Direct current motor parameter on-line measurement system

Abstract

The invention relates to the field of motor driving, in particular to a direct current motor parameter on-line measuring system which comprises a digital control unit, an H bridge, a first analog comparator, a second analog comparator, a first analog-to-digital converter and a second analog-to-digital converter. The first analog comparator and the second analog comparator compare the tail current with a preset current value given by the digital control unit, the comparison result is sent to the digital control unit, the digital control unit carries out calculation processing on the comparison result, accurate detection of the inductance, the resistance, the rotating speed, the average current I and the current ripple of the motor is achieved, the system does not need an additional sensor, and the cost is reduced. Only two comparators are needed, so that the system cost and the installation complexity are greatly reduced; inductance and resistance can be measured at the initial starting stage; and motor current (ripple and average current) and rotating speed conditions are monitored in real time.

Description

Technical Field

[0001] The invention relates to the field of motor driving, and more specifically to an online measurement system for direct current motor parameters. Background Art

[0002] DC motors are widely used and have excellent load and speed regulation performance. In order to monitor the operating status of the motor and improve its working performance, it is usually necessary to measure parameters such as motor inductance, resistance, speed, average current and current ripple, so as to understand the static characteristics and electrical characteristics of the motor, provide important data for the design and analysis of the motor, and ensure the safe and stable operation of the motor; in addition, accurate motor parameters and operating parameters combined with advanced control algorithms can improve the control accuracy and performance of the system.

[0003] Traditional parameter measurement methods include: measuring the resistance of the motor by applying a constant current source or a constant voltage source. The resistance of the motor can be used to calculate the power loss, thermal stability, etc. of the motor; calculating the inductance of the motor by applying an AC signal and measuring the current response of the motor. The inductance of the motor can be used to analyze the dynamic characteristics and response speed of the motor; measuring the motor speed by using an encoder or a Hall sensor. The motor speed is a key performance parameter and is of great significance for controlling the running speed and position of the motor; measuring the motor current by using a current sensor or a current transformer. The motor current is an important parameter reflecting the motor load and working state, and can be used to monitor the working condition of the motor; measuring the ripple of the motor output current by using an oscilloscope or other equipment. The current ripple is an important parameter reflecting the quality of the motor output current and can be used to evaluate the stability and smoothness of the motor output. However, these methods have the problems of cumbersome testing work, high sensor cost, complex installation, inconvenient maintenance, and susceptibility to external interference. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides an online measurement system for DC motor parameters, which can realize accurate detection of motor inductance, resistance, speed, average current I and current ripple.

[0005] The technical scheme is as follows: a DC motor parameter online measurement system, which includes a digital control unit, an H-bridge, a first analog comparator, a second analog comparator, a first analog-to-digital converter, and a second analog-to-digital converter. The H-bridge is connected to a tail current sensing resistor. The first analog comparator and the second analog comparator compare the tail current with a preset current value given by the digital control unit, and send the comparison result to the digital control unit; a PWM control signal enters the digital control unit as an input duty cycle Duty_in, and the digital control unit performs rectification according to the current limiting requirement. The motor current does not exceed the preset current value Itrip provided to the first analog-to-digital converter, and the output effective duty cycle Duty_out after rectification is less than the input duty cycle Duty_in. The inductance L=U is calculated. dc ·t1I trip , where U dc is the bus voltage, t1 is the time it takes for the motor coil current to reach the preset current value Itrip under the action of the power supply voltage, and the resistance R=U dc Duty_outI trip , the current in the PWM decay phase satisfies Substituting the above inductance into the current ripple at any time: Wherein toff is the PWM low level time, vb is the back electromotive force, and the average current in the rectification state is I_mean=Itrip-ΔI / 2; after the speed increases, the rectification state is disconnected, and the preset current value Itrip2 signal provided by the digital control unit to the second digital-to-analog converter is Itrip-ΔI. The second analog comparator compares the current flowing through the tail resistor with the size of Itrip2, and feeds the comparison result back to the digital control unit. The digital control unit can obtain the time T1 less than Itrip2 and the time T2 greater than Itrip2 by timing, and obtain the average current I_mean=Itrip2+(0.5-T1 / Ton)ΔI, wherein Ton is the PWM high level time; when T2 is 0, Itrip2 is updated using Itrip2-ΔI; when T1 is 0, Itrip2 is updated using Itrip2+ΔI, and so on iteratively to obtain the real-time average current: I_mean=Itrip2+(0.5-T1 / Ton)ΔI; the back electromotive force value corresponding to the speed is obtained according to the average current and resistance, and the speed value at any time is calculated.

