Summary:A design method of subdivision driver for two-phase stepping motor based on LMDl8245 driver is given. The working principle of LMD-18245 and the hardware connection and software design of the system are mainly introduced.
Key word:LMDl8245; Stepper motor; Subdivision Driver
Introduction
Stepping motors are widely used in textile machinery such as computerized embroidery machines. The characteristics of such stepping motors are low torque, fast start-up response, low noise, stable operation, good control performance and low cost of the whole machine. Most stepping motors currently used in computerized embroidery machines are five-phase hybrid stepping motors. The purpose is to reduce the step angle and improve control by using a high-phase number stepping motor, but the performance improvement achieved by this method is limited and the cost is relatively high. The subdivision drive technology can greatly improve the operation quality of the stepping motor, reduce the torque fluctuation, suppress the oscillation, reduce the noise and improve the step resolution. If a reactive stepping motor is used, the performance can be significantly improved and the cost of the product can be greatly reduced.
A subdivision driver for two-phase stepping motor is designed by using LMDl8245 stepping motor special drive circuit of National Semiconductor Company.
Features and Functions of 1 LMDl2845
LMDl2845 is a special drive circuit for two-phase stepping motor. Its full-bridge power driver integrates all the circuits needed for bipolar stepping motor drive and control current. It also constructs a combination of bipolar CMOS control and current protection functions on the same device through DMOS power switches on the same monostable structure. At the same time, it uses innovative current detection method to basically eliminate power loss. A LMDl8245 can drive one-phase bipolar stepping motor windings. When its working voltage reaches 55V, the current can reach 3A (peak 6A). Its built-in 4-digit/analog converter provides a digital control of the current of the motor, which simplifies the implementation of full-step, half-step and subdivision drive of the stepping motor. The application of subdivision drive with higher requirements can also be achieved by expanding the DAC externally.
1.1 Main features
The operating voltage can reach 55V and the current can be continuously adjusted at 3A.
Each power switch has a low RDS (on) (usually 0.312);
Built-in clamp diode;
Low loss current detection method;
The motor current is controlled by digital or analogue control.
OTI'[. and CMOS input compatible;
In Ti=155~C~at auto-off;
Overcurrent protection;
Eliminate surge current;
Packaging with 15 pin TO1 220.
1.2 Functional Principles
LMDl8245 mainly consists of circuit breaker amplifier, current induction amplifier, 4-bit DAC, comparator, monostable and protective circuit, etc. Its function and connection diagram are shown in Fig. 1. Here is a brief description of how the main modules work.
(1) Circuit-breaking amplifier
The break amplifier controls and limits the current in the motor coil by managing the feedback drive switches in the power bridge. The power bridge consists of four solid state power switches (S1, S2, S3, S4) and four H-configured connected diodes, as shown in Figure 2. The control circuit detects the coil current of the motor and compares it with the threshold current. When the motor coil current remains below the threshold current, a source switch and a switch-off on the opposite side of the bridge cause the power supply voltage to be added to the coil, while the coil current rapidly increases to 1/cc/R (R is the coil resistance). Once the coil current exceeds the threshold current, the control circuit switches off and off for a fixed turn-off time. During the interruption, the source switch and the opposite diode short-circuit the coil, while the coil current continues to flow and decreases exponentially to zero. In the interruption time, the control circuit turns on the other interruption. And the coil current rapidly increases again to I/cc/R. Repeat the process to achieve current surge action, which limits the coil current to the threshold current. Surge current only occurs when the coil current reaches the threshold current.
(2) Digital to analog converter (DAC)
The function of DAC is to set a threshold voltage at VOACREFD/16 where D is the decimal number equal to the binary number added to M4-M1 and M4 is the bit. In more demanding applications, external: DAC can drive DAC REF input. The DC voltage of DAC REF is 12V, and the suitable voltage range of VDAC REF is 0V~5V.
(3) Comparator, Monostasis and Coil Current Surge Threshold
When the voltage on the pin CS OUT exceeds the output voltage of the DAC, the comparator triggers the monostable state, which provides the turn-off pulse to the control logic once triggered. During the interruption of the pulse, the power sedan car shortens the motor coil, causing a continuous current in the coil and attenuating to zero. By connecting a parallel resistance network between RC and ground, the interruption and pulse time can be set to 1.1RC seconds. To sum up, it is not difficult to get a coil current of about ((VDAC REF D/16)/250). × 10-6 × RS) Surges occur.
Figure 3
2 Driver System Design
By configuring the M4-M1 port in the following different combinations, LMD18245 can operate in single-phase full-step, dual-phase full-step, half-step drive without torque compensation, half-step drive with torque compensation, 1/4 subdivision drive, 1/8 subdivision drive, 1/10 subdivision drive, 1/16 subdivision drive, etc. Different drive modes. Different drive modes can be achieved by adjusting the external jumper or by online programming software. This paper introduces the principle and application of the most commonly used and better performance 10 subdivision driver. The system schematic diagram is shown in Figure 3. The circuit consists of one AT80C2051, two LMD18245 and peripheral circuits.
