stepper_motor_drive (961785), страница 2
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11 shows the phase current of a stepping motorin half-step control with an without phase current increase and the pertinent curves of stap frequencyand torque.6/17Figure 11 : Half Step Driving with Shaping Allowsto Increase the Motor’s Torque toabout 95 % of that of the Full Step.APPLICATION NOTEFigure 12 : Only Two Signals for Full Step Driving are Necessary while Four (six if three-state is neededon the output stages) for half Step.7/17APPLICATION NOTEDRIVE SIGNALS FOR THE MICRO ELECTRONICA direct current motor runs by itself if you supply ifwith voltage, whereas the stepping motor needs thecommutation signal in for of several separated butlinkable commands.
In 95 % of the applications today, the origin of these digital commands is a microprocessor system.In its simplest form, a full-step control needs only tworectangular signals in quadrature. According towhich phase is leading, the motor axis rotates clockwise or counter-clockwise, whereby the rotationspeed is proportional to the clock frequency.In the half-step system the situation becomes morecomplicated.
The minimal two control signals become four control signals. In some conditions as manyas six signals are needed. If the Tri-state-commandfor the phase ranges without current, necessary forhigh motor speeds, may not be obtained from the 4control signals. Fig. 12 shows the relationship between the phase current diagram and the control signal for full and half-step.Since all signals in each mode are in defined relations with each other, it is possible to generate themusing standard logic. However, if the possibility tochoose full and half-step is desired, a good logic implementation becomes quite expensive and an application specific integrated circuit would be better.Such an application specific integrated circuit couldreduce the number of outputs required from a microprocessor from the 6 required to 3 static and dynamic control line.A typical control circuit that meets all these requirements is the L297 unit (fig.
13).Four signals control the motor in all operations :1. CLOCK :The clock signal, giving the stepping command2. RESET :Puts the final level signals in a defined start position3. DIRECTION : Determines the sense of rotation ofthe motor axis.4. HALF/FULL : Desides whether to operate in fullor in half-step.Another inhibit input allows the device to switch themotor output into the Tri-state-mode in order to prevent undesired movements during undefined operating conditions, such as those that could occurduring.Figure 13 : The L297 avoids the Use of Complicated Standard Logic to Generate Both Full and Half-stepDriving Signals Together with Chopper Current Control.RS1 RS2 = 0.5 ΩD1 to D8 = 2 A fast diodes8/17VF ≤ 1.2 V @ i = 2A{ trr ≤ 200 nsAPPLICATION NOTESWITCH-MODE CURRENT REGULATIONThe primary function of the current regulation circuitis to supply enough current to the phase windingsof the motor, even at high step rates.The functional blocks required for a switchmode current control are the same blocks required in switching power supplies ; flip-flops, comparators ; andan oscillator are required.
These blocks can easilybe included in the same IC that generates the phasecontrol signals. Let us consider the implementationof chopper current control in the L297.The oscillator on pin 16 of the L297 resets the twoflip-flops at the start of each oscillator period. Theflip-flop outputs are then combined with the outputsof the translator circuit to form the 6 control signalssupplied to the power bridge (L298).When activated, by the oscillator, the current in thewinding will raise, following the L/R time constantcurve, until the voltage across the sense resistor (pin1, 15 of L298) is equal to the reference voltage input(pin 15, L297) the comparator then sets the flip-flop,causing the output of the L297 to change to an equiphase condition, thus effectively putting a short circuit across the phase winding. The bridge isactivated into a diagonally conductive state whenthe oscillator resets the flip-flop at the start of thenext cycle.Using a common oscillator to control both current regulators maintains the same chopping frequency forboth, thus avoiding interference between the two.The functional block diagram of the L297 and the power stage (L298) are shown in Figure 14 alone withthe operating wave forms.An important characteristics of this circuit implementation is that, during the reset time, the flip-flops arekept reset.
The reset time can be selected by selecting the impedance of the R/C network or pin 16. Inthis way, the current spike and noise across the sen-se resistors that may occur during switching will notcause a premature setting of the flip-flop. Thus therecovery current spike of the protection diodes canbe ignored and a filter in the sense line is avoided.THE RIGHT PHASE CURRENT FOR EVERY OPERATING CONDITIONThe Chopper principle of the controller unit revealsthat the phase current in the motor windings is controlled by two data : the reference voltage at pin 15of the controller and the value of the sense resis-tance at pins 1 and 15 of the L298, that is IL = VREF/RS.By changing VREF it is very easy to vary the currentwithin large limits.
