Design method of LC output filter in PWM (Pulse Width Modulation) driving servo motor system

Abstract

According to the design method of the LC output filter in the PWM driving servo motor system, EMI of a three-phase servo motor system is analyzed, and an EMI signal transmission model is established according to EMI characteristics; converting the EMI signal transmission model into an RLC (Radio Link Control) model; then determining key parameters of the EMI signal transmission model by using an RLC model; and finally, carrying out design verification on the LC output filter based on the key parameters of the EMI signal transmission model. According to the method, key parameters of an EMI signal transmission model are deduced by capturing a current oscillation waveform and performing model conversion; by means of simulation verification, actual measurement and calculation work of LC output filter design is transferred to a simulation environment, the design cost is reduced, and the design efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of filters, and in particular to a design method for an LC output filter in a PWM driven servo motor system. Background Art

[0002] In the aerospace environment, servo motors are usually placed outside the cabin to serve the actuators of the payload. The servo motor and PWM drive controller need to be separated at a certain distance and connected by long cables. The equivalent inductance of the cable and the parasitic capacitance of the windings in this configuration together form an RLC network, which generates current oscillations under the switching action of the PWM signal, thereby causing radiated or conducted EMI signals. These EMI signals may not only damage the motor itself, but also interfere with other electronic equipment in the aerospace payload, such as spectral imaging, communication systems, etc.

[0003] The main means to solve the EMI problem is to use filters for suppression. Designing a suitable filter usually involves modeling and measuring multiple component parameters of the motor and its drive using equipment such as a network analyzer, including resistance, inductance, back electromotive force, fundamental frequency component blocking, turn-to-turn effect, and parasitic capacitance. This process is not only complex, but also time-consuming and labor-intensive.

[0004] LC filters are mainly composed of passive components such as inductors, capacitors and resistors, which use the characteristics of these components to suppress conducted EMI in electronic circuits. Simple LC output filters are designed with LC network topology, where the inductor allows DC and low-frequency signals to pass while blocking high-frequency noise. Passive filters have the advantages of simple structure, low cost, high operating reliability and low operating expenses. They are not limited by hardware and are one of the most widely used harmonic control methods.

[0005] Designing an LC filter is a complex process that requires reasonable matching and calculation based on filtering requirements and component characteristics. The general steps include:

[0006] 1) First, measure the system EMI to obtain the insertion loss required by the EMI filter;

[0007] 2) Then select the appropriate LC output filter topology (such as RC, LC or RLC network);

[0008] 3) Obtain the corner frequency f of the LC output filter c ;

[0009] 4) According to the turning frequency f c You can then select the values ​​of the components in the LC output filter;

[0010] 5) In actual testing, if the designed LC output filter cannot meet the EMI suppression requirements of the system, it is necessary to adjust the component parameters of the EMI filter to meet the suppression requirements.

[0011] The optimal design of LC filters also needs to consider multiple factors such as harmonics, reactive power compensation, grid frequency fluctuations and economic costs.

[0012] In general, the existing filter design technology solutions first need to use network analyzers and other equipment to measure and model multiple component parameters of the motor and its drive. This process requires complex mathematical calculations and a lot of measurement, analysis, testing, and modification work, which is a huge challenge for designers.

[0013] Terminology explanation:

[0014] Electromagnetic interference (EMI): refers to any electromagnetic phenomenon that may degrade the performance of a device, equipment or system during conduction or in the presence of an electromagnetic field accompanied by voltage or current.

[0015] Pulse Width Modulation (PWM): is a modulation technique that controls the average value of the output voltage or current by changing the width of the pulse. Summary of the invention

[0016] The present invention proposes a design method for an LC output filter in a PWM driven servo motor system, which aims to effectively suppress the radiated EMI caused by the PWM signal passing through a long cable, thereby providing a practical and efficient solution for the electromagnetic compatibility of the motor drive system in a space environment.

[0017] To achieve the above object, the present invention adopts the following technical solutions:

[0018] A method for designing an LC output filter in a PWM driven servo motor system, comprising:

[0019] Analyze the EMI of the system and establish an EMI signal transmission model based on the EMI characteristics;

[0020] Convert the EMI signal transmission model into an RLC model;

[0021] Then the RLC model is used to determine the key parameters of the EMI signal transmission model;

[0022] Finally, the design of the LC output filter is verified based on the key parameters of the EMI signal transmission model.

