PWM speed control function of the DC motor

By on April 7, 2010, 3:12 pm

Introduction When the switch is closed, the motor sees 12 Volts, and when it is open it sees 0 Volts. If the switch is opened for the same amount of time it is closed, the engine will see an average of 6 volts, and will run more slowly in consequence. As the amount of time the voltage is increased compared to the amount of time it is turned off, the average speed of motor increases. The switch-off is performed by power MOSFETs. A MOSFET is a device that converts the current very broad and outside under the control of a low voltage level signal. The time it takes an engine to accelerate and decelerate in terms of switching depends on the inertia of the rotor, and how much friction and the load torque is. The chart below shows the speed of a motor is turned on and off fairly slowly: We can see that the average speed is about 150, but it varies a little. If the voltage is turned on quickly enough, he will not have time to change speed much, and the speed is very stable. Is the principle of speed control mode switch. Thus, the speed is set by PWM. Inductors Before turning to a discussion of the circuits, we must first learn something about the action of inductive loads, and inductors. Inductors do not allow current through them to change instantaneously (in the same way the capacitors do not allow the voltage to change instantaneously). The voltage dropped across an inductor carrying a current i is given by the equation where di / dt is the rate of change of current. If the current is suddenly changed by opening a switch or turning off a transistor, the inductor will generate a very high voltage across it. For example, turning off 100 amps at 1 microsecond through an inductor 100 generates 10kV microhenry! The PWM frequency of the PWM signal that results is dependent on the frequency of the ground wave. The frequencies between 20Hz and 18kHz can produce audible cries from the speed controller and motors. Switched on and off the drive MOSFET results in a loss of power shortly. Therefore, more time spent compared to the static switch on and off, the greater will be the sequel “switching losses in the MOSFET. The higher the switching frequency, the more stable is the current waveform in the engines. This waveform is a spike of switching signals at low frequencies but at higher frequencies the inductance of the motor will be good this at an average DC current proportional to the PWM demand. This causes a loss spikiness greatest power in the resistance of the son, MOSFETs, and motor windings of a DC waveform constant current. You can see both graphs. It shows the worst case on-off current waveform, the other the best of cases continues waveform of DC Both waveforms have the same average current. However, when dealing with power dissipation in the parasitic resistances in our engine and cruise control for the DC case: and in case of switching the average power is the waveform switching, two Once power is lost in the parasitic resistances. In practice, the current waveform is not square wave like that, but it remains true that there will be a loss of more power in a waveform non-DC. Choose a rate based on engine characteristics, then we can work mathematically the minimum frequency to achieve this goal. This section is a little math, it would be better to miss it and just use the final equation. The following example shows the equivalent circuit of the motor and the current waveform that the PWM signal turns on and off. This shows the worst case, a 50:50 ratio of PWM, and the current increase is indicated by the engine stopped or stalled, which is also the worst. T is the switching period, which is the inverse of the switching frequency. While holding the falling edge of current signal is given by the equation? is the time constant of the circuit, which is L / R. Thus, the current time T = T / 2 (i1) shall not be less than P% lower at t = 0 (I0). This means that there is a limiting condition: generating PWM signals PWM signals can be generated in a number of ways. It is possible that your radio receiver picks already a PWM waveform from the transmitter. If a microcontroller on the robot, it may be able to generate the waveform, but if you have more than a couple of engines, it may be too much of a burden on resources microcontroller. Several methods are described below. The electronic analog PWM signal is obtained by comparing a triangular wave signal with a continuous signal. The DC signal can vary between the minimum and maximum voltages of the wave triangle. When the voltage waveform is the triangle above the level of DC, the output of the op-amp swings up, and when it is below the low-output balance. On the graph, we can see that if the level was high DC, the pulses would be even slimmer. This uses a scale of resistance against and weighted to generate the triangle wave (in fact, it will generate a sawtooth, but you’ll still get a PWM signal at the end of it). The actual