Stepper motor subdivision control method based on single chip microcomputer and TA8435

1 Stepper Motor 

    The stepper motor is a pure digital control motor, which converts the electrical pulse signal into angular displacement, that is, when a pulse is given, the stepper motor rotates an angle, so it is very suitable for single-chip microcomputer control. In the case of non-overload, the motor speed and stop position only depend on the frequency and number of pulses of the pulse signal, and are not affected by load changes. The motor rotates a step angle. At the same time, the stepper motor has only periodic errors but no cumulative errors, and has high precision.

    Stepper motors have the following characteristics:

    ① The angular displacement of the stepper motor is strictly proportional to the number of input pulses. Therefore, after it rotates one circle, there is no cumulative error and it has good followability.
    
    ② The open-loop CNC system composed of the stepper motor and the drive circuit is simple, cheap and very reliable. At the same time, it can also form a high-performance closed-loop CNC system with the angle feedback link.
    
    ③ The stepper motor has a fast dynamic response and is easy to start and stop, forward and reverse, and change speed.

    ④ The speed can be adjusted smoothly within a fairly wide range, and a large torque can still be obtained at low speed. Therefore, the load can generally be driven directly without a reducer.

    ⑤The stepper motor can only operate by being powered by a pulse power supply and cannot directly use AC power supply and DC power supply.

    ⑥ Stepper motors may experience oscillation and step-out phenomena, and corresponding measures must be taken for the control system and mechanical load.

    Stepper motors have the advantages of simple control and mechanical structure. Figure 1 is a schematic diagram of a four-phase six-wire stepper motor. This type of stepper motor can be used as a four-phase motor or a two-phase motor. It is flexible to use and therefore widely used.　
 

    There are two working modes for stepper motors: full-step mode and half-step mode. Taking a four-phase hybrid stepper motor with a step angle of 1.8° as an example, in the full-step mode, the stepper motor rotates 1.8° for each pulse it receives, and 200 pulses are required for one rotation. In the half-step mode, the stepper motor rotates 0.9° for each pulse it receives, and 400 pulses are required for one rotation. To control the rotation of the stepper motor, pulses must be input to the stepper motor leads in a certain sequence. Taking the above-mentioned four-phase six-wire stepper motor as an example, the control sequence of its half-step working mode and full-step working mode is listed in Table 1 and Table 2.

    When the stepper motor works at a low frequency, it will have the disadvantages of large vibration and noise. If the subdivision method is used, this problem can be well solved. The subdivision control of the stepper motor is essentially to control the current in the stepper motor excitation winding so that the synthetic magnetic field inside the stepper motor is a uniform circular rotating magnetic field, thereby realizing the subdivision of the stepper motor step angle. In general, the amplitude of the synthetic magnetic field vector determines the size of the stepper motor's rotational torque, and the angle between two adjacent synthetic magnetic field vectors determines the size of the step angle. The half-step working mode of the stepper motor implies the working principle of subdivision. [page]

    There are many ways to achieve subdivision. The most commonly used is the pulse width modulation chopper drive method. Most dedicated stepper motor driver chips use this drive method. TA8435 is one of them.　

2 Stepper motor subdivision method based on TA8435H chip 

2.1 TA8435 chip features 

    TA8435 is a single-chip sinusoidal subdivision two-phase stepper motor driver chip produced by Toshiba. The chip has the following features:

    ① Wide operating voltage range (10-40 V);
    
    ② Output current can reach 1.5 A (average) and 2.5 A (peak));
    
    ③ Full step, half step, 1/4 subdivision, 1/8 subdivision operation modes are available;
    
    ④ Pulse width modulation chopper drive mode is adopted;
    
    ⑤ With forward/reverse control function;

    ⑥With reset and enable pins;

    ⑦You can choose to use single clock input or dual clock input.

    As can be seen from Figure 2, TA8435 is mainly composed of 1 decoder, 2 bridge drive circuits, 2 output current control circuits, 2 maximum current limiting circuits, 1 chopper and other functional modules.