[0023] Furthermore, the RLC model is used to determine the key parameters of the EMI signal transmission model, including:

[0024] In the captured oscillating current waveform, two pulses are selected and the period and pulse current peak are measured. Based on the measurement results, the unknown coefficients in formula (9) are calculated by formulas (10), (13), (14) and (17). Based on the calculated coefficients, the values ​​of R, L and C in the RLC model are obtained by combining formulas (18), (19) and (20); where R is the equivalent series resistance of the motor drive cable, L is the equivalent inductance of the motor drive cable, and C is the parasitic capacitance of the motor winding.

[0025] According to Kirchhoff's voltage law, the loop of the RLC circuit is expressed by equation (1):

[0026]

[0027] Where i(t) or i(x) is a function of current, Vin is the input value, switching between 0V and Vin;

[0028] Differentiating both sides of the equation and dividing them by L forms a standard second-order differential equation:

[0029]

[0030] Its characteristic equation is s 2 +(R / L)*s+1 / LC=0, the two solutions of the characteristic equation are calculated using the quadratic formula:

[0031]

[0032] Where α is the Neper frequency in rad / s, which describes the damping rate and is expressed as

[0033]

[0034] ω0 is the resonant radian frequency in rad / s, expressed as:

[0035]

[0036] The step response is underdamped, which means that α 2 Less than ω0 2 , whose radian frequency ω d It is expressed as:

[0037]

[0038] The solution to equation (3) has the following form:

[0039] i(t)=B1·e -αt ·Cos(ω d t)+B2·e -αt ·Sin(ω d t) (7)

[0040] A step function input is made by turning the switch from 0V to Vin, satisfying two initial conditions:

[0041] (1) When t = 0, the current i(t) is zero;

[0042] (2) When t = 0, the voltage on L is Vin, which can be expressed as L·(di(t)) / dt;

[0043] Under these two conditions, find the coefficients B1 and B2:

[0044]

[0045] The current oscillation is expressed as:

[0046] i(t)=B2·e -αt ·Sin(ω d t) (9)

[0047] The vertical axis of the coordinate is the current amplitude, and the horizontal axis is time; T d is the period of oscillation, for T d , coefficient ω d It is derived from the following formula:

[0048]

[0049] The time point at which the nth pulse has a positive peak is defined as t n , the time point when the nth pulse has a negative peak value will be defined as t' n ; t' n t n Add T d half of t' n =t n +π / ω d ;

[0050] The peak-to-peak value of the nth pulse is defined as i p-pn

[0051]

[0052] Select the nth pulse and the mth pulse to be measured, where "m" is the sequence number later than "n";

[0053] i p-pn and i p-pm The ratio is expressed as:

[0054]

[0055] α is solved for:

[0056]

[0057] According to equation (11), B2 is expressed as t n Function of

[0058]

[0059] Calculate t n The measured value of the oscillating current is expressed by equation (9). All peak points have extreme values. Therefore, when t = t n When , the derivative of equation (9) should be zero and can be expressed as:

[0060]

[0061] therefore

[0062]

[0063] Thus, t n According to formula (17), we can get:

[0064]

[0065] Once you get t n , B2 can be calculated by equation (14);

[0066] Therefore, ω d , B2 and t n By T d 、i p-pn and i p-pm Solve the measurement results of ;

[0067] Once you know d , α and B2, the values ​​of R, L and C can be determined (where R, L and C are the equivalent series resistance of the motor drive cable, the equivalent inductance of the motor drive cable and the parasitic capacitance of the motor winding under the RLC model respectively);

[0068] According to equation (8), the calculation formula of L is

[0069]

[0070] Where V in is the PWM swing voltage;

[0071] According to equation (4), R is calculated by the following formula

[0072] R=2·L·α (19)

[0073] According to equations (5) and (6), the calculation formula for C is

[0074]

[0075] In the captured oscillating current waveform, T d It is measured using the vertical cursors in the oscilloscope.

[0076] Furthermore, the design verification of the LC output filter based on the key parameters of the EMI signal transmission model specifically includes:

[0077] Based on the values ​​of R, L, and C determined by the RLC model, a simulation circuit of the EMI signal transmission model is constructed. In the EMI signal transmission model, the sum of the winding equivalent series resistance Resr and the drive cable resistance Rc is 1 / 1.5 of R. Rc can be measured directly on the drive cable. The drive cable equivalent inductance Lc is 1 / 1.5 of L, and the winding parasitic capacitance Cw is 1.5 times of C. Therefore, the values ​​of Resr, Lc, and Cw in the EMI signal transmission model can be determined;

[0078] Then add 1 LC filter on each phase and use the equivalent series resistance (equivalent resistance of the inductor in the filter).

