Based Speed Control PWM DC Motor Drive

Posted on March 18, 2010, 6:49 am

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 motor will see an average of 6 volts, and will run slower accordingly. As the amount of time the voltage is on the increase compared to the amount of time it is extinguished, the average speed increases engine. This on-off switching 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 slow in terms of switching is dependent on the inertia of the rotor, and how much friction and load torque that is. The chart below shows the speed of a motor is switched on and off fairly slowly: We can see that the average speed is around 150, although it varies a little. If the voltage is turned on quickly enough, he will not have time to change a lot of speed, and speed will be fairly stable. This is the principle of control by the mode selector speed. Thus, the speed is set by PWM. Inductors Before continuing to discuss 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 as capacitors do not allow the voltage 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 turn off a transistor, the inductor will generate a very high voltage across it. For example, disabling 100 amps at 1 microsecond through an inductor 100 generates 10kV microhenry! Frequency The frequency PWM signal PWM result depends on the wave frequency ramp. Frequencies between 20Hz and 18kHz may produce audible cries from the speed controller and motors. Each connection and disconnection of the results of controller speed MOSFET in a loss of power shortly. Consequently, the more time switching from the static on and off, the greater will be the result of “loss of the switching MOSFET. The higher the switching frequency, the more stable is the current waveform in engines. This waveform will be a spiky waveform switching to low frequency, but at high frequencies the inductance of the motor will facilitate the observation at a DC average proportional to the PWM demand. This spikiness cause greater loss of power in the resistance of the son, MOSFETs, and motor windings of a DC current signal stable. You can see both graphs. One shows the worst case on-off current wave, the other at best constant DC current signal two waveforms have the same average current. However, when we work on power dissipation in the resistors in our errant motor and inverter for the DC case: And in case of switching the average power is thus in the waveform switching twice power is lost in the stray resistances. In practice, the current signal is not square wave like that, but it remains true that there will be more power loss in a non-DC waveform. Choosing a frequency based on the characteristics of the engine then can we develop mathematically the minimum frequency to achieve this goal. This section is a little math so you may want to miss it and just use the final equation. The following shows the equivalent circuit of the engine, and the waveform as the PWM signal on and off. This shows the worst case, a 50:50 ratio PWM and the current rise is shown for an engine stopped or stalled, also worst case. T is the switching period, which is the inverse of the switching frequency. Just taking the falling edge of current signal is given by the equation? is the time constant circuit, which is L / R. Therefore, the current at t = T / 2 (I1) must not be less than P% lower at t = 0 (I0). This means that there is a boundary condition: So 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, this may be able to generate the waveform, although if you have more than a couple of engines, this may be too much of a burden on the resources of the microcontroller . Several methods are described below. The electronic analog PWM signal is generated by comparing a signal from the triangular wave with a DC signal. The DC signal can vary between the minimum and maximum voltages of the triangle wave. When the triangle voltage waveform is above the level of voltage, the output of the op-see-intensive, and when it is lower, swings low production. From the graph we see that if the DC level gone, the pulses would be even thinner. It uses a counter and a scale of resistance weighted to generate the triangular signal (in fact it will generate a sawtooth, but you’ll still have a PWM signal to the end of it). The actual resistance values that are not available (40k, 80k) can be made with 20k resistors, or close approximations can be used. The 74HC14 is a Schmitt input inverter, which is connected to act as a simple oscillator. The oscillation frequency is about f = 1 / (2. PI. R. C), but not much within a few tens of percent. This square wave flow generated the 4-bit binary 74HC163. All entries preset and clear are disabled, so the outputs QA to QD just rolling around the binary sequence from 0000 to 1111 and a renewal to 0000 again. These outputs, which swing between 0 V and 5 V are introduced into an amplifier was binary weighted section leftmost with the LM324 FLASH 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 in terms of output amp1. The result of this opamp simply multiplies the voltage by – ½, to the positive voltage, and back to logic voltage levels, see the AMP2 output column in the table. Value Count Value amp1 binary output (Volts) AMP2 output (Volts) 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, rightmost opamp compares the voltage with the contribution of the application of voltage, which varies from 0 V to 4. 6875v, where 0V represents 0% PWM ratio and 4. 6875v represents 100% PWM rate. This voltage application range from May-12V to +12 V, but only 0 to 4. 6875 Range will adjust the rate of PWM. The PWM generator chips are available ICs that convert a DC level in PWM. Many of them are designed for use in switching power supplies. Manufacturer of CI normal SGS Thomson SG1524, SG1525. . . SMPS Maxim MAX038 Signal generation Alternatively, a MOSFET driver, which includes a PWM generator can be used. I know of one that is not out yet! SGS Thomson TD340. Digital method The numerical method is to increment a counter, and comparing the counter value with a pre-loaded register value. It is essentially a digital version of the analog method above: The register must be loaded with the PWM level required by a microcontroller. It may be replaced by a simple ADC if the level should be controlled by an analog signal (as would a servo radio control). This method is really useful if a microcontroller is used in your robot, which can easily preload register. Embedded Microcontroller If it which can greatly simplify the process of signal generation. The Hitachi H8S series has up to 16 PWM outputs available, but many other types have two or three. Interfacing with the high power electronics, but there are two sides to electronics: the