2022年步进电机细分控制 .pdf

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1、1/17AN1495APPLICATION NOTE1INTRODUCTIONMicrostepping a stepper motor may be used to achieve one or both of two objectives; 1) increasethe position resolution or 2) achieve smoother operation of the motor. In either case the basic the-ory of operation is the same. The simplified model of a stepper mo

2、tor is a permanent magnet rotor and two coils on the statorseparated by 90 degrees, as shown in Figure 1. In classical full step operation an equal current isdelivered to each of the coils and the rotor will align itself with the resulting magnetic vector alongone of the 45 degree axis. To step the

3、motor, the current in one of the two coils is reversed and therotor will rotate 90 degrees. The complete full step sequence is shown in figure 2. Half step drive,where the current in the coil is turned off for one step period before being turned on in the oppositedirection, has been used to double t

4、he step resolution of a motor. In either full and half step drive,the motor can be positioned only at one of the 4 (8 for half step) defined positions.45 Therefore,the number of steps per electrical revolution and the number of poles on the motor determine theresolution of the motor. Typical motors

5、are designed for 1.8 degree steps (200 steps per revolution)or 7.5 degree steps (48 steps per revolution). The resolution may be doubled to 0.9 or 3.75 degreesby driving the motor in half step. Further increasing the resolution requires positioning the rotor atpositions between the full step and hal

6、f step positions. Figure 1. Model of stepper motorI1I2MICROSTEPPING STEPPER MOTOR DRIVEUSING PEAK DETECTING CURRENT CONTROLStepper motors are very well suited for positioning applications since they can achieve verygood positional accuracy without complicated feedback loops associated with servo sys

7、-tems. However their resolution, when driven in the conventional full or half step modes ofoperation, is limited by the configuration of the motor. Many designers today are seekingalternatives to increase the resolution of the stepper motor drives. This application note willdiscuss implementation of

8、 microstepping drives using peak detecting current control wherethe sense resistor is connected between the bottom of the bridge and ground. Examplesshow the implementation of microstepping drives with several currently available chips andchip sets. REV. 2AN1495/0604名师资料总结 - - -精品资料欢迎下载 - - - - - -

9、- - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 17 页 - - - - - - - - - AN1495 APPLICATION NOTE2/17Figure 2. Full step sequence. Another issue occurs at low operating speeds. At low speeds, both the full and half step drive tendto make abrupt mechanical steps since the time the rotor takes to mo

10、ve to the next position can bemuch less than the step period. This stepping action contributes to jerky movement and mechanicalnoise in the system. Looking at the simplified model of the stepper motor in Figure 1, it can be seenthat if the two coils were driven by sine and cosine waveforms the motor

11、 would operate as a syn-chronous machine and run very smoothly. These sinusoidal waveforms may be produced by a mi-crostepping drive .Microstepping can be implemented in either a voltage mode or current mode drive. In voltage modedrive, the appropriate duty cycle would be generated by the controller

12、 so that the voltage applied tothe coil (Vsupply * duty cycle) is the appropriate value for the desired position. In current modedrives, the winding current is sensed and controlled to be the appropriate value for the desired po-sition. This application note will consider only current mode drive imp

13、lemented using peak detectingcurrent controllers.To understand the microstepping concept, consider the simplified model of the stepper motor as shownin Figure 1. As previously discussed when the two coils are energized with equal currents, the re-sulting magnetic vector will be at 45 and the permane

14、nt magnet of the rotor will align with that vec-tor. However, if the two coils are energized by currents of different magnitude, the resultingmagnetic vector will be at an angle other than 45 and the rotor would attempt to align with the newmagnetic vector. If one coil were driven with a current tha

15、t was twice the current in the second coilthe magnetic vector would be at 30, as shown in Figure 3. For any given desired position, the re-quired currents are defined by the sine and cosine of the desired angle. To implement a microstepping drive, two D/A converters are used to set the current level

16、 in the coilsof the motor, as shown in the block diagram in Figure 4. I1I2I1I2I2I1I21357I1名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 17 页 - - - - - - - - - 3/17AN1495 APPLICATION NOTEFigure 3. Example alignment of microstepingFigure 4. Block Diagram of micr

17、ostepping motor drive. 2MICROSTEPPING WITH THE L6208In a typical application the L6208, which integrates two H-Bridges with the current control, drives abipolar stepper in either full or half step modes. The internal state machine generates the full stepor half step sequence from the clock and direc