The only question is for which purpose and at which conditions.More phase current means more motor torque, butalso higher energy consumption.An analysis of the torque consumption for differentperiods and load position changes shows that thereis no need for different energies.There is a high energy need during the accelerationor break phases, whereas during continuous operation or neutral or stop position less energy has to besupplied. A motor with its phase current continuously oriented at the load moment limit, even with theload moment lacking, consumes needlessly energy,that is completely transformed into heat.Therefore it is useful to resolve the phase current inat least two levels controllable from the processor.Fig.
18 shows a minimal configuration with two resistance and one small signal transistor as changeover switch for the reference input. With anotherresistance and transistor it is possible to resolve 2Bits and consequently 4 levels. That is sufficient forall imaginable causes.Fig. 16 shows a optimal phase current diagram during a positioning operation.9/17APPLICATION NOTEFigure 14 : Two ICs and very Few External Components Provide Complete Microprocessor to Bipolar Stepper Motor Interface.10/17APPLICATION NOTEFigure 15 : Because of the Set-dominant Latch Inside the L297 it is Possible to Hide Current Spikes andNoise Across the Sense Resistors thus Avoiding External Filters.Figure 16 : More Energy is needed During The Acceleration and break Phases Compared the ContinuousOperation, Neutral or Stop Position.11/17APPLICATION NOTEHIGH MOTOR CLOCK RESETS IN THEHALF-STEP SYSTEMIn the half-step position one of the motor phases hasto be without current.
If the motor moves from a fullstep position into a half-step position, this meansthat one motor winding has to be completely discharged. From the logic diagram this means for thehigh level bridge an equivalent status of the input signals A/B, for example in the HIGH-status. For thecoil this means short circuit (fig.
17 up) and consequently a low reduction of the current. In case of highhalf-step speeds the short circuit discharge timeconstant of the phase winding is not sufficient to discharge the current during the short half-step pha-ses. The current diagram is not neat, the half stepis not carried out correctly (fig. 17 center).For this reason the L297 controller-unit generatesan inhibit-command for each phase bridge, thatswitches the specific bridge output in the half-stepposition into Tri-state. In this way the coil can startswinging freely over the external recovery diodesand discharge quickly.
The current decrease rate ofchange corresponds more or less to the increaserate of change (fig. 17 below).In case of full-step operation both inhibit-outputs ofthe controller (pin 5 and 8) remain in the HIGH-status.Figure 17 : The Inhibit Signal Turns Off Immediately the Output Stages Allowing thus a Faster Current Decay (mandatory with half-step operation).12/17APPLICATION NOTEFigure 18 : With This Configuration it is Possible to Obtain Half-step with Shaping Operation and ThereforeMore Torque.MORE TORQUE IN THE HALF-STEP POSITIONA topic that has already been discussed in detail. Sowe will limit our considerations on how it is carriedout, in fact quite simply because of the referencevoltage controlled phase current regulation.With the help of the inhibit-signals at outputs 5 and8 of the controller, which are alternatively active onlywhen the half-step control is programmed, the reference voltage is increased by the factor 1.41 witha very simple additional wiring (fig.
18), as soon asone of the two inhibit-signals switches LOW. This increases the current in the active motorphase proportionally to the reference voltage andcompensates the torque loss in this position. Fig. 19shows clearly that the diagram of the phase currentis almost sinusoidal, in principle the ideal form of thecurrent graph.To sum up we may say that this half-step version offers most advantages. The motor works with poorresonance and a double position resolution at atorque, that is almost the same as that of the fullstep.BETTER GLIDING THAN STEPPINGIf a stepper motor is supposed to work almost glidingand not step by step, the form of the phase currentdiagram has to be sinusoidal.The advantages are very important :-no more phenomena of resonancedrastic noise reductionconnected gearings and loads are treatedwith care-the position resolution may be increased further.However, the use of the L297 controller-unit described until now is no longer possible of the more semplicated form of the phase current diagram theController may become simpler in its functions.Fig.
20 shows us an example with the L6505 unit.This IC contains nothing more than the clocked phase current regulation which works according to thesame principle as L297. The four control signalsemitting continuously a full-step program are nowgenerated directly by the microprocessor. In order toobtain a sinusoidal phase current course the reference voltage inputs of the Controller are modulatedwith sinusoidal half-waves.The microprocessor that controls the direction of thecurrent phase with the control signals also generates the two analog signals.For many applications a microprocessor with dedicated digital to analog converters can be chosen.Eliminating the need for separate D/A circuits.About 5 bit have proved to be the most suitable sud13/17APPLICATION NOTEdivision of the current within one full-step.
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