2.1 System Power Supply
The system is powered by two power sources, one of which uses 8~12V DC to power LM7805, and outputs 5V standard voltage as the signal power supply of the system. Since the rated input voltage of LM18235F driver is 55V DC, when a power supply with more than 45V DC is used, there will be back-EMF caused by motor deceleration, which may increase the power supply voltage and exceed the limit of 55V rated voltage. To enhance protection, an appropriate zener diode can be placed to limit the voltage to less than 55V, or a fuse less than 2.5A can be inserted into the power supply of the motor. Thus, a standard 24V AC transformer can be used, since nearly 35V DC voltage can be generated through diode rectification and capacitor filtering to supply power. The system uses a linear power supply with no voltage regulator. If switching power is used, a large capacitor (10000) will be placed at the output. μ F or larger).
2.2 Step/Direction Connection
AT89C2051's (T0) P3.4 and (T1) P3.5 ports are both defined as GPIO ports and connected to the DIR input of two drives, respectively. The output of P3.4 and P3.5 is programmatically controlled. The actual operation direction of the stepping motor is achieved by changing the input level of INT1 (P3.3). INT0 (P3.2) is used as the input of the stepping pulse. Both the stepping and direction signals are set to "low efficiency". Both of them and the BREAK signals come from the upper computer. Sometimes photoelectric isolation may be required, and the pin pulse width should be at least 2ms. The stepping frequency of the system is 45 kHz. Under 1/8 subdivision driving mode, the speed of the stepping motor with 1.8o step angle can reach nearly 1700 rpm.
Connections for 2.3BRAKE
One of the design features of LMD18245 driver is BRAKE pin, which is used to power off stepping motor.
The pin is at a logical high level which enables braking and braking of the stepping motor. When operating normally, the BRAKE pin is low level or grounded. In this system, the BRAKE pin receives signals from the upper computer to realize the real-time shutdown of the motor.
Setup of 2.4 Current Limiting Resistance
LMDl8245 drives the stepper motor to a current of 3A/phase. The drive current output is set by resistors R5 and R7. The current detection resistance can be calculated using the following formula: R=20000/A (where R is in ohms and A is in amperes). Resistor. Typical values for R5 and R7 are shown in Table 1, requiring the use of 1/4W resistors and not less than 6.6k. Both LMDl8245 drivers must be set to the same current output threshold, and R5 and R7 use the same resistance value. Do not make the output of the drive larger than 3A, otherwise problems will occur. To change the current output flexibly, two variable resistors with 25K resistance can be used to replace R5 and R7 respectively, but R5 and R7 are recommended for safety.
2.5 Drive Mode Selection
Loading a sequence of binary numbers to the M4-M1 port of LMDl8245 in different step stages makes it very easy to achieve full-step, half-step, 1/4 subdivision, 1/8 subdivision, l/10 subdivision and l/16 subdivision drive of a two-phase stepping motor. External jumper switches can be set to load different values to the SELl, SEL2, SEL3 ports and the corresponding software can determine the specific way to drive the stepping motor.
3 Driver System Software Design
The system uses AT89C2051 as its main controller and its instruction system is fully compatible with 5L instruction set. Based on the formula of current variation in uniform subdivision of two-phase bipolar stepping motor: IA=Im-cos θ， IB=Imsin θ， The phase current ratio in the motor and its corresponding binary equivalents on M4-Ml can be calculated for 1/10 subdivision drive. Express the equivalent value as an array:
PDAC [40] =fob000011 11, 0b001011 11, 0bolollll O, 0b01111101,..., 0b00101111};
Table 1 Limited current resistance and output current threshold
PDIR[40]={0b00110000,0b00110000,0b00110000,0b00110000,0b00110000,...,0b00010000l;
The result shows that the software flow of two-phase stepping motor drive control is shown in Figure 4. Initialization settings define the functions of each port, set INT0 as edge trigger, and the rest are GPIO ports: Initialization motor mainly sets the I/O direction of the port before operation, determines the selected subdivision drive mode, turns off all coil power supply and records the correct step position; In the interrupt service program, the step direction is judged and the step position is calculated. According to the step position, the correct value is selected in the array PDAC[401] and: PDIR[40] and sent to the P1 and P3 ports of AT89c2051 to complete a slight step process. Running of the program is aborted by the user.
4 Concluding remarks
The subdivision driver of two-phase stepping motor designed by the author has been better applied in low-end computer embroidery machine. It has low noise, stable operation, good control performance, easy to use, simple design and low cost. It is a high-cost two-phase stepping motor driver.
