is adopted to reduce the ripple of the exciting current and improve the precision of the exciting current; an excitation time sequence circuit is built by adopting a digital signal processor and an optical coupling isolation technology, an ePWM module on/off a chip of the digital signal processor generates an excitation time sequence signal, and the excitation time sequence signal is transmitted to an H bridge driving chip after optical coupling isolation; an H-bridge circuit is built by four N-channel MOS tubes with low on-resistance, so that the on-time and the off-time of the switching tubes are basically consistent; an H-bridge bootstrap drive circuit is built by using a full-bridge drive chip and a bootstrap capacitor, the switching-on and switching-off speed of a switching tube is accelerated, the exciting current follow current time is reduced, the exciting frequency is improved, and the power consumption of an exciting coil drive circuit is reduced.

The working process of the excitation system comprises the following steps: dividing each excitation half period into four stages of current rising, current regulation, current steady state and current continuous flow according to the state of the excitation current; in the exciting current rising stage, the current sampling circuit detects exciting current, exciting current signals are conditioned by the differential compensation circuit and are smaller than a current set value in the PFM controller, the PFM controller outputs pulse square waves with higher frequency and fixed width, at the moment, a direct-current power supply is applied to an exciting coil through an exciting coil driving circuit 8 after being chopped by the PFM controller and filtered by the filter circuit, and the exciting current in the exciting coil rises rapidly; in the field current adjusting stage, when the field current value is larger than the set field current value, the PFM controller outputs a low level, the power switch tube is switched off, the filter circuit provides energy for the field coil, and the field current is reduced; when the exciting current value is smaller than the set exciting current value, the PFM controller outputs a high level, the power switch tube is conducted, the direct-current power supply provides energy for the exciting coil, and the exciting current rises; in the steady-state stage of the exciting current, the PFM controller outputs pulse square waves with lower frequency and fixed width, so that the current in the exciting coil is maintained at a set value; in the excitation current follow current stage, the energy in the excitation coil is absorbed by an energy storage capacitor in the amplitude limiting feedback circuit; after the exciting current follow current stage is finished, the next exciting current rising stage is entered, and the energy storage capacitor in the amplitude limiting feedback circuit charges the exciting coil, so that the rising speed of the exciting current is accelerated, and the energy utilization rate is improved.

The invention has the advantages that:

(1) the constant current control is realized by utilizing a PFM modulation technology, the rising speed of the exciting current is accelerated by utilizing a direct current power supply, and the dynamic adjusting time of the exciting current is shortened by utilizing the characteristic of high transient response speed of the PFM modulation technology; the characteristic of high light load efficiency of the PFM modulation technology is utilized to improve the efficiency of the constant current source, reduce the power consumption of an excitation system, reduce the temperature rise of the system and ensure the reliability of long-term work of the electromagnetic flowmeter.

(2) The invention utilizes the ripple compensation circuit to reduce the exciting current ripple, improve the exciting current precision and improve the signal-to-noise ratio of the output signal of the electromagnetic flowmeter sensor.

(3) The invention utilizes the differential compensation circuit to inhibit the overshoot of the exciting current, reduces the overshoot and dynamic regulation time of the exciting current, enables the exciting current to reach the steady state as soon as possible, and improves the exciting frequency.

Drawings

Fig. 1 is a block diagram of an excitation control system according to an embodiment of the present invention.

Fig. 2 is a block diagram of a constant current control section according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a constant current control section according to an embodiment of the present invention.

Fig. 4 is a schematic diagram of the effect of compensating the excitation current differential according to the embodiment of the present invention.

Fig. 5 is a schematic diagram of a field coil drive circuit according to an embodiment of the present invention.

Fig. 6 is a schematic diagram of an excitation timing circuit according to an embodiment of the present invention.

Fig. 7 is a diagram illustrating a control signal and an excitation current signal according to an embodiment of the present invention.

