Jones on Stepping Motor Control Circuits http://www.cs.uiowa.edu/~jones/step/circuits.html1 of 11 9/14/2005 3:02 PM3. Basic Stepping Motor Control CircuitsPart of Stepping Motors by Douglas W. Jones THE UNIVERSITY OF IOWA Department of Computer ScienceIntroduction Variable Reluctance MotorsUnipolar Permanent Magnet and Hybrid MotorsPractical Unipolar and Variable Reluctance DriversBipolar Motors and H-BridgesPractical Bipolar Drive CircuitsIntroductionThis section of the stepper tutorial deals with the basic final stage drive circuitry for stepping motors.This circuitry is centered on a single issue, switching the current in each motor winding on and off, andcontrolling its direction. The circuitry discussed in this section is connected directly to the motorwindings and the motor power supply, and this circuitry is controlled by a digital system that determineswhen the switches are turned on or off.This section covers all types of motors, from the elementary circuitry needed to control a variablereluctance motor, to the H-bridge circuitry needed to control a bipolar permanent magnet motor. Eachclass of drive circuit is illustrated with practical examples, but these examples are not intended as anexhaustive catalog of the commercially available control circuits, nor is the information given hereintended to substitute for the information found on the manufacturer's component data sheets for the partsmentioned.This section only covers the most elementary control circuitry for each class of motor. All of thesecircuits assume that the motor power supply provides a drive voltage no greater than the motor's ratedvoltage, and this significantly limits motor performance. The next section, on current limited drivecircuitry, covers practical high-performance drive circuits.Variable Reluctance MotorsTypical controllers for variable reluctance stepping motors are variations on the outline shown in Figure3.1:Figure 3.1Jones on Stepping Motor Control Circuits http://www.cs.uiowa.edu/~jones/step/circuits.html2 of 11 9/14/2005 3:02 PMIn Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible forproviding the control signals to open and close the switches at the appropriate times in order to spin themotors. In many cases, the control unit will be a computer or programmable interface controller, withsoftware directly generating the outputs needed to control the switches, but in other cases, additionalcontrol circuitry is introduced, sometimes gratuitously!Motor windings, solenoids and similar devices are all inductive loads. As such, the current through themotor winding cannot be turned on or off instantaneously without involving infinite voltages! When theswitch controlling a motor winding is closed, allowing current to flow, the result of this is a slow rise incurrent. When the switch controlling a motor winding is opened, the result of this is a voltage spike thatcan seriously damage the switch unless care is taken to deal with it appropriately.There are two basic ways of dealing with this voltage spike. One is to bridge the motor winding with adiode, and the other is to bridge the motor winding with a capacitor. Figure 3.2 illustrates bothapproaches:Figure 3.2 The diode shown in Figure 3.2 must be able to conduct the full current through the motor winding, but itwill only conduct briefly each time the switch is turned off, as the current through the winding decays. Ifrelatively slow diodes such as the common 1N400X family are used together with a fast switch, it maybe necessary to add a small capacitor in parallel with the diode.The capacitor shown in Figure 3.2 poses more complex design problems! When the switch is closed, thecapacitor will discharge through the switch to ground, and the switch must be able to handle this briefspike of discharge current. A resistor in series with the capacitor or in series with the power supply willlimit this current. When the switch is opened, the stored energy in the motor winding will charge thecapacitor up to a voltage significantly above the supply voltage, and the switch must be able to toleratethis voltage. To solve for the size of the capacitor, we equate the two formulas for the stored energy in aresonant circuit:P = C V2 / 2Jones on Stepping Motor Control Circuits http://www.cs.uiowa.edu/~jones/step/circuits.html3 of 11 9/14/2005 3:02 PMP = L I2 / 2Where:P -- stored energy, in watt seconds or coulomb volts C -- capacity, in farads V -- voltage across capacitor L -- inductance of motor winding, in henrys I -- current through motor windingSolving for the minimum size of capacitor required to prevent overvoltage on the switch is fairly easy:C > L I2 / (Vb - Vs)2Where:Vb -- the breakdown voltage of the switch Vs -- the supply voltageVariable reluctance motors have variable inductance that depends on the shaft angle. Therefore,worst-case design must be used to select the capacitor. Furthermore, motor inductances are frequentlypoorly documented, if at all.The capacitor and motor winding, in combination, form a resonant circuit. If the control system drivesthe motor at frequencies near the resonant frequency of this circuit, the motor current through the motorwindings, and therefore, the torque exerted by the motor, will be quite different from the steady statetorque at the nominal operating voltage! The resonant frequency is:f = 1 / ( 2 (L C)0.5 )Again, the electrical resonant frequency for a variable reluctance motor will depend on shaft angle!When a variable reluctance motors is operated with the exciting pulses near resonance, the oscillatingcurrent in the motor winding will lead to a magnetic field that goes to zero at twice the resonantfrequency, and this can severely reduce the available torque!Unipolar Permanent Magnet and Hybrid MotorsTypical controllers for unipolar stepping motors are variations on the outline shown in Figure 3.3:Figure 3.3Jones on Stepping Motor Control Circuits http://www.cs.uiowa.edu/~jones/step/circuits.html4 of 11 9/14/2005 3:02 PMIn Figure 3.3, as in Figure 3.1, boxes are used to represent switches; a control unit, not shown, isresponsible for providing the control signals to open and close the switches at the appropriate times inorder to spin the motors. The control unit is commonly a computer or programmable interface controller,with software directly generating the outputs needed to control the switches.As with drive circuitry for variable reluctance motors, we must deal with the
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