[0079] The natural oscillation frequency f0 in the LC filter circuit is also the cutoff frequency f of the LPF (low-pass filter). C Generally speaking, it should be designed to be much lower than the PWM frequency, and a frequency difference of 3 times is safe; on the other hand, the response frequency of the motor must also be considered, and it can be about 10 times the motor response frequency. Combining these two factors, the cutoff frequency f of the system can be determined C .f C and the inductor L in the LC filter f The value of capacitance C f The relationship between the values ​​of is determined by the following formula:

[0080]

[0081] Build a simulation circuit and try different L f and C f value, evaluate the filtering performance, output voltage and charging current, and determine the optimal L value of the LC filter f and C f Theoretically, given the cutoff frequency, find L f and C f There are countless combinations of values. But in fact, some engineering issues need to be considered, such as the volume of inductors and capacitors, the cost of aerospace components, and the rated current and voltage. After comprehensive consideration, the combinations are very limited.

[0082] It can be seen from the above technical solution that the LC output filter design method in the PWM driven servo motor system of the present invention obtains the current oscillation waveform on the servo motor drive cable, determines the motor system network parameters, and then uses the simulation tool to complete the design of the passive filter. Through this method, the measurement and correction work of the actual circuit is minimized, and most of the design work can be completed in a simulation environment.

[0083] Specifically, the present invention analyzes the EMI of a three-phase servo motor system, and establishes an EMI signal transmission model according to the EMI characteristics; converts the EMI signal transmission model into an RLC model; then uses the RLC model to determine the key parameters of the EMI signal transmission model; and finally performs design verification of an LC output filter based on the key parameters of the EMI signal transmission model. The present invention derives the key parameters of the EMI signal transmission model by capturing the current oscillation waveform and model conversion.

[0084] The LC output filter design method proposed in this invention can effectively suppress EMI radiation from the long cables of the motor drive system. By transferring the design process to the simulation environment, the measurement and modification work of the actual circuit is significantly reduced, and the design efficiency is improved. The application of this method in actual systems verifies its effectiveness, especially in the design of customized small motors or compact instruments, providing a flexible and practical EMI suppression solution.

[0085] The present invention is applicable to servo motor systems in aerospace environments and also to some small servo motor systems on the ground. It should be emphasized that in the ground environment, the motor and the controller are generally made into an integrated unit, the housing will shield EMI, and the motor and the driver are usually not connected by a long cable, so no serious current oscillation will occur.

[0086] The key innovative features of the method of the present invention include:

[0087] By capturing the current oscillation waveform and model conversion, the key parameters of the EMI signal transmission model are derived;

[0088] By using simulation verification, the actual measurement and calculation work of LC output filter design is transferred to the simulation environment, which reduces the design cost and improves the design efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 is the control voltage and drive current detection waveform of the embodiment of the present invention;

[0090] Figure 2 is the current damping oscillation on the A-phase conductor of the embodiment of the present invention;

[0091] Figure 3 is a terminal-related equivalent circuit of a three-phase motor winding according to an embodiment of the present invention;

[0092] Figure 4 is an equivalent circuit diagram of a cable and a motor according to an embodiment of the present invention;

[0093] Figure 5 is an equivalent circuit diagram (EMI signal transmission model) of the cable and winding of the embodiment of the present invention;

[0094] Figure 6 is an AC equivalent circuit diagram of an embodiment of the present invention;

[0095] Figure 7 is an RLC equivalent circuit diagram of an embodiment of the present invention;

[0096] Figure 8 It is a typical underdamped current oscillation in the RLC circuit of the embodiment of the present invention;

[0097] Fig. 9 It is the measurement of the current oscillation period of the embodiment of the present invention;

[0098] Fig.10 is a comparison of the step responses of the RLC model and the EMI signal transmission model of the embodiment of the present invention;

[0099] Fig.11 It is a simulation circuit and EMI signal transmission model of the LC filter of the embodiment of the present invention;

[0100] Fig.12 is the square wave response of the LC filter system of the embodiment of the present invention;

[0101] Fig.13 is the waveform of the A-phase current after the filter is installed in the embodiment of the present invention;

[0102] Fig.14 is the amplified waveform of the A-phase current after the filter is installed in the embodiment of the present invention;

[0103] Fig.15 It is a method flow chart of an embodiment of the present invention. DETAILED DESCRIPTION

[0104] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0105] The LC output filter design method for suppressing electromagnetic interference described in this embodiment starts with an actual case, analyzes the EMI of a three-phase servo motor system, and models it according to the EMI characteristics; then uses the RLC model to determine the key parameters of the EMI signal transmission model; finally, the design verification of the LC output filter is performed.