Fig. 8 is a schematic diagram of the flow of the excitation current according to the embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Fig. 1 is a block diagram of an excitation control system of the present invention, and the system includes a dc power supply 1, a PFM controller 2, a filter circuit 3, a ripple compensation circuit 4, a differential compensation circuit 5, an amplitude limiting feedback circuit 6, an excitation timing circuit 7, an excitation coil driving circuit 8, and a current sampling circuit 9. The direct current power supply 1 is a power supply of an excitation control system, the PFM controller 2 is a constant current control core of the system, and the output voltage of the direct current power supply 1 is chopped by adopting a pulse frequency modulation technology; the filter circuit 3 is connected to the rear stage of the PFM controller 2, filters high-frequency noise generated by high-frequency on-off of the power switch tube, stores energy when the power switch tube is switched on, and provides energy for the excitation coil connected with the excitation coil driving circuit 8 when the power switch tube is switched off; the output signal of the filter circuit 3 is sent to the excitation coil driving circuit 8; the excitation coil driving circuit 8 is connected with an excitation coil and realizes excitation current commutation under the drive of two-way complementary rectangular wave signals with dead zones generated by the excitation sequential circuit 7; the ripple compensation circuit 4 acquires ripple signals at two ends of an inductor in the filter circuit 3, and sends the ripple signals at two ends of the inductor to the PFM controller 2 through the coupling capacitor; the amplitude limiting feedback circuit 6 is positioned between the filter circuit 3 and the reference ground, stores electric energy released by the exciting coil when the exciting current commutates, and limits the amplitude of the back electromotive force of the exciting coil; the current sampling circuit 9 converts the exciting current signal into a voltage signal and sends the voltage signal to the differential compensation circuit 5; the differential compensation circuit 5 performs differential compensation on the excitation current signal and then sends the excitation current signal to the PFM controller 2.

Fig. 2 is a block diagram of a constant current control portion of the excitation control system of the present invention, which includes a dc power supply, a PFM controller, a filter circuit, an excitation coil, a current sampling circuit, a differential compensation circuit, and a ripple compensation circuit. The constant current control part of the invention is based on the feedback control of the current error, detect the exciting current in the field coil through the current sampling circuit, the exciting current signal after compensating the circuit through differentiating adds with ripple compensating signal and gets the feedback signal, the feedback signal subtracts with the current set value, when the feedback signal is greater than the current set value, PFM controller outputs the low level, the high-pressure side power switch tube is turned off, the filter circuit provides the energy for the field coil, the exciting current drops; when the feedback signal is smaller than the current set value, the PFM controller outputs high level, the high-voltage side power switch tube is conducted, the direct current power supply provides energy for the magnet exciting coil through the filter circuit, and the magnet exciting current rises, so that the current in the magnet exciting coil is maintained at the set current value.

Fig. 3 is a schematic diagram of a constant current control part of the system of the present invention, which includes a dc power supply 1, a PFM controller 2, a filter circuit 3, a ripple compensation circuit 4, a differential compensation circuit 5, an amplitude limiting feedback circuit 6, and a current sampling circuit 9.

The direct current power supply 1 consists of a high-voltage direct current power supply E1, a decoupling capacitor C1 and a bypass capacitor C2. The high-voltage direct-current power supply E1 provides a power supply for the excitation system, accelerates the rising speed of the excitation current and reduces the rising time of the excitation current.

The PFM controller 2 is composed of a switching power supply chip U1, a turn-on timing resistor R1, and a bootstrap capacitor C3, and serves as a system control core. The switching power supply chip U1 is a synchronous rectification step-down DC/DC converter, and integrates a PFM controller and two power switching tubes on a high-voltage side and a low-voltage side, wherein the power switching tube on the high-voltage side is used for realizing voltage chopping, and the power switching tube on the low-voltage side is used for realizing synchronous rectification, so that a loop is provided for current, and the utilization efficiency of a power supply is improved. The on-timing resistor R1 is used to set the constant on-time of the high side power switch. The bootstrap capacitor C3 cooperates with the switching power supply chip U1 to drive the high-side power switch tube.

