Issue
152 March 2003
2-D
Optical Position Sensor
by
Roger Johnson & Chris Lentz
Surprisingly,
it isn’t too difficult or expensive to build a two-dimensional
optical position measurement system. In this article,
Roger and Chris show you how to do it. All you need
is a PIC, LCD, and some analog gadgets to get moving
on the perfect PSD project.
Start
•
Duolateral Type • Typical
Applications
• Circuitry • Null
and Mode •
Sources and PDF
Position sensitive
detectors (PSD) have been around in various forms for
over 30 years, working quietly in the background of
CD/DVD focus and tracking controls, heat-seeking missile
guidance systems, and metrology instruments such as
range sensors and 3-D laser trackers. Basically, the
PSD gives the analog coordinates of a spot of light
on its surface. We’ve designed many optical measurement
systems that involve lasers, complex lens designs, sophisticated
electronics, computers, and motion control. The PSD
is a small but important part of these systems.
We were wondering
if there was a project that would emphasize PSD technology
without the added complexity of other systems. Hardware
and on-line stores now sell high-quality laser tools
for $40 and up. The basic tool is a laser pointer that’s
integrated with a bubble level (i.e., a laser level).
This laser beam is projected parallel to the base of
the level and establishes a datum; it transfers the
line of sight that’s established by the beam with the
aid of a position detector. In addition, a laser level
can create lines/planes that are plumb and perpendicular
to the level planes, which is ideal for construction
purposes.
The majority
of these tools don’t offer an accurate way to determine
the center of the laser beam, so most people just use
their eyes. This method is adequate for a good number
of construction purposes; however, there are many situations
in which more accuracy is required. Some of the detectors
included with these tools can measure the position of
the laser beam to about one thirty-second of an inch,
but there aren’t any high-accuracy sensors to determine
the exact location of the laser beam. It dawned on us
that such a system would be the perfect PSD project.
In this article,
we’ll describe the operation of a unique optical sensor
and its incorporation into a practical two-dimensional
position measurement system. The sensor is a lateral-effect
photodiode, which is a special member of the family
of optical PSDs.
Our system
consists of a microprocessor, LCD, some analog electronics,
a typical medium-area PSD, and a four-channel, 12-bit
A/D converter. The result is an instrument that has
a position resolution of one-fortieth the diameter of
a strand of human hair. We’ll describe how PSDs operate,
explain how they’re used, and present you with a specific
application. Note that we teamed a PSD with a PIC16F873
microprocessor to produce an instrument that gives the
2-D coordinates of a laser beam on its active surface
with a resolution of 0.0001", an accuracy of 0.001",
and a measurement range of ±0.2". With this sensor
and a laser level, you can measure the straightness,
flatness, angle, centration, and parallelism of virtually
any surface.
To persuade
those of you who want such an instrument, our set of
PCBs and the PSD are offered at cost. The rest of the
project requires inexpensive electronics. You may download
the code and board files for the project from the Circuit
Cellar ftp site.
LATERAL-EFFECT
PHOTODIODE
The lateral-effect
photodiode is a 2-D PSD that generates photocurrents
proportional to the position and intensity of the centroid
of light on the active area. Two-dimensional PSDs have
sensing areas ranging from less than 0.05" in diameter
to more than 1 square inch.
There are
three basic types of PSDs: duolateral, tetralateral,
and pincushion tetralateral. For additional information
about these PSDs, visit the Hamamatsu web site. In addition,
there are several other useful sources on the ’Net.
Use PSD as a keyword and try your own search!