[0006] After adopting the present invention, no additional sensors are needed, only two comparators are needed, which greatly reduces system cost and installation complexity; inductance and resistance can be measured at the initial startup; motor current (ripple and average current) and speed can be monitored in real time. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 It is a schematic diagram of the principle structure of the present invention;

[0008] Figure 2 It is the relationship curve of current and time;

[0009] Figure 3 It is a schematic diagram of the motor current change;

[0010] Figure 4 It is a flow logic diagram of the present invention. DETAILED DESCRIPTION

[0011] The system structure of the present invention is shown in FIG1, where 100 is a digital control unit, 110 is an H-bridge and a load motor. The H-bridge includes a tail current sensing resistor 111. Similarly, the tail current sensing resistor may include an auxiliary filter device, which is not drawn here for simplicity; 101 and 103 are respectively the first analog comparator and the second analog comparator, which respectively compare the tail current size given by the tail current sensing resistor 111 with the preset current value provided by the digital control unit 100 to the first digital-to-analog converter 102 and the first digital-to-analog converter 104, and send the corresponding comparison results to the digital control unit 100. It should be noted that the digital signal provided by the digital control unit 100 to the first digital-to-analog converter 102 and the second digital-to-analog converter 104 will vary with its internal state, so that the first analog comparator 101 and the second analog comparator 103 can complete the comparison of multiple level values.

[0012] When the DC motor is working normally, the voltage across the motor is used to overcome the back electromotive force in addition to overcoming the coil resistance voltage drop, and the remaining voltage is used to overcome the back electromotive force, which is expressed as: U = IR + kω, where I is the average coil current, R is the coil resistance, U is the effective output voltage, k is the back electromotive force constant, and ω is the motor speed. The PWM control signal from an external controller such as an MCU enters the digital control unit 100 as the input duty cycle Duty_in, and the digital control unit 100 performs rectification according to the Itrip current limiting requirement provided to the first digital-to-analog converter 102, so that the motor current does not exceed the preset current value Itrip, and the output effective duty cycle Duty_out after rectification is obviously less than the input duty cycle Duty_in.

[0013] At the moment of starting, the motor speed and back electromotive force are 0, and the coil current rises rapidly under the action of the power supply voltage, reaching Itrip at point A after t1 time. Figure 2 , we get the inductance L = U dc ·t1I trip , where U dc is the bus voltage. The current after point A immediately enters the rectification state. Since the t1 time is very short, the speed ω is basically zero speed. According to U=IR+kω, the coil resistance R=U dc Duty_outI trip, where toff is the PWM low level time, vb is the back EMF, and the maximum current in the rectification state is maintained at Itrip, but the motor gradually accelerates. The current in the PWM decay phase satisfies Substituting the above inductance into the current ripple at any time: Therefore, the average current in the rectification state is I_mean=Itrip-ΔI / 2.

[0014] As the speed gradually increases, the current begins to leave the rectification state at point B. Figure 3 At this time, the Itrip2 signal provided by the digital control unit 100 to the second digital-to-analog converter 104 is Itrip-ΔI. At the same time, in the current driving stage, the analog comparator 103 compares the current flowing through the tail resistor with the size of Itrip2, and feeds the comparison result back to the digital control unit 100. The digital control unit 100 can obtain the time T1 less than Itrip2 and the time T2 greater than Itrip2 through timing. Since the current change amplitude in the PWM driving stage and the adjacent attenuation stage is basically the same, the average current I_mean=Itrip2+(0.5-T1 / Ton)ΔI can be obtained, where Ton is the PWM high level time; as the speed increases, the average current will continue to decrease. When T2 is 0, Itrip2 is updated using Itrip2-ΔI; when T1 is 0, Itrip2 is updated using Itrip2+ΔI. In this way, Itrip2 is updated in real time according to the level state of the second analog comparator 103, and the real-time average current is: I_mean=Itrip2+(0.5-T1 / Ton)ΔI.

