1.0 Introduction
1.1 Background of Study
A stepper motor is an electromagnetic rotary actuator which mechanically converts digital pulse inputs to incremental shaft rotation. The rotation not only has a direct relation to the number of input pulses, but its speed is related to the frequency of the pulses. The motor is able to hold its position ( and its load) between the steps without the aid of clutches or brakes. Thus a stepper motor can be precisely controlled so that it rotates a certain number of steps, producing mechanical motion through a specific distance, and then hold its load when it stops. Furthermore, it can repeat the operation at any prescribed number of times. With the appropriate logic, stepper motors can be bi- directional synchronous, provide rapid acceleration, stopping, and reversal and will interface easily with other digital mechanisms. They are further characterized as having low rotor moment of inertia, no drift and a non cumulative positioning error.
Generally, step motors are operated without feedback in an open-loop manner and often match the performance of more expensive DC servo positioning systems like speed sensor, servo motor ( i.e systems that maintain the speed of motors in a certain limit, even when the load of the output is being varied). Stepper motors may be classified by their motor construction, drive topology, and stepping pattern. There are several different types of stepper motor construction. These include variable reluctance, permanent magnet and hybrid permanent magnet, just like every motor.
Stepper motor consists of permanent magnet rotor and wound stator. The coils of wound stator are called control windings. The rotation of the stepper motor is controlled by switching ON/OFF the current through control windings. Normally, the control windings are exited through driver transisistors. The direction of rotation of the motor can either be reverse of forward by generating the switching sequences in reverse or forward order.
Each of those slight rotations is called a “step,” with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.
Stepper motors are one of the most versatile forms of positioning system. Basically, they are digitally controlled as part of an open loop system.
Their industrial application are in the high speed pick and place equipment and multi-axis machine often directly driving lead screw and ball screws. in the fields of lasers and optics they are frequently used in precision positioning equipment such as linear actuator, linear stages, rotation stages, goniometry and mirror mounts. Other uses are in packaging machinery and positioning of valve pilot stages for fluid control systems.
Commercially, stepper motors are used in floppy disk drivers, flathed scanners, computer printers, plotters, slot machines and many more devices.
The performance of stepper motor is strongly dependent on the driver circuit responsible for communication and current limiting which is also being controlled by switching pulse generator also known as the stepper motors controller or indexer. The pulse generator output command pulses that the motor follows. The pulse generator can also generate sequence of switching pulse at variable rate to drive the motor at variable speed. By altering the frequency of the pulse train, the pulse generator can instruct the motor to accelerate, run at a speed, decelerate or stop. A pulse generator must be present, other wise the motor will not move.
Most applications require that the stepper motor controller manage other control functions as well, including acceleration, deceleration, steps per second and distance. The stepper motor controller can also interface to and control many other external signals. Communication to the stepper motor controller is through an Rs 232 serial port and in some cases an Rs485 port. In either case, the stepper motor controller is capable of receiving high level commands from a host computer and generating the necessary step and direction pulses to the driver. The stepper motor controller includes auxiliary 1/0 for monitoring inputs from external sources such as jog, Home or limit switch.it can also initiate other machine functions through the 1/0 output pins.
Over the years, there have been some challenges faced by stepper motor controller (pulse generators). The commonest
Is when they are used to control drive circuits which are also referred to as constant voltage drive because a constant positive or negative voltage is applied to each winding to set the step positions. However, it is winding current, not voltage that applies torque to the stepper motor shaft. In view of the challenges faced by a pulse generator an embedded micro controllers on the other hand can rectify the challenges faced above. The embedded microcontroller is a self contained system with a processor, memory and peripherals.
The embedded microcontroller system may require minimal memory and program length, with no operating system and no software complexity. They have few instructions for the movement of data between memory on the same chip. (I.e. movement of code and data within the same chip) unlike that micro processor which is between external and internal memory (Movement of code and data from external memory).
1.1 Problem Statement
Over the year, there have been some problems faced by stepper motor controller (pulse generators). The commonest is when they are used to control drive circuits which are also referred to as constant voltage drives because a constant positive or negative voltage is applied to each winding to set the step positions. However, it is winding current, not voltage that applies torque to the stepper motor shaft.
1.2 Objectives of Study
The objective of this seminar is to design a switching pulse generator for stepper motor control using an embedded controller which will solve the challenges faced in the control operation of a stepper motor. The design will include AT89C51. Microcontroller which has an on-chip peripheral function like 1/0 ports, timers/counter, serial communication port which would in turn reduce the complexity of the normal design. The AT89C51 has 1000 write /erase cycle giving the design flexibility in its field programmability.
1.3 Justification of Study
This seminar is of justification to the operation and complexity of stepper motor controllers in relation to the existing ones in use. Its simplicity makes usage very easy because the assembly language use in microcontrollers follow Reduced Instruction Set Computing (RISC) architecture whose instruction set is small. Its high integration functionality makes it look like a stand- alone computer. It also has flexibility in it field programmability.
1.4 Scope of Work
Literature review of the existing works made by other people will be captured as chapter two as part of the remaining scope. Chapter three of the study will be the methodology. System design, development and implementation of the switching pulse generator for a stepper motor using embedded microcontroller will be chapter four of the study. Result s and conclusion will be captured as chapter five.
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