Ask a question Question: Zhuo much, I would like to ask a lot of this low-pass filtering the importance of it, if not, it is not out of the square wave DC yet? I measured with a multimeter plus LC and without LC test out voltage is the same, you can explain the necessity of the low-pass filter is?
^ Half-bridge PWM output circuit | map from "How the elephant put into the refrigerator" tweets illustration ^
Question: Zhuo greatly, I have an idea, the battery voltage increases, then the use pwm modulation, to ensure that in the conventional modulation, the voltage across the motor reaches the rated voltage, so that the motor will have a higher speed. Whether this is reasonable?
Figure ^ | ^ from the network
Analysis of the principle of a decomposition of the complex signal analysis target and appropriate decomposition, the problem does not work for the components may be omitted, the problem grasp contradictions. This method has applications in many disciplines. Objects such as shown in the figure ramp being so stress analysis.
^ Force moving object by analyzing the slope | from FIG. "Signals and Systems" courseware ^
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In all exploded side, the orthogonal decomposition ensures that the signal before and after the decomposition in terms of energy conservation.
There are many ways for signal decomposition. For example, and the AC signal into DC, odd and even components of components, decomposition pulse, harmonic decompose the real and imaginary partial solution and the like. Signal into the DC and AC component is used to decompositions.
DC component is the average of the signal. If the signal is a periodic signal, the DC signal component is defined as the integral over a cycle divided by the period. If the signal is non-periodic signal is averaged over time the length of time approaches infinity when the limit. AC component of the signal is equal to the DC component is subtracted from the signal.
DC and AC signal decomposition
AC component and DC component of the signal is orthogonal, i.e. they are equal to the power and the power of the original signal. Thereby facilitate future power and DC power by measuring a known theoretical signal obtained, calculate the power of the AC component.
Second, the DC and AC PWM signal is a PWM signal decomposition periodic signal within a period, a fixed value of the signal into two values. For example, unipolar PWM signal, two values are 0, E; for a bipolar PWM signal, two values are E, -E. Defined as the proportion occupied in a high cycle duty ratio Duty.
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Unipolar PWM signal
Bipolar PWM signal
If the duty ratio and the peak to peak value is known PWM waveform, it can be determined magnitude of the DC component of the PWM signal. The duty ratio between the DC component and exhibits a linear function. The DC component not related to the frequency f (period T) PWM signal.
Relatively easy to calculate the DC component, and then adding the AC power equal to the power of the original signal according to the DC power, can be analyzed near the corresponding AC component power and rms values.
Third, the inertia (low-pass) Most systems have a dynamic storage system links, e.g. mechanical kinetic energy stored in the rotor of the motor; electromagnetic energy stored in the inductance of the armature; the RC circuit capacitor storing electrical energy. The energy storage system such that there is a smooth part of action of the presence of the input signal: i.e. the signal is rapidly changing greatly weakened; signal for a slow (especially DC signal) it is possible to change the outgoing amplified.
The armature and the rotor in the motor
For example, an AC signal is applied to the DC motor, if the frequency of the signal is relatively high, the alternating current in the inductance of the smoothing action of the motor armature, produced will be small. Torque performance of these alternating current is generated by motions, under the action of the rotor and the mechanical inertia of the transport structure, the average torque output of zero. Such that the real DC motor continues to rotate the application of the DC component of the signal.
It is a multimeter instrument can measure a variety of electrical parameters. It is the amount of time measured DC voltage, the input signal will be low pass filtered to remove the interference signal AC signal, a DC component in the signal retention. Therefore, the PWM output signal using a multimeter measurement time, the size of which can be directly read a straight flow.
Problems Explanation first problem, the need for a PWM signal output LC low-pass filter questioned. The reason is to use a multimeter to measure the voltage before and after LC is the same.
The principle of the circuit analysis, the LC is a second order low-pass filter, i.e. electrical inertia. It can greatly attenuate the AC signal to DC signal no loss. Therefore, before LC is a PWM signal, only the DC component of the PWM, PWM alternating current component is filtered out after LC.
Because multimeter DC voltage profile test only the DC component of the signal, the signal before and after the LC is not be seen. Multimeter AC voltage profile and then the measurement before and after the LC voltage signal, you will find, LC filter with a large AC signal; LC filter after the AC signal is almost zero.
For filtering high-current inductors
LC effect is to filter the AC component of the PWM, for providing a DC voltage adjustable rear DC power circuit, a charging circuit or tank. If not LC filter, PWM signal, the AC component of the DC electric circuit will later have a great interference. If charging was used directly in the back of the storage capacitor, then the capacitance is relatively low due to the impedance of the AC component of the AC voltage in the PWM signal will have a very large alternating current, which will burn in front of the PWM circuit.
