1.3 ANGULAR POSITION
• A variable resistor is used to convert an angle or displacement to resistance/voltage.
• These give absolute position readings.
• Linear resistors are used for measuring linear displacement.
• The basic principle of operation is that a moving wiper (sensor input) moves a contact along a resistor. The ends of the resistor are connected to reference voltages. As the wiper moves the potentiometer acts as a voltage divider and produces a voltage proportional to position.
• rotational potentiometers are the most popular. These may be limited to a fixed range either less than 360, or some number of turns.
• Advantages include,
- typically inexpensive
- easy to use
- very common in a variety of forms, resistances, etc.
• Disadvantages include,
- limited accuracy (there are high cost solutions)
- subject to mechanical wear
• An incremental encoder will produce a set of output pulses, and a direction as it is rotated.
• The encoder contains an optical disk with fine windows etched into it. As the encoder shaft is rotated, the etched disk inside rotates. As it rotates various optical sensors are turned on and off.
• There are two basic types,
absolute - the same shaft position will always give the same position reading
relative/incremental - these just indicate movement, and they require that other circuits or programs be used to track position
• How the openings are etched onto the disk determine whether it is absolute or relative.
• In actual encoders there can be thousands of divisions per rotation.
• With an absolute encoder the output is a binary or gray code number.
• A quadrature pulse counter uses two pulses out of phase and deduces distance and direction.
• These use small magnetic coils to detect positions and behave much like relative encoders.
1.3.4 Practice Problems
1. What is the resolution of an absolute optical encoder that has six tracks? nine tracks? twelve tracks?
(ans. 360°/64steps, 360°/512seps, 360°/4096 steps)