Step Motor Driver For Full Step, Half Step Excitation Mode - Lab Report Example

 Step Motor Driver For Full Step, Half Step Excitation Mode CONTENTS PAGE Summary 2 Introduction 4 Theory 4 Experiment 4 Results 9 Discussion 9 References 10 SUMMARY There are three commonly used excitation modes for step motors; these are the full step, half step and micro stepping [1]. In full step operation, the motor moves through its basic step angle, i.e., a 1.85° step motor takes 200 steps per motor revolution [3]. There are two types of full step excitation modes. In single phase mode, also known as "one-phase on, full step" excitation, the motor is operated with only one phase ( the group of windings) energized at a time [1]. This mode requires the least amount of power from the driver of any of the excitation modes. Using the given state machine states and truth tables, the driver circuit capable of controlling all the three modes of stepper motor excitation was designed using a PIC microcontroller. The result was simulated for each excitation mode and found to agree with the theory. A schematic diagram of the driver circuit is included as fig. 6. 1.0 INTRODUCTION In dual phase mode, also known as “two-phase on, full step” excitation, the motor is operated with both phases energized at the same time [2]. This mode provides improved torque and speed performance. Dual phase excitation provides about 30% to 40% more torque than single phase excitation but does require twice as much power [3]. Half step excitation is alternating single and dual phase operation resulting in steps that are half the basic step angle [1]. Due to the smaller step angle, this mode provides twice the resolution and smoother operation. 2.0 THEORY Stepper motors are devices, which convert electrical impulses into discrete mechanical rotational movements [2]. In a typical stepper motor, power is applied to two coils. The permanent magnet rotor has the same number of pole pairs as the stator coil section [1]. Motors are available in either 2 coils (bipolar) or 4 coils (unipolar) windings [4]. In the unipolar version, the coils are bi-filar (two side by side wires) wound on each stator half and opposite ends of each pair are connected together to form a center tapped coil. With this method, the flux is reversed by powering either one end or the other of the bifilar coil pair with the center connected to common [5]. For a bipolar motor, an external device referred to as an H-Bridge can be used to reverse the polarity of the winding and thus the flux. An H-Bridge can also drive a unipolar motor by not connecting the center tap (common) lead or use only one of the windings in the pair [3]. 3.0 EXPERIMENT 3.1 Design Background Using the given state machine states and truth tables, the driver circuits for all the three modes of stepper motor excitation were designed and simulated. The following are the diagrams showing the truth table and state machine states with actual motor motion. Figure 1: One-Phase, Full Step Figure 2: Two-Phase on, Full Step Figure 3: Half Step The following are the full step and half step sequences to follow in the design process. a) Full Step Sequence Sequence Coil A Coil B 1 + off 2 off + 3 - off 4 off - The positive (+) indicates the coil conducting in the forward direction while one end is positive relative to the ground. The negative (-) sign indicates the coil conducting in the opposite direction [2]. This causes the rotor to rotate in a forward direction and reversing the sequence causes the motor to rotate in the reverse direction [1]. When the motor is stepped according to an 8-part sequence, the result is the half steps sequence. A motor with a step size of 7.5 degrees/step is stepped to obtain motion of 3.75 degrees/step [1]. The half step sequence is as given below. b) Half Step Sequence Sequence Coil A Coil B 1 + + 2 + off 3 + - 4 off - 5 - - 6 - off 7 - + 8 off + 3.2 Materials PIC 16F84 Microcontroller Motor coils, 0.1 Breadboards uF capacitors 5 volt supply 10 K and a 50 Ohm resistor Connector wires Dual Full Bridge Motor Driver IC 3.3 Procedure i) The circuit in fig.5 below is a simple H-Bridge. Q1 and Q2 are connected to the positive motor supply source whereas Q3 and Q4 are connected to ground. ii) Logic is provided to inhibit any programming error, which can cause switching the wrong transistors on at the incorrect time. The timing circuitry is employed to switch on the correct transistors after the others are switched off. Snubber diodes are used to correct the problem of polarity reversal. The circuit in fig.4 depicts a simple H-Bridge with snubber diodes installed. Figure 4: H-Bridge With Snubber Diodes Figure 5: H-Bridge iii) The circuits in fig.4 and fig.5 can easily be implemented by use of the Dual Full Bridge Motor Driver IC. This is the If we use in this design. iv) The designs also require truth tables for the Dual Full Bridge Motor Driver IC and that for microcontroller output. Dual Full Bridge Motor Driver IC Truth Table Enable Input Phase Input Output 1 Output 2 Low High High Low Low Low Low High High High Open Low High Low Low Open Microcontroller Output Table Sequenced for Half-Stepping Sequence Coil A Coil B Coil B Enable RA3 Coil B Phase RA2 Coil A Enable RA1 Coil A Phase RA0 1 High High 0 0 0 1 2 High Off 1 0 0 1 3 High Low 0 1 0 1 4 Off Low 0 1 1 0 5 Low Low 0 1 0 0 6 Low Off 1 0 0 0 7 Low High 0 0 0 0 8 Off High 0 0 1 0 v) Designing and simulation results in a circuit shown by the following schematic diagram in the results section. 4.0 RESULTS Figure 6: Schematic Diagram For The Driver 5.0 DISCUSSION In this design, the no load or constant load accuracy of a 7.5 degree/step motor is within 0.5% noncumulative [2]. It simply implies that the positioning error is the same whether the rotational movement is one step or 1000 steps. Since the step error is noncumulative, it averages itself out to zero within a 4-step sequence, which corresponds to 360 electrical degrees [1]. In the design also a positive voltage on the base of one of the transistors was used. This causes the transistor collector/emitter junction to conduct [3]. When transistors Q1 and Q3 on, the current flows through the motor coil in the forward or + direction. However, when Q2 and Q4 on, the current to flows through the motor coil in the reverse direction. In the experiment, only half step truth table was presented. However, if you compare the half step and full step tables, it's noticed that all of the full step sequences appear in the half step table, even though not in the same order. This information was useful in the circuit design, especially when writing the codes for the microcontroller. 6.0 REFERENCES [1] Hasanien, H. M. (2011). FPGA implementation of adaptive ANN controller for speed regulation of permanent magnet stepper motor drives. Energy Conversion and Management, 52(2), 1252-1257. [2] Shin, D., Kim, W., Lee, Y., & Chung, C. C. (2013). Phase-Compensated Microstepping for Permanent-Magnet Stepper Motors. Industrial Electronics, IEEE Transactions on, 60(12), 5773-5780. [3] Kim, W., Yang, C., & Chung, C. C. (2011). Design and implementation of simple field-oriented control for permanent magnet stepper motors without DQ transformation. Magnetics, IEEE Transactions on, 47(10), 4231-4234. [4] Hasanien, H. M. (2011). FPGA implementation of adaptive ANN controller for speed regulation of permanent magnet stepper motor drives. Energy Conversion and Management, 52(2), 1252-1257. [5] Pal, S., & Tripathy, N. S. (2011). Remote Position Control System of Stepper Motor Using DTMF Technology. International Journal of Control and Automation, 4(2). Read More