resistance values, which are not available (40k, 80k) can be made with 20k resistors, or approximations can be used. The 74HC14 is a Schmitt input converter, which is connected to act as a simple oscillator. The oscillation frequency is about f = 1 / (2. IP. R. C), but it does not matter much within a few tens of percent. This square wave flows generated the 74HC163 4-bit binary counter. All entries in the preset and clear the outputs are disabled, QA to QD just roll the binary sequence from 0000 to 1111 and rollover to 0000 again. These results, which ranged from 0 V to +5 V are introduced into an amplifier was weighted binary section leftmost LM324 op amp with the 80K, 40K, 20K and 10K resistors. The output voltage of this amplifier depends on the value of counting meter and is shown in the table below AMP1 output. The result of this op amp simply multiply the voltage by – ½, to the positive voltage, and back to logic voltage levels, see Amp2 output column in the table. value binary value cons AMP1 output (V) output Amp2 (V) 0 0000 0 0 1 0001 -0. 625 0. 3125 2 0010 -1. 25 0. 625 3 0011 -1. 875 0. 9375 4 0100 -2. 5 1. 25 5 0101 -3. 125 1. 5625 6 0110 -3. 75 1. 875 7 0111 -4. 375 2. 1875 8 1000 -5 2. 5 9 1001 -5. 625 2. 8125 10 1010 -6. 25 3. 125 11 1011 -6. 875 3. 4375 12 1100 -7. 5 3. 75 13 1101 -8. 125 4. 0625 14 1110 -8. 75 4. 375 15 1111 -9. 375 4. The 6875 final, right, op amp compares the voltage with the voltage input demand, which varies from 0 V to 4. 6875v, where 0V represents 0% PWM ratio and 4. 6875v represents 100% PWM rate. This power demand can vary from-12V to +12 V, but only from 0 to 4. 6875 The scope of the report set PWM. chip PWM generator ICs available that should turn a DC voltage PWM. Many of them are designed for use in switching power supplies. SGS Thomson IC Manufacturer normal SG1524, SG1525. . . SMPS Maxim MAX038 signal generator Alternatively, a MOSFET driver, which includes a PWM generator can be used. I only know one who is not out yet! SGS Thomson TD340. Numerical method The numerical method is to increment a counter, and comparing the counter value with a value of the register of pre-loaded. It is essentially a digital version of the analog method above: The register must be loaded with the required level PWM microcontroller. It may be replaced by a simple ADC if the level should be controlled by an analog signal (as he would a servo radio control). This method is really useful if a microcontroller is used in your robot, which can pre-load register easily. If she has embedded microcontroller which can greatly simplify the process of signal generation. Hitachi H8S series has up to 16 PWM outputs available, but many other types have two or three. Interface with high power electronics, but there are two sides to electronics: the side of low power and high-side power. The low power electronics includes any embedded microcontroller, the radio receiver, and PWM generators. The upper power MOSFET includes drivers, MOSFETs themselves, and any valve or pump drivers you may have. Basically anything that is switching currents. Interface with the receiver of the radio you may be able to tap into the PWM signal coming out of the radio before it enters the servo, and use it to drive the input of MOSFET driver. However, it gives you no choice of switching frequency. Furthermore, the knob can generate a voltage to power the PWM generator. A more advanced method if you have a microcontroller on board the robot is to take the PWM signal to the receiver and connected to an input timing of the microphone. The microcontroller must be able to decode the signal, and generating a proportional analog output value (if it has ADC, or if an external ADC is equipped). Another method is to send more advanced data communication via the radio series. The handset radio will need to have a microcontroller in. The microcontroller must read the pots and switches on the handset, and send appropriate commands to the UART. To connect the radio transmitter. At the receiver, the demodulated output is sent to the robot microcontroller UART, and the data is decoded. Current limiting current limit is essential. If the engine is blocked, it can take huge currents that destroy the MOSFETs quickly. The form of current limiting is presented here to measure the current that the engine is taken, and if it exceeds a preset threshold, the MOSFET turn off the bridge. If you have an onboard microcontroller which generates the PWM rate, it would be an advantage if the software can detect the condition of overload, and reduce the rate of PWM, say, 10%. This circuit shows that the upper bridge MOSFET driven by simplicity. The lower MOSFET is not turned off during a current limit. There are only a sense of strength required for each engine, and must be immediately connected to the positive battery terminal. The voltage dropped across the sense resistor is amplified by U1A, which is connected to a differential amplifier circuit. The gain of 480K / 1k is 