[0015] According to the formula U=IR+kω, the speed is proportional to the back electromotive force. According to the average current I_mean=Itrip2+(0.5-T1 / Ton)ΔI, R=U dc Duty_outI trip Substituting in, we can get the back electromotive force value corresponding to the speed, and then calculate the speed value at any time. In specific implementation, first calibrate any speed and output the output duty cycle and average current value corresponding to the speed, and then obtain the back electromotive force constant and U dc The speed value corresponding to any subsequent output duty cycle can be calculated by calculating the bus voltage ratio.

[0016] In summary, the present invention realizes the logic Figure 4:When the motor starts, the current begins to enter the rectification state. The inductance can be obtained based on the timing. After entering the rectification state, the resistance and ripple size can be obtained based on the output duty cycle. As the motor accelerates, the current leaves the rectification state, and the output duty cycle is the input duty cycle. At this time, a constantly changing threshold and comparator are used to track the real-time current and ripple size. Finally, the corresponding speed at any time can be calculated based on the average current.

[0017] The DC motor parameter online measurement system of the present invention has the following advantages: no additional sensors are required, only two comparators are needed, which greatly reduces the system cost and installation complexity; the inductance and resistance can be measured at the initial startup; the motor current (ripple and average current) and speed can be monitored in real time; with the current and speed value system, advanced control and intelligent adjustment can be further realized, and it can be widely used in various occasions that are cost-sensitive but require real-time monitoring and control of motor current or speed, and has good application prospects and economic benefits. It should be noted that this patent is not only applicable to brushed DC motors driven by H-bridges, but its concept is also applicable to parameter measurement and current / speed detection of brushless motors.

Claims

1. A DC motor parameter online measurement system, characterized in that: It includes a digital control unit, an H-bridge, a first analog comparator, a second analog comparator, a first analog-to-digital converter, and a second analog-to-digital converter. The H-bridge is connected to a tail current sensing resistor. The first analog comparator and the second analog comparator compare the tail current with a preset current value given by the digital control unit, and send the comparison result to the digital control unit. The PWM control signal enters the digital control unit as an input duty cycle Duty_in. The digital control unit performs rectification according to the current limiting requirement. The motor current does not exceed the preset current value Itrip provided to the first analog-to-digital converter. The output effective duty cycle Duty_out after rectification is less than the input duty cycle Duty_in. The inductance L=U is calculated. dc ·t 1 I trip , where Udc is the bus voltage, t1 is the time it takes for the motor coil current to reach the preset current value Itrip under the action of the power supply voltage, and the resistance R = U dc Duty_outI trip , the current in the PWM decay phase satisfies Substituting the above inductance into the current ripple at any time: Wherein toff is the PWM low level time, vb is the back electromotive force, and the average current in the rectification state is I_mean=Itrip-ΔI / 2; after the speed increases, the rectification state is disconnected, and the preset current value Itrip2 signal provided by the digital control unit to the second digital-to-analog converter is Itrip-ΔI. The second analog comparator compares the current flowing through the tail resistor with the size of Itrip2, and feeds the comparison result back to the digital control unit. The digital control unit can obtain the time T1 less than Itrip2 and the time T2 greater than Itrip2 by timing, and obtain the average current I_mean=Itrip2+(0.5-T1 / Ton)ΔI, wherein Ton is the PWM high level time; when T2 is 0, Itrip2 is updated using Itrip2-ΔI; when T1 is 0, Itrip2 is updated using Itrip2+ΔI, and so on iteratively to obtain the real-time average current: I_mean=Itrip2+(0.5-T1 / Ton)ΔI; the back electromotive force value corresponding to the speed is obtained according to the average current and resistance, and the speed value at any time is calculated.

Images