The DC motor control system with TL494 as the core is introduced, using PWM technology. The H-bridge DC motor control system based on TL494 can simplify the circuit structure, strong driving capacity, low power consumption and convenient control, and stable performance.
Since the DC motor has good starting, braking and speed control, it has been widely used in various aspects such as industrial and aerospace. With the development of power electronics, pulse width modulation (PWM) speed control technology has become a common speed regulation method for DC motors, with high speed regulation accuracy, fast response speed, low speed range and low power consumption. The H-bridge circuit is used as the power drive circuit of the drive, and the four quadrant operation of the DC motor can be easily achieved, including forward rotation, forward braking, reverse, reverse braking, has been widely used in the modern DC motor servo system. .
1, DC motor PWM speed control principle
It is well known that the DC motor speed formula is:
DC motor speed control can be divided into excitation control method and armature voltage control method. Excitation control methods are very useful, and most applications use the armature voltage control method. With the advancement of power electronic technology, the armature voltage can be achieved by a variety of ways, wherein the pulse width modulation (PWM) is commonly used to change the speed control method of the armature voltage. The method is to adjust the armature voltage U of the electric motor to adjust the DC motor by changing the ratio of the electrical electrical electrical travel and the power generation period (ie, the duty ratio), thereby controlling the motor speed.
The core component of the PWM is a voltage-pulse width converter that modulates the pulse width according to the control command signal to control the on-time communication time of the high power transistor with the width of the command change, and realizes two Control of end voltage.
The voltage-pulse width converter structure is composed of a triangular wave generator, an adder, and a comparator. Triangular wave generator is used to generate a triangular wave UT of a certain frequency, which adds a signal UI + UT with the input command signal UT, and then feeds to the comparator. The comparator is an operational amplifier working in an open-loop state with extremely high open loop gain and limiting switching characteristics. The weak changes in the signal difference between the two inputs will cause the comparator to output the corresponding switch signal. In general, the comparator is negative input, and the signal UI + UT is input from the positive terminal. When UI + UT "0, the comparator outputs a full level of positive level; when UI + UT" 0, the comparator outputs a full-amplified negative level.
Figure 1 Voltage-pulse width comparator
The modulation process for the voltage-pulse width converter to the signal waveform is shown in Figure 2. Due to the limit characteristics of the comparator, the output signal US is unchanged, but the pulse width varies with the change of the UI, the frequency of the US is determined by the frequency of the triangular wave.
When the command signal UI = 0, the output signal US is a rectangular pulse equal to the positive and negative pulse width. When UI "0, the positive pulse width of the US is greater than the negative pulse width. When UI "0, the positive pulse width of the US is smaller than the negative pulse width. When UI "UTPP / 2 (UTPP is the peak of triangular waves), US is a positive DC signal; when UI" UTPP / 2, US is a negative DC signal.
Figure 2PWM pulse width modulation waveform
2, DC motor drive control total flow chart
The DC motor drive control circuit is divided into control signal circuitry, pulse width modulation circuit, drive signal amplifying circuit, H-bridge power drive circuit, etc., and the control flow is as shown in FIG.
Figure 3 DC motor drive control total flow chart
As can be seen from Fig. 3, the motor logic control signal is first transmitted from the single chip, mainly including the motor operating direction signal DIR, the motor speed signal PWM, and the motor brake signal Brake, and then pulse width modulation by TL494, the output signal drives the H bridge. The power circuit drives the DC motor. The H bridge is made of four high-power enhanced field effectors, and its function is to change the steering of the motor and amplify the drive signal.
3, TL494 pulse width modulation circuit
3.1, TL494 Each pin function
In a circuit that implements the motor PWM control, the system uses a TL494 chip that consists of a reference voltage generating circuit, an oscillation circuit, a intermittent period adjustment circuit, two error amplifiers, pulse width modulators, and output circuits. A total of 16 feet, the functional structure is shown in Figure 4.
The TL494 chip is widely used in single-ended dual-tube type, half-bridge, full bridge switching power supply. Sippoint resources are:
◆ Integrate all pulse width modulation circuits.
◆ Online linear serrated wave oscillator, external oscillation elements are only two (a resistor and a capacitor).
◆ Built-in error amplifier.
◆ Substitute 5V reference reference voltage source.
◆ Adjustable dead time.
◆ Built-in power transistors provide 500mA driving capabilities.
◆ Push or pull two output methods.
3.2
TL494 is a fixed frequency pulse width modulation circuit, a linear serrated wave oscillator, and an oscillation frequency can be adjusted by an external resistance and a capacitor, and its oscillation frequency is as follows:
Figure 4TL494 Interface
The width of the output pulse is achieved by comparing the positive electrode zigzag voltage on the capacitor CT to another two control signals. Power output tubes Q1 and Q2 are controlled by or non-gates. When the clock signal of the double trigger is low, it will be selected, i.e., only when the serrated voltage is greater than the control signal. When the control signal is increased, the width of the output pulse will be reduced.
The control signal is input external to the integrated circuit, and all the way to the dead time comparator, all the way to the input of the error amplifier. The dead time comparator has a 120mV input compensation voltage, which limits the minimum output dead time, about 4% of the serrated cycle, when the output is connected, the maximum output duty cycle is 96%, while the output termination reference Usually, the duty cycle is 48%. Additional dead time can be generated on the output pulse when the dead time control input is connected to the input voltage (between 0-3.3.).
The chip has the characteristics of strong anti-interference ability, simple structure, high reliability, and low price.
3.3, circuit design based on TL494 push-pull output
The specific implementation circuit of the control system is shown in Figure 5. System power drives use MOSFET with high input impedance, which can be driven directly from crystal triode. The 13 feet of TL494 are used to control the output mode. In this system, select the end input to a low level. At this time, the TL494 inner triggers Q1 and Q2 are inseparable, the two outputs are the same, the frequency and the oscillator frequency are the same, and the maximum duty cycle is 98%.
Figure 5 is based on the circuit design of the TL494 push-pull output.
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