ASME B89.4.21.1-2020 pdf download
ASME B89.4.21.1-2020 pdf download.Environmental Effects on Coordinate Measuring Machine Measurements.
The types ofceilingand method ofluminaire mountingare significantfactors in the distribution ofthe thermal energy developed in the lighting system. In dimensional measurement, the usual type of luminaire mounting is suspended, surface mounted, or recessed. In some locations, luminous or louvered ceilings may be used. For suspended luminaires, approximately40% ofthe total energyis dissipated to convective aircurrents in the room. The inputbalance is indicated in all directions to be reflected or absorbed and reradiated. Since nearlyall ofthe inputenergyremains within the occupied space, the thermal factor is generallytaken as unity(a value of 1 with no units). On the other hand, for surface-mounted luminaires, the heat is transferred by radiation, convection, and conduction. Upper surfaces of the luminaire absorb energy and transfer it by conduction into the ceiling. Since many ceilings are made of acoustical materials and hence are insulators, the temperatures within the luminaire will be elevated. Consequently, the lower surfaces will radiate and convect to the space below at a higher rate. Unless the ceiling is a good absorber and can reradiate into space above it, essentially all of the input energy will remain in the occupied space. Recessed luminaires will distribute some portion of the input wattage above the suspended ceiling. Asaresult,theenergyactuallytransferredtotheroomis lessthanthetotal,andthis resultsinalowerthermalfactorfor the room. The energy transferred to the space above the ceiling may contribute to the overall problem, however, since many times this space is partofthe air system for the laboratory. Luminous or louvered-ceiling lighting systems behave very much like a suspended luminaire in their transfer ofenergy since mostofthe plastics or glasses used in this type of ceiling are good absorbers of infrared.
The lightfrom the luminaires in itselfis a heatproducer, notbyheatingthe airas convection sources do, butbyraising the temperature of any surface that absorbs it. This can become a significant problem in a dimensional measurement process because most objects, e.g., dark surface plates and measuring machines, are better absorbers of light than reflectors of light. In many cases, this can cause a degree or more temperature differential between the air and the object. It is recommended that the general lighting system in a dimensional measurements location be left on at all times to stabilize the heatload in the occupied space from this source. This is also importantbecause ofthe possible detrimental effect the lower air temperature may have on the lighting system, particularly if fluorescentlampsareused.Inconduction,heatistransferredbymeansofthemolecularinteractionbetweenadjacentmolecules.Thisinteractionisprimarilydependentonthetemperaturedifferenceandtheresistanceofthematerialtoheattransfer.Theresistancetoheattransferisdependentonthenatureanddimensionsoftheheattransfermedium.Whileallheattransferproblemsare dependent on the difference in temperature, thegeometry,andthephysicalpropertiesoftheobjectbeingmeasured,inconductionheattransfer,theobjectbeingmeasuredisinalmostallcasesasolid.Thereareseveralwaystocorrelatethegeometry,physicalproperties,andtemperaturedifferenceofanobjectwiththerateofheattransferthroughtheobject. In conduction heat transfer, the most common means of correlation is through Fourier’s law of heat conduction.
Covers may be used for a variety of reasons, including shielding the machine from short-term thermal effects or contamination from dirt, providing electromagnetic interference (EMI) shielding, providing safety shielding, styling, and cosmetic impact. If the environment contains airborne particulates, protected guideways (bearing and scale covers) may be beneficial. Covers may be beneficialinshieldingthemachinestructurefromcyclictemperaturefluctuationsinthemachine’senvironment.Iftherearecomponentsunderthecoversthatgenerateheat,thisinternallygenerated heatinside the CMM could be trapped under the covers, potentially increasing the thermal resonance ofthe machine. The structures under the covers may have changed by 1°C to 2°C during the warm-up and measurement processes. If a steel scale had heated 2°C, the scale length would change about 23 μm over a length of 1 m. Additionally, bending may occur if a temperature gradient is induced in the machine structure. Machines with active temperature compensation would compensate for this change in length. Internal components that dissipate variable energy, such as servo motors or linear actuators, dissipate heat as a function of the measurement duty cycle. For example, if a measurement cycle has a series of y-axis moves with little or no x- and z-axis motion, the heat dissipation from the y-axis actuator may influence one or more machine scales and could induce a thermal gradient in the machine structure. Therefore, the influence of internally generated variable heat dissipation may be more difficult to correctfor than constant heat sources that may equilibrate