The filter circuit 3 is composed of an inductor L1 and a capacitor C4. The inductor L1 and the capacitor C4 are used for filtering high-frequency noise generated by high-frequency switching of the power switch tube, reducing voltage ripples applied to the magnet exciting coil, storing energy when the high-voltage side power tube is conducted, and providing energy for a load when the low-voltage side power tube is conducted.

The ripple compensation circuit 4 is composed of a resistor R2, a capacitor C5, and a capacitor C6. The resistor R2 and the capacitor C5 are ac-coupled to the feedback pin of the switching power supply chip U1 through the capacitor C6 with a ripple that is in phase with the voltage ripple. Ripple voltage DeltaV_FBThe calculation formula is shown as formula (1), wherein E1 is the voltage value of the high-voltage direct-current power supply, V_OFor the output voltage of the filter circuit 3, TON is the constant on-time of the high-side power switch tube.

The differential compensation circuit 5 is composed of an integrated operational amplifier U2, resistors R3, R4, R5, R6 and a capacitor C7. Exciting current signals are sent to a feedback pin of a switch power supply chip U1 after in-phase amplification and differential compensation, so that overshoot and dynamic adjustment time of the system is reduced, and the excitation frequency of the system is improved; the transfer function g(s) of the differential compensation circuit is shown in formula (2), wherein the resistors R3, R4, R5 and R6 determine the dc gain of the signal, and the resistors R5 and R6 and the capacitor C7 determine the ac gain and the leading phase of the signal.

s is the Laplace operator.

The amplitude limiting feedback circuit 6 consists of a diode D1, a voltage stabilizing diode Z1 and a capacitor C8. The diode D1 is used to prevent the exciting current from flowing back to the circuit in the front stage of the diode D1, and thus protect the circuit. The voltage stabilizing diode Z1 plays a role in amplitude limiting of the counter electromotive force of the exciting coil of the H bridge, and therefore the high-end MOS tube of the H bridge is guaranteed to work normally. The capacitor C8 is used for storing the electric energy released by the exciting coil and charging the exciting coil when the exciting current rises, so that the rising speed of the exciting current in the exciting coil is accelerated, and the utilization efficiency of the system is improved.

The current sampling circuit 9 is composed of a current sampling resistor Rs. The current sampling resistor Rs is connected in series with the excitation coil and is used for converting an excitation current signal into a voltage signal.

And (3) constant current process: converting exciting current into voltage through a current sampling resistor Rs, adding the voltage to ripple compensation signals after differential compensation, and sending the voltage to a feedback pin of U1, wherein when the voltage sent to the feedback pin is less than reference voltage, the U1 controls an internal high-voltage side MOS tube to be conducted for a constant time, and exciting voltage is applied to an exciting coil after being filtered by an inductor and a capacitor so that exciting current rises; when the voltage sent to the feedback pin is greater than the reference voltage, the U1 controls the internal high-voltage side MOS tube to be turned off, the filter inductor and the capacitor supply power to the excitation coil, and the excitation current is reduced, so that the excitation current is maintained at a set value.

Fig. 4 is a schematic diagram illustrating the differential compensation effect of the exciting current of the excitation control system of the present invention, wherein a in fig. 4 is a waveform diagram of the exciting current without compensation, and b in fig. 4 is a waveform diagram of the exciting current after compensation. When the exciting current is positive as shown in fig. 4, the excitation is called forward excitation; when the excitation current is negative, it is called negative excitation. t0-t6 is an excitation period, and comprises positive excitation processes t0-t4 and negative excitation processes t4-t 6. The forward excitation comprises excitation current rising t0-t1, excitation current regulation t1-t2, excitation current steady state t2-t3 and excitation current follow current t3-t 4. As shown in a in fig. 4, when the high voltage dc power supply E1 is excited, the excitation current rises rapidly, and there are problems of large overshoot of the excitation current, long dynamic adjustment time, short steady-state time, and the like, which are not favorable for increasing the excitation frequency. As shown in fig. 4 b, the compensated field current adjustment t1 '-t 2' is less than the uncompensated field current adjustment t1-t 2. Therefore, differential compensation is adopted, current overshoot and dynamic regulation time are reduced, the exciting current quickly enters a steady state, and the exciting frequency is improved.