[0106] The following are specific instructions:

[0107] Electromagnetic Interference Analysis and System Modeling

[0108] The switching action of PWM causes current oscillation, which generates EMI radiation through the long cables of the motor drive system. Therefore, there must be an AC signal transmitted along the cable. Use an oscilloscope and current probe to capture the current signal through phase a in the cable. Figure 1 The measurement results are shown.

[0109] In this test, the input of channel 1 (C1) is a current probe on the phase a conductor in the cable. C2 is a voltage probe that tests the logic input of phase a in the driver IC, C3 tests the logic control voltage of phase B, and C4 tests the logic control voltage of phase C. Channel 1 is set to AC input, while the other channels are set to DC. It can be seen that each switching action will generate current ripple. Expand the waveform, such as Figure 2 As shown, the ripple is found to be a damped oscillation with a maximum peak-to-peak amplitude of 350mA, a period of 150ns, and a duration of about 1.5μS.

[0110] exist Figure 2 In the figure, since the dead time of the driver IC is 2.5μS, the oscillation starts 2.5μS after the TTL signal on Phase A (C2) rises. It can be concluded that the current oscillation is a response to the PWM switching action.

[0111] In addition, as an AC signal, the response to a rising edge or a falling edge is the same in amplitude and frequency. In order to simplify the analysis, the present invention only considers the rising edge of the A phase.

[0112] In various studies of industrial motors, many different circuit models are used to describe the winding behavior of a passive 2-port network consisting of a resistor (R), an inductor (L) and a capacitor (C). The most ideal circuit form of the required model should represent the main physical phenomena present in the motor windings. Since the focus of this study is on the current oscillations in the cables, resonances within the windings do not need to be considered. Therefore, if Figure 3 As shown, the windings are modeled using a lumped terminal-related equivalent circuit.

[0113] exist Figure 3 In the figure, Rw is the winding resistance, Lw is the winding inductance, Ci is the inter-winding capacitance, and Cs is the winding self-capacitance. In the present invention, the windings are connected in a Y shape, that is, terminals U2, V2 and W2 are connected at one point. Therefore, the three inter-winding capacitors on the right side can be eliminated.

[0114] The cable between the drive port and the motor can be modeled as an RLC network. The output port of the drive is modeled as a SPDT switch. Since the signal under investigation is the response to the rising edge of Phase A, the other two phases are grounded. Figure 4 The equivalent circuit of the system is shown.

[0115] exist Figure 4 In the figure, Rc is the series resistance of the cable; Lc is the inductance of the cable. In the figure, the three capacitors of Ci are connected in a triangle. They can be combined with the three Cs capacitors. The lamp capacitor is marked as Cw. In addition, the capacitor Cw of the winding is attached with an equivalent series resistor. These resistors are marked as Rw. Therefore, the EMI signal transmission model is established, such as Figure 5 shown.

[0116] exist Figure 5 The values ​​of Rw and Lw can be obtained from the specifications of the motor, which are 15Ω and 10mH respectively in this case. Therefore, the cutoff frequency of the branch circuit composed of Rw and Lw is 239Hz. Since the frequency of the current oscillation signal captured on the cable is about 6MHz, it cannot pass through the branch of Rw and Lw.

[0117] In this circuit, only Cw can pass high-frequency AC signals. The 6MHz oscillation signal is generated by the LC circuit, which consists of the equivalent inductance of the cable (L C ) and the parasitic capacitors (Cw) of the windings, and pass through the high-frequency path. The high-frequency path consists of Rc, Lc, Resr, and Cw. Therefore, Lw and Rw can be deleted from the diagram without any impact on the analysis of high-frequency oscillation. The AC equivalent circuit is shown in Figure 6 shown.

[0118] Figure 6 The circuit shown can be converted into a simple RLC equivalent circuit as Figure 7 shown.

[0119] exist Figure 7 In the circuit, the value of C is equal to two-thirds of Cw. The value of R is 1.5 times the sum of Rc and Resr, and the value of L is 1.5 times Lc. i(t) is the current in response to the Vin step function. In the actual circuit, i(t) is the oscillation on phase A, such as Figure 2 shown.

[0120] Therefore, two models are established. One is the RLC model, such as Figure 7 As shown in Figure 1, it is used to analyze AC signals. The other is the EMI signal transmission model, such as Figure 5 As shown, it is used to analyze AC and DC signals simultaneously.

[0121] Determination of RLC model and EMI signal transmission model parameters

[0122] In the RLC model, the values ​​of R, L, and C are difficult to measure on a real system. However, since i(t) can be measured by an oscilloscope, the values ​​of R, L, and C can be calculated using the waveform of i(t), as shown in Figure 2 shown.