side of low power and high-side power. The low power electronics board includes a microcontroller, the radio receiver, and PWM generators. The upper part includes the power MOSFET drivers, the MOSFETs themselves, and any valve or pump drivers you may have. Basically anything that is large switching currents. Interfacing with the radio receiver control You may be able to tap into the PWM signal coming out of radio receiver before it enters the servo, and use this drive for entry to the driver MOSFET. However, it gives you no choice of switching frequency. Alternatively, the knob can generate a voltage to power the PWM generator. A more advanced method, if you have a micro-controller on board the robot to take the PWM signal radio receiver and connect it to a clock input of the microphone. The microcontroller must be able to decode this waveform, and generate a value proportional analog output (if it has ADC, or if an external ADC is installed). Another more advanced method is to send serial data communications through radio. The combined radio command will need to have a microcontroller in. The microcontroller must read the pots and switches on the handset, and send orders out of its proper UART. This connects to the transmitter. At the receiver, the demodulated output is sent to the robot microcontroller UART, and the data is decoded. Current limitation of current limiting is absolutely essential. If the engine stalled, it may take considerable currents destroy the MOSFET very 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, turning the MOSFET off the bridge. If you have a micro-controller board which generates the PWM rate, it would be an advantage if the software could detect most of the state and reduce the ratio of PWM by, say, 10%. This circuit shows the MOSFET just above the bridge being driven for simplicity. Lower MOSFETs are turned off during a current limit. There is only one sense resistor required for each engine, which must be immediately connected to the battery positive terminal. The voltage dropped across the sense resistor is amplified by U1A, which is connected to a differential amplifier circuit. The gain of this is 480k / 1k is 480. This is an important benefit because the voltage dropped across the sense resistor is very low. The output of the differential amplifier is strongly low-pass filter by RxCx. This is because there will be lots of noise from the engine, and we do not want to limit the current if we do not need it. D13 is present to ensure that no negative peaks may affect the following systems. U2B compares the filtered signal with a predefined value (here represented by V5), and if the current is too high (the signal is greater than V5), U2B turn Q1 and Q2 ravaging the PWM signal from PWM generator . 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 around immediately, but it takes a few milliseconds. This stops the MOSFET be quickly activated and deactivated. A simulation of the part of current limiting circuit is shown in the diagram below. The threshold voltage V5 was chosen to define a current limit of 30 amps. The square wave is the voltage (PWM MOSFET gate voltage), and neglect the waveform is the drain (motor) current. The bit bristly at the top of the wave slopey is when the current limiter is switched in and out. Some channels you may see an example of current flowing through MOSFET main power by placing a MOSFET power much lower in parallel with it. This works well, but the problem is the actual current limit depends on the value of RDS (on) MOSFET. If Rds (on) was only half the value we expect it to be, then twice as much power would pass before the limiter circuit took effect. Also the RDS (on) value depends very much on the current through the MOSFET, and temperature. Any variation in RDS (on) will change the current limit. RDS (on) figure is cited as a maximum value on the datasheet, but this is not a safe design parameter. This means that it is not within defined limits which are published on the sheet. For example, guarantees of CMOS digital logic that the output voltage, Vo, will be between Vcc-0. 5V and Vcc, and this figure may be used to design circuits that are based on this figure. However, with RDS (on), we only know that it will be between 0 and value to the document. 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 resistance is much safer. Feedback Speed Control To stop a robot swerved in an arc when you decided to go forward, you need to control the speed of reaction of the engine. This means that the actual speed of each wheel is measured and compared to all other wheels. Obviously to go straight, the motor speed must be equal. However, this does not necessarily mean that the application rate for each engine must be the same. The engines have different amounts of friction, and therefore a “more severe”, the car will require a greater demand for speed to go as fast as an engine more free-running. A schematic diagram of a feedback controller analog speed is shown below, the demand for speed is a voltage which is fed to the PWM generator for motor A. This engine drives A at a speed dependent voltage demand. The speed of the engine A is sampled using an optical encoder. It has a frequency output that is proportional to motor speed. If we assume that the motor B is already running at a certain speed, then the optical encoder on its shaft will produce a frequency too. The phase comparator compares the two frequencies, effectively comparing the speeds of two engines. Its output is a signal that becomes bigger than the two input frequencies further apart. If both frequencies are the same, it returns zero. The integrator adds the output of phase comparator to everything before leaving. For example, if the output of the integrator was above 3 volts, and its entry is 0 volts, then its output will be 3 volts. If his contribution has changed to -1 volts, then its output would change to 2 volts. Suppose that the engine B works more slowly than A. engine and the output of phase comparator will be positive, and the output of the integrator will start to rise. The engine speed then B will increase. If it is passed to a higher speed than an engine, then the comparator output becomes negative phase, 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 a motor, and the robot will move 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 will always be a direct motor (motor in this case A), and others will have their speed locked to it. Note that if the motor is direct drive faster, or more outspoken than others, then when it is driven at its fastest speed (PWM signal is always ON), while the other engines will never be able to follow, and the robot deviate again. It is preferable to directly drive the slower motor.