Stepper motors are a major component of many robotics applications. The complexity of any engineering system’s structure depends on many factors. Examples of these for the design of stepper motor (SM) applications are: the level of the technical progress in the particular application, manufacturing technology requirements, safety and environmental factors, equipment and machines used in the process, economic considerations. The systems approach
is imperative in that it coordinates the elements of a SM application, the elements of other engineering applications of the project as well as the elements of the involved economic infrastructure.
The diagram which illustrates the coordination between these systems activities is shown in Figure 1. Table 1 provides a short description of the SM intercommunications labeled on the diagram.
The electrical engineering (EE) section represents all aspects of the electrical segment of a SM application. Applying systems categories, the authors consider the EE section as a system. However, the EE system consists of various subsystems that differ by function and by content.
These subsystems are:
- Electric Machines subsystem – responsible for research, design, selection, installation, adjustment, operation, and maintenance of a stepper motor as a rotating machine;
- Digital Circuits subsystem – responsible for the sequential excitation of motor windings in response to signals from the control subsystem.
- Electronic Power Driver subsystem – responds to excitation signals from the digital circuit. This subsystem is also responsible for handling of coil currents and suppression of inductive voltage spikes.
- Control subsystem – responsible for control system design, optimization, stability and performance analysis.
|1||SM drive requirements, such as coil current, coil voltage, motor winding information, step speed.|
|2||Digital interface for excitation control.|
|3||igital interface for step-command pulses and other control signals.|
|4||Utilization of specific electrical signals in the control subsystem provided through the external connections. The control subsystem will be controlling specific electrical signals to achieve the desired control specifications.|
|5||SM design features, such as size, inertia, number of steps, angle per step are inputs for the control subsystem design and operation.|
|6||Command pulse specification; SM drive requirements, such as step speed.|
|7||Requirements for motor speed, precision, number of steps, sequence of operations.|
|8||Requirements for dimensions, paths for electric wiring, information on mechanical characteristics of the load (mass, moment of inertia among others).|
|9, 11, 13, 14||Utilization of available electrical components manufactured by vendors and suppliers.|
|10||Influence of manufacturing requirements on the control systems design. Slight changes in manufacturing procedures can make large impacts on the control system.|
|12||Information on mechanical limitations of the SM and mechanical connections of the load to the SM for control purposes.|
To demonstrate the concepts being used in the SM applications of the Electrical Engineering Technology (EET) curricula, the authors developed a fully-functional demonstration unit. The block diagram of the unit is presented in Fig. 2.
The Stepper Motor control circuit contains the following functional blocks:
- Computer Printer Port Interface.
- Electrical Isolation.
- Power Drivers and Current Limiting.
- Coil Activation Indicator.
The computer printer port provides the logic level stimulus, which, when conditioned, will cause the stepper motor to rotate. The stator coils of the stepper motor must be energized in the proper sequence. The program being executed on the computer (PC) is the sequence generator. It supplies the proper levels and timing necessary to cause shaft rotation. The flow-chart of the program is presented in Fig. 3.
Opto-isolators are used to protect the PC’s printer port from being damaged by voltages that might be present in the driver circuit. They also effectively isolate the PC from unwanted noise generated by switching currents. The power transistors provide drive current directly to the motor coils. They are high current-handling devices. The opto-isolators have a darlington pair output that is sufficient to cause the power transistors to turn on. The light-emitting diodes (LEDs) provide a visual indication of coil activity. This allows observation of the step sequence when the motor is rotated at low speed.