42.6 MICROBOT OVERVIEW

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The Microbot is a small 5 axis robot, using stepper motors. The controlling computer is in the base, and uses a 6502 microprocessor. This computer has two serial ports (RS232) which can be used to connect to dumb terminals, or PCs. There is a Basic like programming language which allows users to directly control the robot, or write programs to do so. Inside the robot, the stepper motors drive the robot through the use of gears, and pulleys. This arrangement allows for a great deal of backlash in the robot. (Try pushing the robot, and you will notice the play).

The use of stepper motors has a number of implications. Common stepper motors divide one rotation of a shaft into 200 divisions. The motor controller can then accurately position the shaft. This means once the absolute position of the robot has been determined, a certain number of motor steps, in either direction will result in a known position. This mode of control means that feedback sensors are not required for the robot (although a calibration step is required before motion). The stepper motors are not very strong, and if their `slip torque' is overcome, then they will lose their position, and without feedback the robot position will be in error until recalibrated.

The only feedback to the controlling computer is for the gripper closed. There is a small spring loaded contact switch which will indicate when the is a large force between the gripper fingers. This allows objects of unknown size to be picked up, but not broken.

A teach pendant is available with the Microbot. This teach pendant allows a user to program the robot without the use of languages, or other computers. The teach pendant allows the user to move joints, then record robot positions. When the robot positions are played back in sequence, they can perform some valuable manipulation.

In general this robot is very simple, and of no industrial use, but it displays many important considerations for full size robots.

Practice: Try programming the robot to pick up a block from one location, move it to another, then move it back again. You will notice some of the problems the robot has picking up the block.