480. It is a very important gain because the voltage dropped across the sense resistor will be very low. The output of the differential amplifier is strongly low-pass filter by RxCx. Because there will be lots of noise from the engine, and we do not want to limit the current if we do not need. D13 is present to ensure that no negative spikes can affect the following systems. U2B compares the filtered signal with a preset value (represented here by V5), and if the current is too high (the signal is superior to V5), U2B will turn on Q1 and Q2 which PWM signals PWM generator clips. This will force the MOSFET driver to turn the MOSFET off. Q1 should be repeated four times, one for each channel MOSFET driver, but all four transistors can be driven from U2B. D11, R14 and C4 make sure that the MOSFET does not turn on immediately, but takes a few milliseconds. This prevents the MOSFETs be quickly activated and deactivated. A simulation of the part of current limiting circuit is indicated in the diagram below. The threshold voltage V5 was chosen to set a current limit of 30 amps. The square wave is the voltage (PWM MOSFET gate voltage), and the waveform is botched flight (motor) current. The tricky bit at the top of the steep waveform is when the current limit is set on or off. Some channels you can see the example of current flowing through the main power MOSFET by placing a power MOSFET much lower in parallel with it. It works OK, but the problem is the actual current limit is based on the value of RDS (on) MOSFET. If RDS (on) was only half the value we expect it to be, then twice more can pass before the current limit circuit took effect. Also the RDS (on) value depends on a lot of current through the MOSFET, and temperature. Any change in RDS (on) will change the current limit. RDS (on) figure is cited as a maximum value of the leaf, but this is not a design parameter safe. This means that it is not within specified limits, which are published on the datasheet. For example, CMOS digital logic guarantees that the output voltage, Vo, will be between Vcc-0. 5V and Vcc, and that this figure may be used for the design of circuits based on this figure. However, with RDS (on), we only know that it will be between 0 and the value. We can not count on a minimum value of it, however, is the minimum value that controls the current limit. Therefore, using a separate shunt resistor is a much more secure. Feedback speed control to stop a robot lurches in an arc when you want to go forward, you must have the speed of servo motor. This means that the actual speed of each wheel is measured and compared to all other wheels. It is clear to go straight, the engine speed must be equal. However, this does not necessarily mean that the application speed for each engine must be the same. The engines will have different amounts of friction, and thus an engine “rigid”, it will demand greater speed to go as fast as a more free-running engine. A diagram of speed control is analog return below, the demand for speed is a DC voltage which is fed to the PWM generator for motor A. The engine drives one to one dependent on the speed voltage demand. The speed of motor A is sampled using an optical encoder. It has an output frequency which is proportional to motor speed. If we assume that the motor B is already running at a certain speed, the optical encoder on its shaft will also produce a frequency. The phase comparator compares the two frequencies, effectively compare the speeds of both engines. Its output is a signal that becomes larger than the two input frequencies further apart. If both frequencies are the same, it has an output of zero. The integrator adds the output of the phase comparator to all that was before its release. For example, if the integrator output was previously 3 volts, and its entry is 0 volts, then its output will be 3 volts. If entry changed -1 volts, then its output would be 2 volts. Suppose that B motor is running more slowly than motor A. Then, the output of the phase comparator will be positive, and the output of the integrator will start to increase. The speed of motor B will then increase. If it is passed to a higher speed than the motor A, then the phase comparator output becomes negative, and the output of the integrator will start to decline, reducing engine speed B. In this way, the speed of motor B is kept the same as the speed of motor A, and the robot goes in a straight line (as long as its wheels are the same size!). This method can be extended to use any number of wheels. An engine that will always be driven directly (in this case motor A), and others will have their speed stuck at it. Note that if the direct drive motor is faster, or more free-running than the others, then when it is driven at its fastest speed (the PWM signal is always ON) and other engines will never be able to follow and yet the robot to deviate. It is therefore preferable to directly drive the slower the engine.

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