2.2 TA8435 subdivision working principle 

    In Figure 3, at the first CK clock cycle, the decoder turns on the bridge drive circuit, and the current flows from VMA through the motor coil and then through RNFA to form a loop with the ground. Due to the effect of the coil inductance, the current gradually increases, so the voltage on RNFB also increases. When the voltage on RNFB is greater than the voltage at the positive end of the comparator, the comparator turns off the bridge drive circuit, the current on the motor coil begins to decay, and the voltage on RNFB decreases accordingly; when the voltage value is less than the forward voltage of the comparator, the bridge drive circuit is turned on again, and the cycle continues. The current rises and falls continuously to form a sawtooth wave, and its waveform is shown in the first section of the IA waveform in Figure 3. In addition, since the chopper frequency is very high, generally tens of kHz, its frequency is related to the selected capacitor. Under the action of OSC, the current sawtooth ripple is very small, and the output current can be approximately considered to be DC. At the beginning of the second clock cycle, the output voltage Ua of the output current control circuit reaches the second stage, and the forward voltage of the comparator also corresponds to the voltage of the second stage. Therefore, the current flowing through the stepper motor coil also rises from the first stage to the second stage 2. The current waveform is shown in the second part of Figure IA. The working principle of TA8435 in the third and fourth clock cycles is the same as that in the first and second clock cycles, except that the comparator forward voltage is increased. The output current waveform is shown in the third and fourth parts of Figure IA. In this way, a step current is finally formed, and the current added to coil B is shown in Figure 3 IB. In one clock cycle of CK, the current flowing through coil A and coil B works together, and the stepper motor runs one microstep.

2.3 Application of stepper motor 

    Figure 4 is a schematic diagram of the microcontroller connected to TA8435 to control the stepper motor. Pins M1 and M2 determine the motor's rotation mode: MI=0, M2=0, the motor runs in full step mode; M1=1, M2=0, the motor runs in half step mode; M1=0, M2=1, the motor runs in 1/4 subdivision mode; M1=1, M2=1, the motor runs in 1/8 step subdivision mode. CW/CWW controls the direction of motor rotation, and the maximum frequency of CK1 and CK2 clock inputs cannot exceed 5 kHz. By controlling the frequency of the clock, the motor rotation rate can be controlled. When REFIN is high, the output voltage of NFA and NFB is 0.8 V, and when REFIN is low, the output voltage of NFA and NFB is 0.5 V. These two pins control the input current of the stepper motor. The relationship between the current size and the external resistance of the NF terminal is: Io=Vref/Rnf. In Figure 4, set REFIN = 1, select the rated current of the stepper motor to be 0.4 A, select 1.6 Ω, 2W high-power resistors for R1 and R2, and do not connect the O and C wires. The stepper motor is used as a two-phase bipolar, and the output torque of the stepper motor can be increased when the four-phase is used as a two-phase. The fast recovery diodes D1 to D4 are used to discharge the winding current. [page]

    The following is a program to control a stepper motor using TA8435, which realizes the function of controlling the stepper motor to rotate clockwise using 1/8 subdivision. Timer 1 is used to output pulses to TA8435 to control the speed of the stepper motor.

    #include<8051. h>
    
    #define REFIN P1_5;
    
    #define M2 Pl_4}

    #define-M1 Pl_3;
    
    #defineCW Pl_2;
    
    #define CLK2 Pl_1;
    
    #deflne CLKl P1_O;
    
    void C10cklrqHandler（void）interrtlDt 3
    
    //Timer 1 interrupt routine
    
    ｛CLK2=!CLK2；
    
    TH0=0xef;
    
    TL0=0xff;
    
    void main（）
    
    ｛ PO=0x00；
    
    TMOD=0x00;
    
    EA=1：
    
    ETl=h
    
    TRl=1：
    
    REFlN=0；
    
    M2=O：
    
    M1=1：
    
    CLKl=1：
    
    CW=
    
    1．for（；;）；
    
    ｝
    
3 Conclusion 
    
    This paper introduces the characteristics of stepper motor and the working principle of TA8435 chip. The use of subdivision can improve the control accuracy of stepper motor and reduce the vibration and noise of stepper motor. Therefore, when working at low frequency, 1/4 subdivision or 1/8 subdivision mode can be selected to reduce the vibration and noise of the system. When the system needs to work at high speed, the subdivision mode may not reach the required speed. At this time, full step or half step mode can be selected. At high speed, in full step or half step working mode, the stepper motor runs stably, with small vibration and noise. It is quite easy to switch between subdivision, half step and full step working modes of TA8435. Using TA8435 to control stepper motor has the characteristics of low price, simple control and reliable operation, so it has high promotion value and broad application prospects.