This section covers all types of engines, ranging from basic circuits needed to control a variable reluctance motor, the H-bridge circuitry needed to control a motor pole permanent magnet. Each class drive circuit is illustrated by practical examples, but these examples are not intended to be an exhaustive catalog of the control circuits commercially available, or the information contained herein intended to substitute for information found on the manufacturer data sheets for components parts mentioned. This section covers only the most elementary circuits of control for each engine category. All these circuits assume that the power of the engine provides a control voltage does not exceed the rated motor voltage, which substantially limits the performance of the engine. The next section, the circuit current limited disk covers circuits practice high-performance disk. Variable reluctance controllers typical reluctance stepper motors variable are variations on the contour of Figure 3. 1: Figure 3. 1 in Figure 3. 1, the boxes are used to represent the switches, a control unit, not shown, is responsible for providing control signals to open and close the switches at the appropriate times to run the engines. In many cases, the control unit is a computer or programmable interface controller, with software directly generating the outputs needed to control the switches, but in other cases, an additional control circuit is introduced, sometimes free ! The windings of motors, solenoids and similar devices are all inductive loads. As such, the current in the motor winding can be switched on or off instantly without involving infinite tension! When the command switch of a motor winding is closed, allowing current to flow, the result of this is a slow rise in current. When the command switch of a motor winding is opened, the result of this is a point of tension that can seriously damage the switch unless care is taken to address them appropriately. There are two basic ways to cope with the peak voltage. One is to bridge the motor winding with a diode, and the other is to bridge the motor winding with a capacitor. Figure 3. 2 illustrates two approaches: Figure 3. 2 The diode in Figure 3. 2 must be capable of full flow through the motor winding, but it did lead briefly when the switch is off, as the current through the winding declines. If diodes relatively slow, as the common family 1N400X are used in conjunction with a quick switch, it may be necessary to add a small capacitor in parallel with the diode. The capacitor in Figure 3. 2 presents design problems more complex! When the switch is closed, the capacitor discharges through the switch to ground, and the switch must be able to handle the brief surge of discharge current. A resistor in series with the capacitor or in series with the power will limit the current. When the switch is open, energy stored in the motor winding to charge the capacitor to a voltage well above the supply voltage and the switch must be able to withstand this tension. To solve for the size of the capacitor, we equate the two formulas for the energy stored in a resonant circuit: P = C V2 / 2 R P = I2 / 2 Where: P – stored energy in seconds or watt coulomb volt C – capacity in farads V – voltage across capacitor L – inductance of the winding engine, Henry I – current in motor winding Solving the minimum size of capacitor required to avoid overvoltage on the switch is easy enough: C> L I2 / (Vb – Vs) 2 where: Vb – the breakdown voltage of the VS switch – the voltage variable reluctance inductor have a variable that depends on the angle of the shaft. Therefore, the worst case, the design must be used to select the capacitor. In addition, the motor inductances are often poorly documented, if at all. The capacitor and the motor winding, in combination, form a resonant circuit. If the control system drives the motor at frequencies near the resonant frequency of this circuit, the motor current through the motor windings and, therefore, the torque exerted by the engine will be very different from the torque of the steady state at nominal operating voltage! The resonant frequency is: F = 1 / (2 (SC) 0. 5) Again, the resonance frequency for an electric motor with variable reluctance depends on the angle of a tree! When a variable reluctance motor is used with pulse exciting near resonance, the oscillating current in the motor winding leads to a magnetic field that goes from zero to twice the resonance frequency, which can seriously reduce the Couple available! Controllers unipolar permanent magnet motors and the typical hybrid stepper motors unipolar are variations on the contour of Figure 3. 3: Figure 3. 3 in Figure 3. 3, as shown in Figure 3. 1, the boxes are used to represent the switches, a control unit, not shown, is responsible for providing control signals to open and close the switches at the appropriate times to run the engines. The control unit is typically a computer or programmable interface controller, with software directly generating the outputs needed to control the switches. As for the control circuits for variable reluctance motors, we must deal with the inductive stroke occurs when each of these switches is turned off. Again, we may shunt the kick induction using diodes, but now, 4 diodes are required, as shown in Figure 3. 