It is necessary to increase the LC filter circuit of the above charging half-bridge circuit. In order to reduce the attenuation of the filter circuit to the output DC component, which requires the use of a small internal resistance of the inductor, the inductor current is large.
Analysis for the second problem, it is known, the motor torque acting in the running DC component of the PWM waveform, there is no relation to the AC component. Therefore, no matter how the operating voltage of the driver circuit, i.e. the peak PWM output high, simply by adjusting the duty cycle so that the same average output voltage, the generated torque of the motor on the same.
Accordingly, the battery does not need to be boosted.
A PWM signal driving the motor car models
Additional battery boost circuit will also bring additional power losses, increase the output impedance of the drive circuit. This is far less directly connected to the battery in the current driving circuit can provide power. Therefore, additional battery boost for car models is superfluous. Experimental Verification Experiment 1: DC and AC components using the PWM waveform multimeter multimeter range measuring DC and AC voltage of the PWM signal is known, the foregoing verification signal decomposition theory.
Application of the motor driver chip MC33886 unipolar PWM waveform output frequency of 2kHz. MC33886 power is 8V, so the PWM output waveform peak is about 8V. PWM duty cycle of about 50%. According to the foregoing analysis, this time should be the DC component of the signal is approximately 4V.
Experimental PWM waveform output circuit and
Using FLUKE multimeter range measuring DC voltage PWM waveform display output measured value 3.988V, which is very close to theoretical values.
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^ Movable FIG measured DC component of the PWM waveform, and is very close to the theoretical value ^
AC voltage measurement profile table FLUKE AC component of the PWM waveform. AC component power of the signal plus the DC component of the signal power is equal to the total power, can know the theoretical analysis for the PWM signal of 50% duty cycle experiments, the effective value of the AC component which should be equal to 4V. Here eliminating the need for analysis.
However, the actual FLUKE AC effective value multimeter display 4.42V, with a difference of about 10% of the theoretical value. This is due to the normal handheld multimeter AC sine wave profile is calculated by measuring the effective value of the signal from the peak measurement is not true RMS measurement, a large error is generated. True RMS AC desktop replacement table to re-measure the AC signal of the PWM, the measured value of 3.998V, is very close to the calculated value.
PWM AC multimeter range measuring AC signal, with 10% of the theoretical value of the error
None Experiment 2: Observation on the effect of PWM direct link capacitor filter extracts the DC component of the PWM waveform, it is necessary to filter out the AC component therein. Are LC circuit inductance can be relatively bulky get rid of it?
Experiment, directly outputs the PWM signal using a 1000uF capacitor filtering. By an infrared camera to observe the temperature change of the output power of the integrated circuit chip.
The image of the infrared temperature test circuit
Initially, a pin with a separate capacitor circuit. After the circuit is powered, hand pin 1000uF capacitor according to the output PWM pad, then you can see the current limiting regulated DC power supply circuit and an output current at saturation. The following figure shows a sharp rise in the temperature of the output integrated chip MC33886.
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^ PWM output circuit caused by direct heating using a capacitive filter ^
Extracting the DC component of the PWM signal can not be directly to the filter capacitor loading on the PWM output signal. Since the capacitance is low resistance to the AC signal, the AC voltage of the PWM signal is generated in a large component of the alternating current in the capacitor, the output circuit causing thermal damage.
In order to limit the alternating current, inductance required limits. Here inductance also becomes smoothing inductor, whichever PWM output alternating voltage smoothing means. Clearly, L the larger the value, the smaller the AC current, resulting in less power loss. Since the actual inductance of the DC resistance is always present, as more bypass, it will increase DC resistance. Therefore, the inductance value needs to be a compromise value based on the actual impedance bypass inductor, PWM frequency, and the value of the capacitor.
None **** extending discussed previously discussed DC motor car on car models, if a booster circuit to provide power to it is in fact superfluous. However, in an actual electric vehicle, it tends to improve the efficiency of the motor and a step-up circuit provides a high voltage source for the motor. Although the arrival of some DCDC boost circuit power loss, but also improve the overall efficiency of the car.
^ Electric car battery configuration | Figure from the network ^
When PWM DC motor drive, although there is no contribution to the AC component of rotational torque, but it produces vibrations sometimes reduce the friction in the motor static. Cars particularly when in the upright normal state, the low speed forward and reverse operation. Reduce static friction will reduce the impact of non-linear component of the system control.
The higher frequency PWM theory, the time post-filtering, the desired L, value C will be lower, thereby reducing the volume of the components, the driving motor, the vibration generated noise is smaller. However, PWM frequency is increased, switching loss will have a great power output stage. Some circuit chip specifies the maximum PWM frequency, such as the MC33886 required output frequency is not higher than 10kHz. Also makes high frequency PWM current output capacity of the output circuit is decreased.
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