18、tion inputs. 1 Although at first glance it is not obviousthat the L6208 may be used in a microstepping application, it is possible since the current controlcircuits have separate reference inputs.To implement a microstepping application, a variable voltage proportional to the desired output cur-rent

19、 must be applied to each of the reference pins. In the block diagrams above, the two requiredD/A converters provide the required voltages. A simple and inexpensive alternative to a D/A con-O AO BFull Step positionIA= IBHalf Step positionIA= -1, IB =0IA=2 * IBD/APWMHBridgeD/APWMHBridge名师资料总结 - - -精品资

20、料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 17 页 - - - - - - - - - AN1495 APPLICATION NOTE4/17verter chip is to use a counter/timer in the microprocessor to generate a PWM output for each phaseand pass this through a voltage divider and low pass filter to get the desired v

21、oltage. The Vref inputvoltage is equal to the microprocessor power supply voltage times the divider ratio of the resistordivider times the PWM duty cycle. Figure 5 shows the connection between a microcontroller andthe L6208. The complete circuit schematic for the power section is shown in appendix A

22、.Since the L6208 includes an internal phase generation circuit, this circuit must be synchronized tothe externally provided reference voltages. Again a simple solution is possible. The initial state ofthe decoding logic after reset is known and may be used as the starting state. After applying a res

23、etto the L6208, either at power up or by forcing a reset from the microprocessor, the full-scale voltageis applied to both Vref pins to align the stepper motor to the known state that corresponds to one ofthe full step positions. Once the motor is aligned, the references can be reduced to 70.7%, whi

24、chis the correct value for the currents for the 45-degree position in the microstepping sequence. Afterthe motor is aligned the microcontroller can move through the sine/cosine table to generate the ap-propriate reference levels to move in either direction. The software also has to set the appropria

25、tedirection on the CW/CCW pin and generate a clock pulse for each phase reversal that is required.This occurs whenever the phase crosses a 90 boundary in the sine table. By operating the L6208in the full step mode and providing clock signals at the appropriate time, the decoding logic will out-put t

26、he correct phase information for the bridges. Using the L6208 in the half step mode with theappropriate clock signals can improve the performance at the zero cross over of the current, as willbe discussed later. Figure 5. Circuit connections for the L6208Figure 6 shows the operating waveforms when u

27、sing the L6208 in full step mode and varying thereference inputs to achieve microstepping. Trace 1 is the clock input to the L6208. Traces 2 and 3on the plot are the VrefA and VrefB inputs applied to the L6208. Trace 4 is the motor current in chan-nel B. Although the current has the discontinuities

28、near zero that are typical of a peak detection cur-rent control method, the resulting output matches the desired sine wave reasonably well. 5.1K20K0.15.1K20K0.1VREFBVREFACLOCKCW/CCWRESETOUTnOUTnOUTnPWM OUTPWM OUTL6208Micro名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - -

29、第 4 页，共 17 页 - - - - - - - - - 5/17AN1495 APPLICATION NOTEFigure 6. Microstepping waveforms: Typical OperationFigure 7. Microstepping waveforms: Current can not follow desired sine wave3SPEED LIMITATIONSSince the motor coil is primarily an inductance, the rate of current change in the coils is limit

30、ed bythe L/R time constant of the motor. As the motor is operated at higher speeds, the L/R time constantof the motor limits the rate of current change and the current can no longer follow the desired sinewave. Figure 7 shows the motor current at a higher rotational frequency. On this scope trace, w

31、esee two effects. First, the filter on the reference voltage is starting to roll off the reference signal and名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 17 页 - - - - - - - - - AN1495 APPLICATION NOTE6/17second, the motor current is limited by the motor time

32、constant and it begins to look more like atriangle waveform than the desired sine wave. Although moving the pole of the filter on the refer-ence voltage will make the reference signal appear more ideal, it will have little effect on the motorcurrent at this point since the motor current is primarily

33、 limited by the L/R characteristics of the mo-tor. When approaching this point, the motor will run smoothly in full step mode and the micropro-cessor could easily change to full step drive.If the step rate is increased further, the motor will stall when the current can no longer reach a valuelarge e