4: Figure 3. 4 The diodes are needed because the motor winding is not two independent inductors, it is a single center operated inductor with center tap at a fixed voltage. This acts as an autotransformer! When one end of the motor winding is pulled down, the other end will fly up, and vice versa. When a switch opens, the inductive bounce will drive this end of the motor winding to the positive supply, where it is blocked by the diode. The opposite end will fly down, and if it was not floating at the voltage at the time, it will fall under the ground, reversing the voltage across the switch to this end. Some switches are immune to such turnarounds, but others may be seriously damaged. A capacitor may also be used to limit the voltage bounce, as shown in Figure 3. 5: Figure 3. 5 The rules for sizing the capacitor in Figure 3. 5 are the same as the rules for sizing the capacitor in Figure 3. 2, but the resonance effect is quite different! With a permanent magnet motor, if the capacitor is driven at or near the resonant frequency, the couple will rise to as much as twice the torque at low speed! The resulting torque curve over-speed may be quite complex, as illustrated in Figure 3. 6: Figure 3. 6 Figure 3. Figure 6 shows a peak in the available torque to the electric resonance frequency, and a valley at the mechanical resonance frequency. If the frequency of electrical resonance is placed appropriately above what would have been cut-off speed of the engine using a diode-PF, the effect may be a considerable increase in the rate cut effective. The mechanical resonance frequency depends on the couple, so if the frequency of mechanical resonance is nowhere near the electric resonance, it will be moved by the electric resonance! In addition, the width of the mechanical resonance depends on the local slope of torque versus speed curve, if the torque decreases with speed, mechanical resonance will be sharper, while if the couple climbs at a speed, it will be broader or even split into several resonant frequencies. Practice Unipolar and Variable Reluctance Drivers In the circuit above, the details of necessary switches have been deliberately ignored. Any technology switching, toggle switches to operate power MOSFET! Figure 3. 7 contains some suggestions for implementation of each switch, a motor winding and a diode protection included for guidance: Figure 3. 7 Each of the switches in Figure 3. 7 is compatible with a TTL input. The 5 volt power supply used for logic, including the 7407 driver is open-sensors used in the figure must be well regulated. The engine power, typically between 5 and 24 volts, needs only minimal regulation. Note that these circuits are switching power supply suitable for driving solenoids, DC motors and other inductive loads and for driving stepper motors. SK3180 transistor shown in Figure 3. 7 is a Darlington power with a current gain over 1000: thus, the 10 milliamps passing through resistance of 470 ohms is more than sufficient to allow the transistor to switch a few amperes of current in motor winding. The stamp of 7407 used to drive the Darlington May be replaced by a chip high voltage open collector can sink at least 10 milliamperes. In the case where the transistor fails, the pilot high voltage open collector is used to protect the rest of the logic circuits from the power of the engine. The IRC IRL540 shown in Figure 3. 7 is a transistor having a force field. It can handle currents up to about 20 amps, and it decomposes nondestructive 100 volts, therefore, this chip can absorb inductive spikes without diode protection if it is attached to a large enough heat sink. This transistor has a switching time very fast, so the protection diodes must be comparable or faster bypassed by small capacitors. This is especially important with the diodes used to protect the transistor bias against cons! In the case where the transistor fails, the Zener diode and 100 ohm resistor protect TTL circuits. The resistance of 100 ohms also acts to slow a little time on the switching transistor. For applications in which each motor winding draws less than 500 milliamps, the family ULN200x tables Darlington Allegro Microsystems, also available as DS200x from National Semiconductor, and as the Motorola MC1413 table Darlington will drive the motor windings or multiple of other inductive loads directly from the inputs. Figure 3. Figure 8 shows the pinout of the chip ULN2003 widely available, a table of 7 Darlington transistors with TTL compatible inputs: Figure 3. 