[0123] According to Kirchhoff’s voltage law (KVL), the loop of the RLC circuit can be expressed by Equation 1:

[0124]

[0125] Where i(t) or i(x) is a function of current and Vin is the input value, switching from 0V or 30V.

[0126] Differentiating both sides of the equation and dividing them by L forms a standard second-order differential equation:

[0127]

[0128] Its characteristic equation is s 2 +(R / L)*s+1 / LC=0, the two solutions of the characteristic equation can be calculated using the quadratic formula:

[0129]

[0130] Where α is the Neper frequency in rad / s, which describes the damping rate and can be expressed as

[0131]

[0132] ω0 is the resonant radian frequency in rad / s, which can be expressed as:

[0133]

[0134] review Figure 3 , the step response is underdamped, which means that α 2 Less than ω0 2 Its radian frequency ω d It is expressed as:

[0135]

[0136] The solution to equation (3) has the following form:

[0137] i(t)=B1·e -αt ·Cos(ω d t)+B3·i -αt ·Sin(ω d t) (7)

[0138] In the present invention, a step function input is performed by turning the switch from 0V to 30V, and two initial conditions should be met:

[0139] (1) When t = 0, the current i(t) is zero;

[0140] (2) When t = 0, the voltage on L is Vin, i.e. 30 V, which can be expressed as L·(di(t)) / dt.

[0141] Under these two conditions, find the coefficients B1 and B2:

[0142]

[0143] Therefore, the current oscillation can be expressed as:

[0144] i(t)=B2·e -αt ·Sin(ω d t) (9)

[0145] Figure 8 shows a typical step response of an RLC series circuit, which is an underdamped oscillation consisting of several pulses. Each pulse is a sine wave that starts at zero, rises to a positive peak, falls to a negative peak, and then returns to zero. The amplitude of these pulses decreases over time.

[0146] exist Figure 8 In the figure, the vertical axis is the current amplitude. The horizontal axis is time. d is the period of oscillation. d , coefficient ω d It is derived from the following formula:

[0147]

[0148] Figure 8 The coordinates in the waveform do not exist in the actual waveform. Measuring the absolute amplitude is inaccurate. Instead, the peak-to-peak amplitude of the pulse can be measured because the peak point is easier to locate on the oscilloscope.

[0149] like Figure 8 As shown in the figure, the time point at which the nth pulse has a positive peak is defined as t n The time point at which the nth pulse has a negative peak value will be defined as t' n . t' n t n Add T d half of t' n =t n +π / ω d .

[0150] The peak-to-peak value of the nth pulse is defined as i p-pn

[0151]

[0152] Select the nth pulse and the mth pulse to be measured, where "m" is the sequence number later than "n".

[0153] i p-pn and i p-pm The ratio can be expressed as:

[0154]

[0155] Therefore, "α" is solved:

[0156]

[0157] Fig. 9 The measurements of the 3rd and 5th pulses in one example are shown.

[0158] Once the “ω d ” and “α”, only one coefficient B2 in equation (9) is unknown. According to equation (11), B2 can be expressed as t n function.

[0159]

[0160] It looks like t n As has been determined, B2 can be calculated. However, since it is impossible to accurately locate the oscillation starting point, t n It is difficult to measure directly on an oscilloscope.

[0161] exist Fig. 9 The waveform of the oscillation signal is magnified. It can be seen that the starting pulse is not neat. This is because the driver needs a very short time, usually tens of nanoseconds, to reduce the output resistance from R off Switch to R on . The series resistance is not stable for a short time. In addition, some small oscillations caused by parasitic capacitors and inductors on the board occur for a short time at the beginning of the PWM pulse and are superimposed on the oscillating signal. The small oscillations distort the starting pulse. This is also the reason why the first one or two pulses cannot be selected for measurement.

[0162] It can be calculated that t n The oscillating current is expressed by equation (9), and all peak points have extreme values. Therefore, when t = t n When , the derivative of equation (9) should be zero and can be expressed as:

[0163]

[0164] therefore

[0165]

[0166] Thus, t n It can be obtained by formula 17:

[0167]

[0168] Once you get t n , B2 can be calculated by equation (14).

[0169] Therefore, ω d , B2 and t n By T d 、i p-pn and i p-pm The measurement results are solved.

[0170] Once you know d , α and B2, the values ​​of R, L, and C can be determined.

[0171] According to equation (8), the calculation formula of L is

[0172]

[0173] Where V in is the PWM swing voltage, which in this case is 30 volts.

[0174] According to equation (4), R can be calculated by the following formula

[0175] R=2·L·α (19)

[0176] According to equations (5) and (6), the calculation formula for C is

[0177]

[0178] Next, the calculation process in an actual project is explained.

