How Laser Shows Work - Scanning System
The most important part of laser projection system are the galvos, more commonly called scanners. Graphics, animations, abstracts and dynamic beam effects are generated by X-Y scanning of the laser beam using galvanometer scanners.
galvanometer (often abbreviated to galvo) is a current-sensitive device
that operates in a similar manner to an analogue meter. In an analogue
meter, a small coil of wire is wound around a lightweight aluminium bobbin
that is suspended in the gap of a permanent magnetic field by means of
pivots and bearings. The coil has a thin needle attached which extends over an
indicating face which is is the part of the analogue meter which you see.
Usage Note: The words galvo or galvanometer refers to the basic galvanometer itself, without an attached mirror. Scanner refers to a galvanometer that has a mirror attached to it such that it can deflect a laser beam by applying appropriate control signals.
The type of galvos used in laser scanning systems typically use a moving magnet or soft iron rotor. Since the shaft has to reciprocate (move back and forth) many thousands of times per second, it would be very difficult to build a rugged device that used a moving coil due to the flexing of the wires that would be needed to supply the current to the coil. Laser scanners are build "inside out" from the typical meter movement described above; the coils are wound on the outside pole pieces (armature), and a magnetic or soft iron rotor, mounted in small precision bearings and suspended in the gaps of the pole pieces, moves the shaft with the mirror. The shaft has a spring or torsion bar to return the rotor to the central at-rest position when no current is applied.
Types of scanners
There are two major types of scanners used in laser light shows - open loop and closed loop. A scanner is open loop if no position detection device is used, and closed loop if a position detection device is used.
Open loop scanners are typically built as shown in the diagram above. The rotor has a shaft running through it with top and bottom bearings. The shaft protrudes outside the housing of the scanner so that a mirror can be attached to the shaft.
The closed loop scanner adds an additional element. The shaft is fitted with a position detector that can determine where the scanner is within it's range of motion. In modern high-speed scanners, the position detector uses an optical system with a vane that occludes the light from an LED falling onto a photo sensor. As the shaft moves through it's range of motion, more or less light falls on the photo sensor generating a signal that is proportional to the position of the shaft and it's attached mirror.
In the diagram above, the LED is shown in
green and the photo detector in light blue with an occluding vane attached
to the shaft between then. The closed loop scanner amplifier is also
more complex than the typical open loop scanner amp.
So far, we have covered only one axis of scanning - one scanner can only take a laser beam and deflect it in a single plane drawing a line. In order to create images, we need to control the position of the laser beam, both horizontally (X axis) and vertically (Y axis). Laser projectors use a pair of scanners mounted orthogonally (at right angles to one another) to control X and Y axis deflection.
The laser beam first encounters the X (horizontal)
scanner. This deflects the beam at right angles to it's line of
travel and upwards onto the Y mirror. If the X scanner were fed a sine
wave, the movement of the mirror would draw a line on the Y (vertical)
mirror placed above the X mirror. The Y mirror takes the line drawn
by the X mirror and moves it vertically. If the Y scanner were fed
with the same sine wave as the X scanner, the projected image would be a line at a 45
degree angle. If the two sine waves were out of phase, a circle
would be drawn.
In addition to the scanners, a separate device is used for blanking -- rapid on/off control of the laser beam. In an un-blanked system, all letters in a word (or all parts of an image) drawn by the laser are joined together like cursive script. In a blanked system, the letters are individual (not connected) like printing. In actual fact the letters are still joined by a line however the laser is turned off as it jumps between segments by the blanking device so that the join line is not visible.
The most common form of blanking for many years, was to
send the beam through a third scanner which deflected it to a pair of
small mirrors set at right angles to one another (a corner reflector).
The corner reflector sent the beam back to the scanner and from there it
was deflected into the X-Y scan pair. This arrangement
formed an optical switch as any movement of the scanner caused misalignment
of the beam through the corner reflector, turning the beam off. This
method is still used in some high power laser projectors as there is a
limitation to the amount of power a PCAOM cell can handle, and the corner reflector
has little optical loss.
More technical information about scanners can be found on the Scanning Systems page in the Laser Show Systems section Backstage.
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