Fig. 5 is a schematic diagram of an excitation sequential circuit 7 of the excitation control system of the invention, and the excitation sequential circuit 7 is composed of a digital signal processor chip U4 and an optical coupling isolation circuit. The optical coupling isolation circuit is composed of an optical coupling isolation chip U3, resistors R7, R8, R9, R10, R11, R12, R13, R14, a triode T1 and a triode T2. The excitation timing signals PWM1 and PWM2 are two-way complementary rectangular waves with dead zones generated by an on-chip and peripheral ePWM module of the digital signal processor chip U4. R7 and R14 are pull-up resistors, R8 and R13 are base resistors of triodes T1 and T2, and R9, R10, R11 and R12 are current-limiting resistors. The triodes T1 and T2 work in saturation conducting and turn-off states, when an excitation timing signal PWM1 or PWM2 is at a low level, the triode T1 or T2 works in a turn-off state, a light emitting diode in the optical coupling isolation chip U4 is in a turn-off state, the phototriode is in a turn-off state, and the CON1 or CON2 is at a high level; when the excitation timing signal PWM1 or PWM2 is at a high level, the transistor T1 or T2 operates in a conducting state, the light emitting diode inside the optocoupler isolation chip U4 is in a conducting state, the phototransistor is in a conducting state, and the CON1 or CON2 is at a low level. The triodes T1 and T2 are used for improving the current output capability of the pin U4 of the digital signal processor chip, and the optical coupling isolation is used for protecting the digital signal processor chip U4.

Fig. 6 is a schematic diagram of the exciting coil driving circuit 8 of the excitation control system of the present invention, and the exciting coil driving circuit 8 is composed of an H-bridge 8.1 and an H-bridge driving circuit 8.2. The H bridge 8.1 consists of four voltage control devices, namely N-channel MOS tubes Q1, Q2, Q3 and Q4, and protective diodes are connected in parallel between drain electrodes and source electrodes in the N-channel MOS tubes Q1, Q2, Q3 and Q4. The drains of the high ends of the H-bridge, namely Q1 and Q4, are powered by a constant current control circuit based on PFM control, and the sources of the low ends of the H-bridge, namely Q2 and Q3, are connected with the ground through a current sampling resistor Rs. The source of Q1 is connected to the drain of Q2 and connected to the CD1 terminal of the driving excitation coil L, and the source of Q4 is connected to the drain of Q3 and connected to the CD2 terminal of the driving excitation coil L. The H-bridge driving circuit 8.2 comprises a driving chip U5, resistors R15, R16, R17, R18, capacitors C9 and C10. When CON1 is at a high level or a low level, the driving chip U5, in cooperation with the resistor R15 and the capacitor C9, controls the voltages of the gate and the source of the MOS transistor Q1, so that the MOS transistor Q1 operates in a saturated on or off state; the driving chip U5 controls the voltage of the gate and the source of the MOS transistor Q3 by matching with the resistor R18, so that the MOS transistor Q1 operates in a saturation on or off state, thereby realizing the linked switch control of the MOS transistors Q1 and Q3. When CON2 is at a high level or a low level, the driving chip U5, in cooperation with the resistor R16 and the capacitor C10, controls the voltages of the gate and the source of the MOS transistor Q4, so that the MOS transistor Q4 operates in a saturated on or off state; the driving chip U5 controls the voltage of the gate and the source of the MOS transistor Q2 by matching with the resistor R17, so that the MOS transistor Q2 operates in a saturation on or off state, thereby realizing the linked switch control of the MOS transistors Q2 and Q4.