[0179] In the captured waveform, T d It is measured using the vertical cursors in the oscilloscope, such as Fig. 9 shown.

[0180] exist Fig. 9 The duration of the five pulses is 770ns, T d It is 154ns.

[0181] Select the 3rd and 5th pulses for measurement. Use the horizontal cursors to measure their peak-to-peak amplitudes. p-p3 174mA, i p-p5 It is 82mA.

[0182] According to T d =154ns, i p-p3 =174mA, i p-p5 =82mA and V in =30V, the coefficients in equation (9) are obtained from equations (10), (13), (14) and (17).

[0183]

[0184] Then, according to equations (18), (19) and (20), the values ​​of R, L and C in the RLC model can be calculated:

[0185]

[0186] As mentioned above, in the EMI signal transmission model, Cw is 1.5 times C, Lc is 1 / 1.5 of L, and Resr+Rc is 1 / 1.5 of R. Therefore, Cw is 267pF, Lc is 2.24μH, and Resr+Rc is 10.9 ohms. Rc can be measured directly on the cable and is 0.2 ohms. Then Resr is calculated to be 10.7 ohms. Therefore, based on these determined values, a simulation schematic diagram of the EMI signal transmission model is established and the simulation tool The layout is carried out in Fig.10 Shown on the left.

[0187] The signal generator is used to simulate a 30KHz PWM signal on phase A. Switches S1 and S2 are placed on the simulated circuit to change the initial state of phases B and C. A current probe is placed on the wire of phase A to measure the current oscillation. The RLC model is also laid out on the right side for comparison with the EMI signal transmission model.

[0188] The simulation results are displayed on a virtual oscilloscope. Channel B is the current oscillation in the EMI signal transmission model; Channel C is the oscillation in the RLC model.

[0189] Comparing the waveforms of channel B and channel C, it is found that the oscillations in the RLC model and the EMI signal transmission model are the same. This verifies the equivalence of the RLC model and the EMI signal transmission model in terms of AC. The simulated waveforms are also consistent with Fig. 9 The actual waveforms shown are the same. It verifies the analysis and calculations.

[0190] Next, an EMI suppression filter was designed using the EMI signal transmission model and simulation tools.

[0191] Design of EMI suppression filter

[0192] The purpose of establishing the above two models and determining all the parameters in these models is to help design EMI suppression filters. Through the EMI signal transmission model and its parameters, all components of the filter can be modified and tested with simulation software. Otherwise, the design work will require a lot of measurement, analysis, testing and modification of the actual circuit. Sometimes, miscalculation may damage the motor.

[0193] As mentioned above, the EMI generated in the RLC network is an oscillation in response to the switching signal. In order to reduce the response, a low-pass filter (LPF) is usually used to filter the high-frequency elements of the switching signal, that is, to reduce the dv / dt ratio. The LC circuit consists of an inductor and a capacitor, and is an ideal choice for making an LPF because of its simple structure, few components, and low additional power consumption.

[0194] This LC filter always produces two oscillations, one is the natural oscillation at the frequency determined by the filter inductor and capacitor; the other is the forced oscillation at the frequency determined by PWM. If the natural frequency is close to the operating frequency, resonance will occur in the circuit where the voltage and current are too high. If the resonance phenomenon occurs, the natural frequency of the filter should be designed to be much higher than the PWM frequency or much lower than the PWM frequency. Generally speaking, three times the frequency difference is safe. Since the PWM frequency is 30kHz in this case, the natural frequency of the filter should be higher than 100kHz or lower than 10kHz. As an LPF, a lower frequency is preferred to reduce noise. Therefore, the natural frequency of the LC filter should be lower than 10KHz, which is also the cutoff frequency of the low-pass filter.

[0195] On the other hand, the motor response frequency is 238Hz, which is determined by the inductance and resistance of the winding. In order to avoid affecting the control characteristics, the minimum frequency of the LC filter is specified as 2.5K, which is about 10 times the motor response frequency.

[0196] The natural oscillation frequency (f0) in the LC circuit is also the cutoff frequency (f C ), which is determined by the following formula:

[0197]

[0198] Where L is the value of the inductor in the LC filter and C is the value of the capacitor of the LC filter.

[0199] The inductor of the LC circuit is connected in series with the winding, and its inductance cannot be too large so as not to affect the characteristics of the winding. Experimentally, in this case, the inductor cannot exceed one tenth of the winding inductance, that is, 1mH.

