HP 5501A Operating And Service Manual page 75

2.37 Air Turbulence
One of the most important factors to be considered during the installation of the Laser Trans-
ducer System is air turbulence. Air turbulence, or inhomogeneity of the air in the optical
measuring path, is usually caused by variations in air temperature. The major effect of air
turbulence i s to reduce the amount of signal at the receiver. This reduction is due to either a
physical deflection of the laser beam or a degradation of the coherence of the beam.
If
the air
turbulence conditions become excessive, this could resu It in a complete loss of measurement
signal. This loss of signal i s detected by the interface electronics.
In uncontrolled environments such as a machine shop, the effects of air turbulence can be
minimized by protecting the laser beam with covers of some type. Since this will undoubtedly
be done anyway for protection against beam breakage caused by cutting fluid or metal chips,
air turbulence effects will usually not be a significant installation factor in typical shop
environments.
One application where serious consideration must be given to air turbulence is in temperature
controlled environments. Although it would appear that such an environment would be ideal,
temperature controlled areas often exhibit greater air turbulence than non-controlled areas.
This turbulence is caused by a non-uniform air temperature resulting from the mixing of new
air from the temperature control unit with existing air. Since air is a poor heat conductor, any
attempt to change the temperature of the environment by heating and cooling the air causes
non-uniform air temperature. O n a long term basis, this provides good measurement con-
ditions regarding thermal expansion effects. However, the short term fluctuations can cause
measurement signal degradation in the Laser Transducer System.
Protection against air turbulence problems which occur in controlled environments depend
largely on the specific application. For systems such as integrated circuit photomask cameras
in small closely controlled rooms, it may be sufficient to provide constant air flow over the
measurement paths. In other cases, such a s measuring machines, protecting the laser beams
with covers prevents air turbulence effects from interfering with the measurement.
One source of air turbulence which can affect not only the Laser Transducer System but also
the accuracy of the machine itself is localized heat sources (e.g., motors, pumps, etc.) located
on or near the machine. Make every effort to shield the measurement path from these types
of heat sources. Note that a local heat source which can affect the Laser Transducer System
enough to cause measurement signal loss also tends to degrade the geometric accuracy of the
machine through warping or bending. Therefore, consideration should be given to thermally
isolating the heat source from the machine as well as the measurement path.
2.38 Laser Beam and Optics Protection
In almost all positioning systems, some type of protection is generally provided for the pro-
tection of the displacement transducer whether it is a leadscrew, glass scale, or Laser Trans-
ducer System. This prevents metal chips or cutting fluid from interfering with the measure-
ments. In the case of the Laser Transducer System, it can also provide additional protection
against unintentional laser beam blockage and air turbulence problems. I n addition, the
optical components usually require protection to prevent contamination of the optical sur-
faces by oil or cutting fluid. In many applications which are "clean", no protection at
all
may
be needed.
I f protection of the laser beam and optical components i s required, there are two general
types; moving component protection and non-moving component protection. Since the 5501A
Laser Transducer and 10780A Receiver are housed in NEMA-12 type enclosures, no protection
for these devices is needed except in the most severe environments.