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Stepper-Motor Operation and Interfacing Fundamentals Prepared by: P. David Fisher and Diane T. Rover Ampere’s Law & Biot-Savart Law An electrical current I in a ware causes (induces) a magnetic field B. The direction of B is given by the “right-hand rule”. I R B Magnetic Fields for a Long Thin Wire For a long thin wire, the strength of the magnetic field B a distance R from the wire is B = (U0I)/(2R) (1) where 0 = permeability of vacuum, 0 = 4x10-7 henry/meter The Solenoid A coil of wire with N turns creates a magnetic field B in the direction illustrated, where B = kNI, (2) with k being a constant. Hence, B is proportional to N and I. I B 1 Magnetic Field Lines st_mot_3.doc The Electromagnet N (North) (South) S A B Iron Core If you place a compass and in the vicinity of the iron core, you would discover that one end (say A) would be similar to the “South Magnetic Pole” of the earth, while the other end (say B) would be similar to the Earth’s “North Magnetic Pole”. Two important properties of electromagnets are the following: 1. All Electromagnets are dipoles; i.e., they have a North Pole (N) and a South Pole (S). 2. The position of the Poles (at A or B) is determined by the direction of the current I and the direction of the winding. 2 st_mot_3.doc Basic Model for a Stepper Motor Consider the four electromagnets physically arranged as illustrated. P1(L11,L12) A B P4(L41,L42) A B A L31 R31 B P2(L21,L22) B A P3(L31,L32) where R11 L11 i11 i31 va R12 L12 L32 i12 R32 Windings for P1 & P3 i32 vb L21 R21 L41 R41 1 i21 i41 vc R22 L22 L42 i22 R42 Windings for P2 & P4 i42 vd 3 st_mot_3.doc Controlling Magnetic Polarities with Winding Voltages (va, vb, vc and vd) The magnetic polarities of the electromagnets can be controlled by varying the winding voltages va, vb, vc and vd. Consider the following two cases. Case I – P1 and P3 P1(L11,L12) A B P(L31,L32) A B P1(L11,L12) A B P3(L31,L32) A B N A B S S A B N S A B N N A B S ill > 0 va = 5V il2 = 0 vb = 0V i3l > 0 va = 5V i32 = 0 vb = 0V ill = 0 va = 0V il2 > 0 vb = 5V i3l = 0 va = 0V i32 > 0 vb = 5V Note: Winding voltages va and vb control the polarities for electromagnets P1 and P3. 4 st_mot_3.doc Case II – P2 & P4 P2(L21,L22) A B P4(L41,L42) A B P2(L21,L22) A B P4(L41,L42) A B N A B S S A B N S A B N N A B S i2l > 0 vc = 5V i22 = 0 vd = 0V i41 > 0 vc = 5V i42 = 0 vd = 0V i2l = 0 vc = 0V i22 > 0 vd = 5V i4l = 0 vc = 0V i42 > 0 vd = 5V Note: Winding voltages vc and vd control the polarities of electromagnets P2 and P4. 5 st_mot_3.doc Controlling Stepper-Motor State Transitions The “state” of a stepper motor can be controlled by controlling the winding voltages of the electromagnets. Consider the following example. P1 va = 0V Present State P1 & P3 vb = 5V B N P4 B S N B P2 S B vc = 5V C P2 & P4 vd = 0V P3 P1 Next State va = 5V P1 & P3 vb = 0V B S P4 B S N B P2 N B vc = 5V C P2 & P4 vd = 0V P3 Note: The polarities of electromagnets P1 and P3 can be reversed by simultaneously changing va from 0V to 5V and vb from 5V to 0V. 6 st_mot_3.doc Important Questions and Conclusions With respect to the previous example, answer the following questions. 1. Assume that the stepper motor is in its “initial state.” If a compass is positioned with the pivot point of its needle at point C, in what direction would the needle point? 2. Assume that the stepper motor is in its “next state.” If a compass is positioned with the pivot point of its needle at point C, in what direction would the needle point? 3. Did the compass needle move clockwise or counter clockwise? 4. What voltages do we need to change to have the compass needle rotate in the opposite direction? 5. How many “steps” does it take to make a 360 rotation? 6. How might you add the number of steps for a 360 rotation? Identify two distinct approaches. 7. Why might you want to add steps? 8. What are the engineering design considerations that must be addressed as a new stepper-motor assembly is designed for a new commercial application? 7 st_mot_3.doc Stepper-Motor Interface Circuit Model There are a number of significant challenges facing the computer engineer who must interface a stepper motor to a microcontroller. For example, consider the following transient circuit response problem. S1 RS1 VS RS1 = Shunt Resistance of Switch R11 + VS1 i11 L11 + VL11 - RS1 >> R11 Case 1: Switch closes at t = 0 i11(0-) = i11(0+) = VS/RS 0A i11(t) = (VS/R11)e-t/, where = R11/L11 VL11(t) = Vse-t/ (3) (4) (5) Case 2: Switch Opens at t = 0 Because currents through an inductor cannot change discontinuously, ill(0+) = i11(0-) = VS/R11 (6) Applying Kirchhoff’s Voltage Law (KVL) around the loop at time t = 0+ yields: -VS + VS1 + R11i11 + VL11 = 0 -VS + RS1i11 + R11i11 + VL11 = 0 VL11 = VS – (RS1 + R11)i11 VS – (RS1/R11)VS, when RS1 >> R11 VL11 -(RS1/R11)VS -(1M/0.1k)VS -104VS (7) (8) (9) (10) These large voltages will destroy the solid-state switch. 8 st_mot_3.doc Diode Protection There exists a very standard solution to the problems which arise due to the desire to rapidly switch electrical currents in circuits containing inductive loads. The following example illustrates the solution. The winding of an electromagnet can be modeled as a resistance in series with an inductance, as illustrated in the figure. Under computer control, current i11 is to be controlled by controlling voltage v1. As we saw with the stepper-motor example, i11 will assume one of two steady-state values—i.e., i11 = 0A and i11 = VS/R11. In the circuit illustrated, diode D1 protects the interface logic from large transient voltages. v1 Interface Computer Logic D1 R11 i11 L11 Case 1: v1 = 0V The voltage drop across the diode is v1 = 0V, and the diode is turned off (an open circuit). Also, i11 = 0A. Case 2: v1 = VS, where VS > 0V The voltage drop across the diode is v1 = VS, and the diode is turned off (an open circuit). Also, i11 = VS/R11. Case 3: At t = 0-, v1 = VS, where VS > 0V. Then at time t = 0, the interface logic switches and presents a high impedance to the rest of the circuit. At time At t = 0+, the current i11 = VS/R11 and passes through the diode. The diode is forward biased with v1 = -0.7V. With time, i11 drops to 0A, v1 returns to 0V and the diode is turned off (an open circuit). This is the solution to only one interfacing problem. Another common problem is the fact that actuators, such as the stepper motor, do not operate at standard “logic voltages.” This problem will be discussed as we investigate the electrical properties of a specific stepper motor and its computer-interface requirements. 9 st_mot_3.doc