51 single chip microcomputer and TA8435 stepper motor subdivision control

1 Stepper motor
Stepper motor is a pure digital control motor. It converts electrical pulse signal into angular displacement. That is, when a pulse is given, the stepper motor rotates an angle. Therefore, it is very suitable for single-chip microcomputer control. Under non-overload conditions, 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 non-accumulative errors and 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, when it rotates a circle, there is no cumulative error and it has good followability.

2) The open-loop CNC system composed of stepper motors and drive circuits is simple, cheap, and very reliable. At the same time, it can also form a high-performance closed-loop CNC system with angle feedback links.


3) The dynamic response of stepper motors is fast, and it 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, so it is generally possible to directly drive the load without a reducer.

5) Stepper motors can only run with pulse power supply, and cannot directly use AC power supply and DC power supply.

6) Stepper motors have 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 mechanical structure. Figure 1 is the 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 degrees as an example, in the full-step mode, the stepper motor rotates 1.8 degrees for each pulse it receives, and 200 pulses are required for one rotation. In the half-step mode, the stepper motor rotates 0.9 degrees 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 excitation winding of the stepper motor 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 rotation 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 contains the working principle of subdivision.

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 (10-40V);

2) Output current can reach 1.5A (average) and 2.5A (peak);

3) Full step, half step, 1/4 subdivision, 1/8 subdivision operation modes are available;

4) Pulse width debugging chopper drive mode is adopted;

5) With forward/reverse control function;

6) With reset and enable pins;


7) Single clock input or dual clock input can be selected.

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 continues to rise and fall to form
a sawtooth wave. 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 comparator forward voltage 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, and 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 stages, except that the comparator forward voltage is increased again. 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, under the joint action of the current flowing through coil A and coil B, the stepper motor runs a subdivision step.

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: M1 = 0, M2 = 0, the motor operates in full step mode; M1 = 1, M2 = 0, the motor operates in half step mode; M1 = 0, M2 = 1, the motor operates in 1/4 subdivision mode; M1 = 1, M2 = 1, the motor operates in 1/8 step subdivision mode, CW/CWW controls the motor's rotation direction, the maximum frequency of the CK1 and CK2 clock inputs cannot exceed 5KHz, and the control clock frequency can control the motor's rotation rate. When REFIN is high, the output voltage of NFA and NFB is 0.8V, and when REFIN is low, the output voltage of NFA and NFB is 0.5V. These two pins control the input current of the stepper motor, and the relationship between the current and the external resistance of the NF terminal is: IO = Vref/Rnf. In Figure 4, set REFIN = 1, select the stepper motor rated current of 0.4A, R1, R2 use 1.6 ohm, 2W high-power resistors, O, C two lines are not connected. The stepper motor is used as two-phase bipolar, and the output torque of the stepper motor can be increased when the four-phase is used as two-phase. The D1-D4 fast recovery diode is used to discharge the winding current.

The following is the program for controlling the stepper motor using TA8435, realizing the function of controlling the clockwise rotation of the stepper motor using 1/8 subdivision, and using timer 1 to output pulses to TA8435 to control the speed of the stepper motor.

3 Conclusion

This article introduces the characteristics of stepper motors and the working principle of TA8435 chip. The use of subdivision can improve the control accuracy of stepper motors and reduce the vibration and noise of stepper motors. 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 for TA8435 to switch between subdivision, half step and full step working modes. Using TA8435 to control stepper motors has the characteristics of low price, simple control and reliable operation, so it has high promotion value and broad application prospects.