[0200] Inductors always have parasitic capacitance. For an inductor of 1mH / 1A, its parasitic capacitance is usually tens of pF. The parasitic capacitance of the inductor in the LC circuit cannot be ignored because it can bypass transient signals. The transient signal comes from the switch, passes through the parasitic capacitor of the inductor in the LC filter, and then reaches two branches, one is the capacitor in the LC filter, and the other is the cable and winding. Although the transient signal passing through the cable is small, there is also a risk of electromagnetic interference problems. Therefore, the capacitor in the LC filter should be large enough to shunt most of the transient current before it is transmitted to the cable. Experimentally, the value of the filter capacitor should be at least 100 times the value of the parasitic capacitor in the winding. In this case, the minimum value of the filter capacitor is set to 27nF.

[0201] Therefore, the design of the LC filter in the present invention sets three constraints:

[0202] 1) The L value should not exceed 1mH;

[0203] 2) C value should not be less than 27nF;

[0204] 3) The product of L and C should satisfy the cut-off frequency between 2.5KHz and 10KHz.

[0205] Choosing different cutoff frequencies will result in different filter characteristics. Even for a certain frequency, different combinations of L and C will produce different effects.

[0206] When the PWM signal through the LPF reaches the load, most of the AC element of the current should be absorbed by the filter. The AC current alternately flows into and out of the filter's capacitor, charging and discharging the capacitor. Although in theory the current will not actually do anything, it will still consume some power due to the equivalent series resistor within the capacitor. The AC current also produces an AC voltage on the capacitor, which is superimposed on the DC output voltage of the load. The small inductance of the filter results in a large AC current; the small capacitance results in a high AC output voltage. When designing the filter, you also need to consider the AC current and AC voltage.

[0207] In order to evaluate the performance of different cutoff frequencies and different LC combinations, a lot of calculations are required. The practical method is not to calculate, but to build a simulation circuit and try different values ​​in a virtual environment. Fig.11 A system schematic diagram based on the EMI signal transmission model is shown.

[0208] exist Fig.11 In, L f and C f To form an LC filter, R f YesL fThe equivalent series resistance is 1 ohm. An LC filter is placed between the switch and the cable. A signal generator generates a 30V / 30KHz square wave to simulate PWM. An analog oscilloscope is used to capture the waveform of certain signals. Channel A of the oscilloscope is connected to the output of the PWM. Channel B tests the output current fed to phase A of the motor. Channel C tests the AC output voltage, and Channel D tests the AC current charging and discharging C. f .

[0209] Through simulation tools and EMI signal transmission models, different frequencies and different LC combinations were implemented in a virtual environment. After checking all simulation results and considering several engineering issues, the filter parameters were finally determined to be L f =1000μH (rated current is 900mA), C f =330nF (rated voltage is 100V). The cut-off frequency of the LC filter is 8.76KHz. The simulation results are as follows Fig.12 shown.

[0210] exist Fig.12 In the example, the current fed to the cable has a ripple of 1.5 mA at 30 kHz (Channel B); f The charging current on the MOSFET is 260mA (Channel C); and the AC amplitude of the output voltage is 4V at 30kHz (Channel D).

[0211] As mentioned above, EMI comes from the current oscillation transmitted in the cable, which has a frequency of 6MHz and a maximum amplitude of 350mA. Through the LC filter, the current oscillation is suppressed to 1.5mA and the high-frequency components are removed. EMI should have been fully filtered.

[0212] The control characteristics of the servo motors were also tested and no changes were found after the filters were installed. Fig.13 and Fig.14 The captured waveform of the actual signal on the circuit is shown. The test configuration is the same as Figure 1 and Figure 2 The configuration is the same as in .

[0213] contrast Figure 1 and Figure 2 , Fig.13 and Fig.14 , it can be seen that the current oscillation has been filtered out.

[0214] The specific process of the method of the present invention is as follows: Fig.15 As shown;

[0215] In general, the embodiments of the present invention do not need to use network analyzers and other equipment to measure and model multiple component parameters of the motor and its driver, nor do they need to consider the resonance factor in the winding; they only need to measure the drive current of the system, saving time and effort. At the same time, debugging and evaluation are completed in a simulation environment, reducing costs.

[0216] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for designing an LC output filter in a PWM driven servo motor system, characterized in that: The following steps are included: Analyze the EMI of the three-phase servo motor system and establish an EMI signal transmission model based on the EMI characteristics; Convert the EMI signal transmission model into an RLC model; Then the RLC model is used to determine the key parameters of the EMI signal transmission model; Finally, the design of the LC output filter is verified based on the key parameters of the EMI signal transmission model.