8 The basic resistance of each transistor Darlington corresponds to standard bipolar TTL outputs. Each darlington NPN is wired with its emitter connected to pin 8, designed as a grounding prong, each transistor in the package is protected by two diodes, short-circuiting the emitter and collector, protection against reverse voltages through the transistor, and one connecting the collector to the pin 9, and if the pin 9 is connected to the positive power of the engine, this diode will protect the transistor against inductive spikes. ULN2803 chip is essentially the same as the ULN2003 chip described above, except that a 18-pin package, and contains 8 DARLINGTONS, allowing one chip to be used to drive a pair of common magnet pole permanent and variable reluctance motors. For motors drawing less than 600 milliamps per winding, power quad UDN2547B pilot made by Allegro Microsystems will take care of all 4 common windings of stepper motors unipolar. For motors drawing less than 300 milliamps per winding, Texas Instruments SN7541, 7542 and 7543 drivers double power, are a good choice, both options include some logic with the pilot power. Motors and bipolar H-Bridges things are more complex permanent magnet bipolar stepper motors because they have no center taps on their windings. Therefore, to reverse the direction of the field produced by a motor winding, we need to reverse the current in the coil. We could use a double-pole double-throw electromechanical switch to do the electronic equivalent of such a change is called an H-bridge and is described in Figure 3. 9 As with the unipolar drive circuits discussed above, the switches used in the H-bridge must be protected against voltage spikes caused by off in a motor winding. This is usually done with diodes as shown in Figure 3. 9. It is noteworthy that H-bridges are applicable not only to control stepper motors bipolar, but also for control of DC motors, push-pull solenoids (those divers with permanent magnets) and many other applications. With 4 switches, the H-bridge has 16 basic modes of operation possible, including 7 short-circuiting the power! The following operating modes are of interest: forward mode, switches A and D closed. Reverse mode, the switch B and C closed. These are the usual modes of operation, allowing current to flow of food through the motor winding and beyond the earth. Figure 3. 10 shows forward mode: Figure 3. 10 fast decay mode or freewheel mode, all switches open. Any current flowing in the motor winding will work against voltage track, plus two drops of diode current to decay rapidly. This method offers little or no effect dynamic braking of the motor rotor, so that the rotor side freely if all motor windings are powered in this mode. Figure 3. 11 illustrates the flow of current immediately after switching from forward mode to perform a rapid decline. Figure 3. 11 ways to slow decay modes or dynamic braking. In these modes, during May to circulate through the motor winding with minimal resistance. Consequently, if the current flows in a winding engine when one of these modes is entered, the current decay slowly, and if the motor rotor turns, it will induce a current which will act as a brake on the rotor . Figure 3. 12 illustrates one of many useful slow decay modes, D with switch closed and if the motor winding has recently been running mode ahead, the state of the switch B may be open or closed: Figure 3. H-12 Most bridges are designed so that the logic necessary to prevent a short circuit is at a very low level in the design. Figure 3. 13 illustrates what is probably the best arrangement: Figure 3. 13 Here, the following operating modes are available: XY ABCD 00 0000 rapidly decreasing 01 1001 before 10 0110 Reverse 11 0101 slow decay The advantage of this arrangement is that all relevant modes are preserved and that they are encoded with a minimum number of bits, it is important when using a microcontroller or a computer system to drive the H-bridge because many numbers of these systems have little bits available for the parallel output. Unfortunately, few integrated H bridge chips on the market have such a simple control system. Practice Bipolar Circuits There are a number of H-bridge driver integrated market, but it is always useful to examine the implementations of discrete components to understand how an H-Bridge Works. Antonio Raposo (RDA @ Cybill. INESC. Pt) proposed the H-bridge circuit of Figure 3. 14; Figure 3. 14 inputs X and Y for this circuit can be powered by open-collector TTL outputs as in the unipolar DARLINGTON-based drive circuit in Figure 3. 7. The motor winding will be supplied if exactly one of the inputs X and Y is high and precisely one of them is low. If both are low, both pull-down transistors are off. If both are high, both pull-up transistors are off. Consequently, this simple circuit is the engine braking mode dynamics in both the 11 and 00 states, and does not offer a way of cabotage. The circuit of Figure 3. 14 consists of two identical halves, each of which may be properly described as a push-pull driver. Half Term The H-bridge is sometimes applied to these circuits! It is also interesting to note that a half H-bridge circuit is quite similar to the output circuit drives used in TTL logic. In fact, TTL tri-state line drivers such as the 74LS244 and 74LS125A can be used as a half H-bridges for small loads, as shown in Figure 3. 