Fig. 7 is a schematic diagram of a control signal and an excitation current signal of an excitation control system according to the present invention, where CON1 is a control signal of MOS transistors Q1 and Q3, CON2 is a control signal of MOS transistors Q2 and Q4, SW is a signal output by a PFM controller, and I is an excitation current waveform diagram. When the control signal is at a high level, the MOS tube is conducted; when the control signal is at low level, the MOS tube is turned off. In the excitation current rising stage, the frequency of the SW signal is higher; in the steady-state stage of the exciting current, the frequency of the SW signal is lower.

Fig. 8 is a schematic view showing the flow direction of the excitation current in the excitation control system according to the present invention. The excitation process is explained in conjunction with fig. 7 and 8. At the rising stage of the exciting current from t0 to t1, an exciting control signal CON1 is at a high level, CON2 is at a low level, MOS tubes Q1 and Q3 are switched on, Q2 and Q4 are switched off, the exciting current in the exciting coil is detected through a current sampling resistor Rs, the exciting current signal passing through a differential compensation circuit 5 and a ripple signal of a ripple compensation circuit 4 are added to obtain a feedback signal, the feedback signal is smaller than a current set value, a PFM controller 2 outputs a pulse with a fixed width at a higher frequency, at the moment, a direct current power supply 1 is chopped by the PFM controller 2 and filtered by a filter circuit 3, and the pulse is applied to the exciting coil through an exciting coil driving circuit 8 to enable the exciting current to rise; in the excitation current adjusting stage of t1-t2, when the excitation current value is larger than the set excitation current value, the PFM controller 2 outputs a low level control signal, the power switch tube is turned off, the filter circuit 3 provides energy for the excitation coil, and the excitation current is reduced; when the exciting current value is smaller than the set exciting current value, the PFM controller 2 outputs a high-level control signal, the power switch tube is conducted, the direct-current power supply 1 provides energy for the exciting coil, and the exciting current rises; in the steady-state stage of the exciting current from t2 to t3, the PFM controller 2 outputs a fixed-width pulse with a lower frequency, so that the current in the exciting coil is maintained at a set value; in the excitation current freewheeling stage from t3 to t4, the excitation current cannot change suddenly due to the inductive load characteristic of the excitation coil, and the excitation current feeds back energy to the capacitor C8 in the amplitude limiting feedback circuit 6 through the MOS tube protection diode. When the capacitance value of the capacitor C8 is small, the voltage value of the capacitor C8 is higher than the high-voltage direct-current voltage value due to the fed back energy, and the energy is fed back to the excitation coil when the next excitation current rises, so that the rise speed of the excitation current is accelerated, and the energy utilization rate is improved.

In the excitation period, the excitation current loop of the section t0-t3 is constant current output → D1 → MOS tube Q1 → excitation coil CD1 → excitation coil CD2 → MOS tube Q3 → current sampling resistor Rs → reference ground, as shown in path I in FIG. 8; in the section t3-t4 of the freewheeling process, the freewheeling loop is the excitation coil CD2 → the MOS transistor Q4 protection diode → the capacitor C8 → the reference ground → the current sampling resistor Rs → the MOS transistor Q2 protection diode → the excitation coil CD1, as shown in the path ii in fig. 8; the excitation current loop of the section t4-t5 is constant current output → D1 → MOS tube Q4 → excitation coil CD2 → excitation coil CD1 → MOS tube Q2 → current sampling resistor Rs → reference ground, as shown in path III in FIG. 8; during the freewheel process segment t5-t6, the freewheel loop is the field coil CD1 → the MOS transistor Q1 protection diode → the capacitor C8 → the reference ground → the current sampling resistor Rs → the MOS transistor Q3 protection diode → the field coil CD2, as shown by the path iv in fig. 8.