34、nough to produce the required torque. Figure 8 shows a typical current waveform when themotor has stalled. The almost pure triangular current waveform is similar to the triangular waveformthat would result if the motor were being driven in the full step mode at this step rate. At this oper-ating poi

35、nt the current is entirely controlled by the L/R time constant of the motor and no choppingis occurring. Figure 8. Waveforms when motor has stalled4SLOW VS. FAST DECAY MODEWhen implementing current controlled motor drives, the designer has a choice of the recirculationpath the current flows in durin

36、g the off time. Figure 9 shows the two recirculation options imple-mented in the L6208. Applying the chopping to only one side of the bridge allows the current to re-circulate around a low voltage loop, in the upper transistors with the L6208. Since the rate of changeof the current is controlled pri

37、marily by the L/R time constant of the motor, the current decays rela-tively slowly, hence the designation of slow decay mode. However applying the chopping to bothsides of the bridge results in the current recirculating back to the power supply and a higher voltageacross the coil, hence a fast deca

38、y mode. The L6208 also implements a type of synchronous rec-tification that turns on the MOS transistor in parallel with the conducting diode to reduce the powerdissipation. 1The selection of the decay mode influences the operation of a microstepping drive in several ways.The most obvious is the mag

39、nitude of the ripple current. Drives implemented using the fast decaymode will have, for the same off time or chopping frequency, a higher ripple current than drives im-plemented using a slow decay mode. This difference in itself is not significant for most stepper mo-名师资料总结 - - -精品资料欢迎下载 - - - - -

40、- - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 17 页 - - - - - - - - - 7/17AN1495 APPLICATION NOTEtor drives. Issues with the stability of the current control loop are discussed elsewhere 3.When microstepping at a relatively high speed, the selection of the decay mode affects the abilityof th

41、e drive to follow the desired current level. At any time, the rate of change of current is deter-mined by the inductance of the motor and the voltage across the coil. In the slow decay mode, thevoltage across the coil during the off time is only the drop across one transistor and one diode sothe cur

42、rent changes very slowly. As the desired current level is lowered, it is the rate of change dur-ing the off time that determines how quickly the current transitions to the new level. At low speeds,the effect may not be too noticeable. However, at higher speeds, the motor current cannot decayfast eno

43、ugh to follow the desired decreasing slope of the sine wave. During this time the currentchange is limited by the time constant imposed by the motor inductance and the slow decay pathand can remain higher than the set value. The current will continue to decay at the slow rate until aphase reversal o

44、ccurs, at which point the bridge reverses, applying the full supply voltage acrossthe coil, effectively putting the bridge in a fast decay mode and the current will decay quickly to zero.Selecting the fast decay mode can improve the ability of the drive to follow fast decreases in thecurrent. The wa

45、veforms in Figure 6 are achieved using the fast decay mode.The ability of the drive to increase current on the upper slope of the sine wave is not affected by thechoice of the decay mode since the voltage applied to the coil during the on time is the same.Figure 9. PWM current control decay modes.tV

46、AIOUTtVBttVBtVAtIOUTSlow DecayFast DecayONTimeOFFTimeONTimeOFFTimeON TimeOFF Time inSlow-Decay RecirculationOFF Time inFast-Decay RecirculationVSA B VSA B VSA B ON ON OFF ON RSENSERSENSERSENSEIOUT IOUT IOUT 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 17 页 -

47、- - - - - - - - AN1495 APPLICATION NOTE8/175MINIMUM CURRENT ISSUESWhen operating a chopping current control that has a minimum duty cycle, the current cannot betaken below a level that is effectively set by the motor resistance and the minimum duty cycle. Con-stant off time controls, like the L6208,

48、 have a minimum on time that is set primarily by the propaga-tion delays from the end of the off time until the comparator detects a current above the thresholdand retriggers the monostable putting the bridge in the recirculation mode again. This minimum ontime and the off time set by the monostable

49、 set a minimum working duty cycle for the circuit. Whenthis duty cycle is applied to the motor, a current will be established. If a reference corresponding toa current lower than this minimum is set on the input, the circuit will detect that the motor current isabove the reference. However, since th

50、e IC is already operating at its minimum duty cycle, the cur-rent can not go any lower and thus will not reach the current level desired by the reference level.The minimum duty cycle in other controllers can some times be adjusted. The minimum on time inthe L6506, for example, is set by the width of