2. The method for designing an LC output filter in a PWM driven servo motor system according to claim 1, characterized in that: The RLC model is used to determine the key parameters of the EMI signal transmission model, including: In the captured oscillating current waveform, two pulses are selected and the period and pulse current peak are measured. Based on the measurement results, the unknown coefficients in formula (9) are calculated by formulas (10), (13), (14) and (17). Based on the calculated coefficients, the values ​​of R, L and C in the RLC model are obtained by combining formulas (18), (19) and (20); where R is the equivalent series resistance of the motor drive cable, L is the equivalent inductance of the motor drive cable, and C is the parasitic capacitance of the motor winding. According to Kirchhoff's voltage law, the loop of the RLC circuit is expressed by equation (1): Where i(t) or i(x) is a function of current, Vin is the input value, switching between 0V and Vin; Differentiating both sides of the equation and dividing them by L forms a standard second-order differential equation: Its characteristic equation is s 2 +(R / L)*s+1 / LC=0, the two solutions of the characteristic equation are calculated using the quadratic formula: Where α is the Neper frequency in rad / s, which describes the damping rate and is expressed as ω0 is the resonant radian frequency in rad / s, expressed as: The step response is underdamped, which means that α 2 Less than ω0 2 , whose radian frequency ω d It is expressed as: The solution to equation (3) has the following form: i(t)=1· -αt ·os(ω d t)+2· -αt ·in(ω d t)(7) A step function input is made by turning the switch from 0V to Vin, satisfying two initial conditions: (1) When t = 0, the current i(t) is zero; (2) When t = 0, the voltage on L is Vin, which can be expressed as L·(di(t)) / dt; Under these two conditions, find the coefficients B1 and B2: The current oscillation is expressed as: i(t)=2· -αt ·in(ω d ·)(9) The vertical axis of the coordinate is the current amplitude, and the horizontal axis is time; T d is the period of oscillation, for T d , coefficient ω d It is derived from the following formula: The time point at which the nth pulse has a positive peak is defined as t n , the time point when the nth pulse has a negative peak value will be defined as t' n ; t' n t n Add T d half of t' n =t n +π / ω d ; The peak-to-peak value of the nth pulse is defined as i p-pn Select the nth pulse and the mth pulse to be measured, "m" is the sequence number later than "n"; i p-pn and i p-pm The ratio is expressed as: α is solved for: According to equation (11), B2 is expressed as t n Function of Calculate t n The measured value of the oscillating current is expressed by equation (9). All peak points have extreme values. Therefore, when t = t n When , the derivative of equation (9) should be zero and can be expressed as: therefore Thus, t n According to formula (17), we can get: Once you get t n , B2 can be calculated by equation (14); Therefore, ω d , B2 and t n By T d 、i p-pn and i p-pm Solve the measurement results of ; Once you know d , α and B2, the values ​​of R, L and C can be determined, where R, L and C are respectively the equivalent series resistance of the motor drive cable, the equivalent inductance of the motor drive cable and the parasitic capacitance of the motor winding under the RLC model; According to equation (8), the calculation formula of L is Where V in is the PWM swing voltage; According to equation (4), R is calculated by the following formula R=2·L·α (19) According to equations (5) and (6), the calculation formula for C is In the captured oscillating current waveform, T d It is measured using the vertical cursors in the oscilloscope.

3. The method for designing an LC output filter in a PWM driven servo motor system according to claim 2, characterized in that: The design verification of the LC output filter based on the key parameters of the EMI signal transmission model specifically includes: Based on the values ​​of R, L, and C determined by the RLC model, a simulation circuit of the EMI signal transmission model is constructed. In the EMI signal transmission model, the sum of the winding equivalent series resistance Resr and the drive cable resistance Rc is 1 / 1.5 of R. Rc can be directly measured on the drive cable. The drive cable equivalent inductance Lc is 1 / 1.5 of L, and the winding parasitic capacitance Cw is 1.5 times of C. Therefore, the values ​​of Resr, Lc, and Cw in the EMI signal transmission model can be determined. Then add 1 LC filter on each phase and use equivalent series resistance; The natural oscillation frequency f0 in the LC filter circuit is also the cutoff frequency f of the LPF, i.e. the low-pass filter. C On the one hand, it should be designed to be much lower than the PWM frequency, and a frequency difference of 3 times is safe; on the other hand, the response frequency of the motor must also be considered, which can be about 10 times the motor response frequency. Combining these two factors, the cutoff frequency f of the system can be determined C ;f C and the inductor L in the LC filter f The value of capacitance C f The relationship between the values ​​of is determined by the following formula: Build a simulation circuit and try different L f and C f value, evaluate the filtering performance, output voltage and charging current, and determine the optimal L value of the LC filter f and C f value.

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