Stepper motor control circuit

Several dedicated integrated circuit drivers are available for four-phase stepper motors. SAAl027 is one of the commonly used ones. It is characterized by a wide operating voltage range of 9.5V to 18V and a large output drive current of up to 500mA. It is suitable for controlling four-phase full-step stepper motors. Figure 4 is the appearance and pin function diagram of SAAl027. Figure 5 (↓ below) is its internal principle block diagram and basic application. 　　In fact, the integrated circuit has three buffer inputs, and each buffer input controls a two-bit (four-state) synchronous reversible counter. Its output is fed to a transcoder. Then use four outputs to control the four transistors of the output stage. The output stage operates in open collector mode. The winding coil of the motor is connected in series to the collector. In order to prevent the reverse electromotive force from damaging the transistor, a reverse diode is connected in parallel at both ends of the winding. Special attention should be paid to the fact that pins 13 and 12 of the integrated circuit are pins through which large currents flow. A small current flows through pins 14 and 5. Pin 5 and pin 12 must be grounded during use. Usually the positive 12V is directly connected to pin 13, and then connected to pin 14 through the R1-C1 decoupling circuit. The positive voltage must also be sent to pin 4 via Rx. The role of Rx is to determine the maximum output drive current capacity of the four transistors. The size of Rx can be calculated by the following formula; Rx=(4E/I)-6 where E is the power supply voltage and I is the desired maximum phase current of the motor. When using 12V, when the Rx value is 420Ω, 180Ω or 78Ω), the maximum output current is 100mA, 200mA or 350mA respectively. The SAA1027 integrated circuit has three input control terminals: count, mode and reset. The reset terminal is usually high level. Each transition of the counter from low level to high level will cause the integrated circuit to change state. All working statuses are listed in Table 3. 　　At any time, the sequence repeats every fourth step. But when the reset terminal is low level, it can be reset to the starting state. When the mode control input is low, the sequence repeats in one direction (usually clockwise). On the contrary, when the mode control end is high level, the sequence is repeated in the other direction (counterclockwise rotation). Figure 6 is the driving and test circuit of SAAl027. 　　This circuit is used for hybrid four-phase stepper motors with rated currents up to 300mA. The motor can use SW3 for manual single-step testing, or SW2 for automatic step testing via the 555/7555 astable oscillator. SW4 can control the direction of the motor. SW5 is used for reset control test. Using SW1 and RV1 potentiometers, the working speed of the astable circuit can be changed within a wide range. When the 1st gear is set, it is low-speed control, and the frequency range is from 5Hz to 68Hz. When SW2 is in the 2nd and 3rd gears, the oscillation frequency is 10 times and 100 times that of the 1st gear respectively. The total speed control range is from 6-8500 rpm. Figure 6 is a basic circuit. There are several variations according to different usage occasions. Figure 7 is an interface circuit between a stepper motor and a microprocessor. The output port of a computer or microprocessor, usually when the terminal drive voltage is lower than 1V, it is in a logic 0 state; when it is higher than 3.5V, it is in a logic 1 state. This kind of logic is called positive logic. However, the circuit in Figure 7 is the opposite of the above. Therefore, when the stepper motor input terminal transitions from high level to low level, the working state changes. The reset terminal is reset with high level. When the mode input terminal is low level, the motor rotates forward; when it is high level, the motor rotates reversely. The maximum output current of the circuit design in Figure 6 is 300mA. If you want to extend the current by 5A, use the two circuits in Figure 8. Each phase of a stepper motor requires an external drive circuit. A four-phase stepper motor requires four such additional circuits. The circuit in Figure 8(a) is used to drive a four-phase stepper motor, and four such additional circuits need to be added. The circuit of Figure 8(a) is used to drive four completely independent windings. The circuit of Figure 8(b) is for a stepper motor with a common point in the windings. The function of D1 and D2 is to prevent the back electromotive force of the motor from damaging the output stage transistor.