In the embodiment, the power consumption of the excitation control system of the electromagnetic flowmeter based on differential compensation PFM modulation of the present invention is analyzed and compared with the excitation control system based on linear power supply in the prior art and the power consumption of the excitation control system of the electromagnetic flowmeter based on PWM modulation of current error control under the conditions of excitation frequency of 6.25Hz, excitation voltage of 24V and excitation current of 200mA, and the comparison results are shown in table 1 below:

TABLE 1 excitation System Power consumption analysis

Through comparison, the electromagnetic flowmeter excitation control system based on the differential compensation PFM has the advantages that the constant current source efficiency is highest, and the power consumption of the excitation system is lowest.

The invention is not to be considered as limited to the specific embodiments shown and described, but is to be understood to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Claims

1. An electromagnetic flow meter excitation control system based on differential compensation PFM modulation is characterized in that: the device comprises a direct current power supply (1), a PFM controller (2), a filter circuit (3), a ripple compensation circuit (4), a differential compensation circuit (5), an amplitude limiting feedback circuit (6), an excitation time sequence circuit (7), an excitation coil driving circuit (8) and a current sampling circuit (9);

the direct current power supply (1) is a power supply of an excitation control system;

the PFM controller (2) is a system constant current control core, and the output voltage of the direct current power supply (1) is chopped by adopting a PFM modulation technology;

the filter circuit (3) is connected to the rear stage of the PFM controller (2) to filter high-frequency noise generated by high-frequency on-off of the power switch tube, and meanwhile, the filter circuit stores energy when the power switch tube is switched on and provides energy for the magnet exciting coil connected with the magnet exciting coil driving circuit (8) when the power switch tube is switched off;

the output signal of the filter circuit (3) is sent to the excitation coil driving circuit (8);

the excitation coil driving circuit (8) is connected with an excitation coil, and the excitation current commutation is realized under the drive of two-way complementary rectangular wave signals with dead zones generated by the excitation time sequence circuit (7);

the ripple compensation circuit (4) acquires ripple signals at two ends of an inductor in the filter circuit (3), and the ripple signals at two ends of the inductor are sent to the PFM controller (2) through a coupling capacitor;

the amplitude limiting feedback circuit (6) is positioned between the filter circuit (3) and a reference ground, stores electric energy released by the exciting coil when the exciting current commutates, and limits the amplitude of the back electromotive force of the exciting coil;

the current sampling circuit (9) converts an exciting current signal into a voltage signal and sends the voltage signal to the differential compensation circuit (5);

and the differential compensation circuit (5) carries out differential compensation on the excitation current signal and then sends the excitation current signal to the PFM controller (2).

2. The excitation control system of an electromagnetic flowmeter based on differential compensation PFM modulation as claimed in claim 1, wherein: the PFM controller (2) consists of a switching power supply chip U1, a turn-on timing resistor R1 and a bootstrap capacitor C3; the switching power supply chip U1 is a synchronous rectification step-down DC/DC converter and adopts a pulse frequency control technology; the on-timing resistor R1 sets the constant on-time of the high-voltage side power switch tube; the bootstrap capacitor C3 is matched with the switching power supply chip U1 to drive the high-voltage side power switch tube; constant current control is realized, system efficiency is improved, and system power consumption and system temperature rise are reduced.

3. The excitation control system of an electromagnetic flowmeter based on differential compensation PFM modulation as claimed in claim 1, wherein: the ripple compensation circuit (4) consists of a resistor R2, a capacitor C5 and a capacitor C6; a resistor R2 and a capacitor C5 in the circuit are used for generating ripples which are in phase with inductive current in a filter circuit, and the ripples are reduced by alternating current coupling of the capacitor C6 to a feedback pin of a switching power supply chip U1.

4. The excitation control system of an electromagnetic flowmeter based on differential compensation PFM modulation as claimed in claim 1, wherein: the differential compensation circuit (5) is composed of an integrated operational amplifier U2, resistors R3, R4, R5, R6 and a capacitor C7; the exciting current signal is amplified and subjected to differential compensation and then sent to a feedback pin FB of a switch power supply chip U1, so that the overshoot and dynamic adjustment time of the exciting current is reduced, and the exciting frequency of the system is improved.