Climatic Chamber for the Credibility Evaluation of Profilometric Measurements Construction and Control

The paper shows the construction and control of a climatic chamber used for the credibility evaluation of the profilometic measurements. The paper presents a control model using a PID controller and a solution for air circulation. The data regarding the influence of environmental temperature changes on the three-dimensional representation of the surface realized by means of a tactile profilometer were presented. This work shows the temperature characteristics of a climatic chamber.


INTRODUCTION
The surface topography is generated in the last stages of the manufacturing processes. Materials contact, sealing, friction, a lubricant retention, a wear resistance and other functional properties are related to the surface topography. Surface roughness is very important and often analysed. Many roughness measuring instruments give absolute measurement of surface heights. The assessment of profiles was employed since the early 1930s [23]. These systems basically involved the use of the mechanical stylus.
The vertical movements of a tip, which follows the roughness profile, are amplified and digitised to extract the roughness parameters. The profile (2D) measurement is typically quicker, simpler and easier to interpret than the areal (3D) measurement. However, most surface interactions are areal in nature. The 3D surface topography is obtained by collecting parallel scanning profiles with a defined step. The 3D surface parameters are more reliable than the 2D profile parameters. Somicronic, a small company near Lyon, delivered a prototype 3D stylus system to the Ecole Centrale de Lyon in 1990 [1].
There are a lot of factors affecting uncertainty in the surface geometry measurement using the stylus (tactile) technique. They are caused by environment, measuring equipment, measured object, software and stylus. One can distinguish various kinds or errors using the stylus instruments like the errors typical for the interaction between a stylus tip and a surface, the errors caused by the digitisation process and the errors obtained during data processing [15,17].
The lateral resolution of the stylus instrument depends mainly on the size of a stylus tip. The fundamental parameter in the transfer function of the measurement system is the radius of the tip curvature. The tip geometry acts as a low pass filter by removing high spatial frequencies of the signal [12,14,31]. The stylus flank angle also affects its resolution. However, the slope limitation is not a problem for most types of smooth surfaces.
The slower the stylus moves, the finer details can be resolved. However, a low stylus speed is the main disadvantage in the areal surface measurement. For a comparatively high speed the stylus flight is possible -it is the potential for the stylus to lose contact with the surface. The parameters that affect this phenomenon are: the stylus speed, the stylus force on the surface, the damping constant and surface characteristics [3, 16, 22,]. The spiral sampling can be used to reduce the measurement time [9]. The stylus method is criticised because it is claimed to damage the surface. However, the effect of the plastic deformation is usually not serious [25,27].
The effect of a temperature change on the results of the profile measurement is negligible, since the measurement time is low. However, during the measurements of surface topography with the use contact profilometer, the impact of temperatures changes of environment on measurement accuracy and mapping of the topography of measured surface was noticed. The temperature changes of the environment in which the device works influence the thermal expansiveness of the profilometer structure which conditions the interferences during the measurements of surface topography [11,28].
In order to determine the geometric distortion of a profilometer as a function of the temperature changes of environment, a climatic chamber used for the credibility evaluation of profilometric measurements was constructed. The purpose of the chamber was to simulate the real working conditions of the measuring device in a controlled way [13]. The studied object placed inside the climatic chamber was HOMMEL Etamic T8000 contact profilometer (Fig.1).
The control of the temperature inside the chamber, the change of its values in a determined way and monitoring of response of the contact profilometer on external thermal extortion, allow the compensation of the impact of external thermal disturbances during the measurement of surface topography.

CONSTRUCTION AND CONTROL
The climatic chamber shown in the picture (Fig.  2) was built from PVC boards, in addition, it was equipped with thermal isolation made of expanded polystyrene, separated from the basic construction by an air gap. The important element of the climatic chamber construction for the credibility evaluation of yjr profilometric measurements is the use of the circulation system and the air dosage diffusion system as well as a thermal-storage stove for the prepared warm and cold air. Cooling and heating of air was conducted with the use of Peltier cells located in the walls of the thermal-storage stove of climatic chamber (Fig.3) [2,30]. The climatic chamber is equipped with the additional upper perforated board which steadily distributes cold air to the inside of the chamber and a passage which channels hot air from the above of profilometer drives. The air flows are controlled by a servo valve.
The temperature regulation system inside the chamber was based on ATmega 2560 microprocessor, DS 18B20+, DHT22 temperature sensors  and executive peripheral systems like servo vales and the set of relays. In a control system PID controller, the mathematical form of which is expressed by the equation (1); was used. The signals from three temperature sensors located inside the measured part of climatic chamber, two sensors in air preparatory part and a sensor placed outside climatic chamber used for measurements of external temperature of environment were carried to the controller. The task of the sensor placed outside the climatic chamber was the early activation of the temperature regulation system inside isolated part of the chamber in case of changes in the environment temperatures. (1) where: S t -control value, K p -gain, T i -integration time, T d -derivation time, X b -the control deviation at time t, X p -the control deviation at time t-1 The climatic chamber to assess the accuracy of profilometer measurements can operate in two modes. The first mode is to stabilize and maintain the determined temperature inside the main part of the measuring chamber -where the profilometer is placed [21]. The second mode controls the temperature changes in time, inside the measuring part of the chamber according to the determined process or designated periodic function. The control system allows the setting of periodic function, amplitude and the frequency of temperature changes inside the chamber. The change of the temperature setting is within the range 18-26°C. The simulation of activities and selection of controller parameters were carried out in the Matlab Simulink programme (Fig.4) [8,20]. Temperature regulation inside the prime part of the chamber is performed with the addition of a pre-determined amount of warm and cold air, prepared in a thermal storage stove beforehand (Fig.5). The air inside the chamber is dispersed and continually circulates, which homogenises the distribution of temperature in the whole volume of the chamber.

SUMMARY AND DISCUSSION OF OBTAINED RESULTS
The study on the climatic chamber characteristics for the credibility evaluation of profilometric The inside of the climatic chamber was heated to to a temperature higher than ambient, followed by searching of leakiness in the isolating layer with the use of FLIR T620 thermal imaging camera (Fig.6) [5]. The next step in determining the characteristics of operating conditions inside the climatic chamber was to determine the features concerning the accomplishing set temperature (higher from environment temperature) as a function of time (Fig. 7) [4,10,29]. The characteristics of the cooling system and the time after which cooling of the inside of the climatic chamber from dedicated operating range follows and re-heating were determined as well. (Fig. 8) and (Fig. 9). These data made it possible to choose the correct parameters of the PID controller [6,7,18,19,24,26,32].

CONCLUSIONS
The use of climatic chamber allowed providing stable operating conditions for the contact profilometer from the independent changes of the environment temperature. This is an important aspect which provides the reduction of external disruptions during the measurements of surface topography. The climatic chamber for the credibility evaluation of mapping of the surface topography allowed for simulating the variable conditions of the profilometer environment, which allowed determining the response of the measuring device system. This information will contribute to the creation of an error correction algorithm of surface topography as a function of changing temperatures. The applied construction solutions and microprocessor control system allow the operation of the climatic chamber in different variants, which significantly expands its application in experimental studies.