Servo - June 2009

SIMULATION

Figure 1: An image of a real stepper motor adds realism to the elevator simulation.

physical construction of a mechanical system. In most cases, though, students would learn much more if they could forego construction and concentrate their efforts on learning to control the motor, interrogating the sensors, and developing an appropriate system-level algorithm.

One possible solution is to give each student a pre-built system so they can begin experimentation immediately. As you might expect, though, constructing, maintaining, and storing the necessary equipment usually prevents this from happening.

It was this type of educational problem that originally inspired the robot simulator in RobotBASIC. The simulator allows programmers to concentrate on algorithm development instead of physical construction. Fortunately, RobotBASIC’s simulation capabilities extend well beyond robotic applications. Double-buffered video output (for flicker-free animation), bitmap manipulation, an easy-to-use 3D graphics engine, matrix math, extensive mathematical

functions, and many other features make it easy for even a mid-level programmer to develop professional looking simulations.

Let’s examine one option for simulating an elevator project controlled by a stepper motor. The output from the program is shown in Figure 1. Notice that an actual picture of a stepper motor has been integrated into the graphic display. A 3D pointer allows the still image to move in a realistic manner and seemingly control a 2D elevator (the motor drives a spool that winds and unwinds a cable attached to the elevator).

You can control the simulated stepper motor by sending it binary codes (1000, 0100, 0010, and 0001) in the proper sequence through an interface subroutine. The floor and elevator buttons, along with the limit switch at the bottom of the shaft, allow a complete elevator algorithm to be developed without any physical construction. Programming the system is surprisingly realistic because the programmer is forced to control the simulated motor in the same manner as a real motor. Standard stepping was used in this example program but it could easily be modified to handle half-stepping or full-torque drive.

Once you accept the idea of creating simulations for a virtual laboratory, the options are endless. Figure 2 shows a simulation of a DC motor suitable for teaching the fundamentals of speed control. The motor reacts as expected when the associated mass and friction parameters are changed. The programmer controls the motor by specifying the duty cycle to be applied just as they would with a real world motor controller.

Figure 3 shows another possibility. Two pointers are attached to two potentiometers which — in the simulation — are attached to eight-bit A/D converters. A DC gearhead motor (controlled by direction and duty cycle parameters) drives the rear potentiometer. The front potentiometer pointer can be moved with the mouse.

In the demo program, the motor is simply turned on at a low speed with the proper direction. It is immediately obvious to the student or programmer that the motor is not moving fast enough to keep up with the manually moved pointer. If the program is changed to increase the speed, they see that the motor does, in fact, move quicker but the increased speed causes it to regularly overshoot its destination. In only a few minutes, the need for a more sophisticated control algorithm becomes apparent, thus