15: Figure 3. 15 This circuit is effective for driving motors with a maximum of about 50 ohms per winding tension less than about 4. 5 volts with a supply of 5 volts. Each tri-state buffer in the LS244 can run about twice the current it can source and internal resistance of the buffer is sufficient, which take their course, to share power equitably among drivers who are executed in parallel. This drive motor for all relevant states obtained by the driver in Figure 3. 13, but these states are not coded as efficiently: XYE Mode – 1 rapid disintegration disintegration 000 slow 010 forward 100 reverse 110 The second slow decay mode DB, XYE = 110, provides a marginally weaker than the first braking due to the fact that the LS244 drivers may fall more power than they can source. The microprocessor (formerly Telcom Semiconductor) TC4467 Quad CMOS driver is another example of a general purpose driver that can be used as 4 semi-independent H-bridges. Unlike the old drivers, the data sheet for this driver even suggests the use of applications for motor control, with supply voltages of 18 volts and 250 milliamps per winding engine. One problem with the chips on the market for controlling stepper motors is that many of them have lifetimes market relatively short. For example, the Seagate IPxMxx series of H-bridge chips double (IP1M10 by IP3M12) was very well thought out, but unfortunately it seems that Seagate made these when they used stepper motors to position the head in Seagate. The Toshiba TA7279 dual H-bridge driver is another excellent choice for engines with another of less than 1 amp, but again, it seems to have been made for internal use. The SGS-Thompson (and others) L293 dual H-Bridge is a direct competitor for bullets above, but unlike them it does not include protection diodes. The L293D chip, introduced later, is pin-compatible and includes these diodes. If the earlier L293 is used, each motor winding must be resolved through a bridge rectifier (1N4001 equivalent). Using external diodes allows a series resistor to be placed in the path of recirculation current to accelerate the decomposition of the current in a coil when the engine is off, this may be desirable in certain applications. The family L293 offers an excellent choice for driving small bipolar stepper development of one ampere by the motor winding up to 36 volts. Figure 3. 16 shows the pinout common chip L293D and L293B: Figure 3. 16 This chip may be regarded as semi-independent 4-H bridges, has allowed two or as two full H bridge. This is a SMD power package with 4 pins, 5, 12 and 13 to conduct heat to the circuit board or an external heat sink. The SGS-Thompson (and others) L298 dual H-bridge is quite similar to the above, but which is capable of handling up to 2 amps per channel and is packaged as a component of power, as with the LS244, it is security of the H son of two bridges in the package 298 in one of the 4-H-AGP bridge (the data sheet for this chip provides specific advice on how to do). A warning is appropriate on the 298, the switches of this chip very fast, fast enough that the current protection diodes (equivalent 1N400X) does not work. Instead, use a diode as the BYV27. The National Semiconductor LMD18200 H-bridge is another good example, which manages up to 3 amps and has full protection diodes. While H built bridges are not available for very high currents or very high voltages, it is well designed components on the market to simplify the construction of bridges in H from separate switches. For example, International Rectifier sells a range of half H-bridge drivers, two of these chips over 4 MOSFET switching just build a bridge H. The IR2101, IR2102 and IR2103 are based H half bridge drivers. Each chip has 2 inputs to directly control the two switching transistors on a leg of a H-bridge The IR2104 and IR2111 have the same logic output side to control the switches of an H-bridge, but they also understand the logic input side, in some applications, may reduce the need for external logic. In particular, the 2104 includes an enable input, so 4 2104 chips plus 8 switching transistors can replace an L293 with no need for additional logic. The data sheet microprocessor (formerly Telcom Semiconductor) TC4467 family of CMOS quad driver includes information on how to use the drivers in the family to drive the power MOSFET H-bridges at speeds up to 15 volts. A number of manufacturers make complex H-bridge chips that include a circuit current limiting, which are the subject of the next section. It is also interesting to note that there are a number of 3-phase bridge drivers on the market, able to move Y or delta configured 3-phase permanent magnet steppers. Few of these engines are available, and these chips are not developed with steppers in mind. However, the Toshiba TA7288P, sees GL7438, the TA8400 and TA8405 are clean, and 2 chips of this type, with one of 6 half-bridges ignored, will strictly control 5-winding 10 steps per revolution motor.