1.1.1 TemperaturemeasurementMeasurement oftemperature has a very important role in machine tool behaviour.
Temperature isthe physical property measured by sensing technology.Measurement systemprinciple can be placed into two categories contact measurement and non-contactmeasurement 22.214.171.124 Non-contactTemperature measurementInfraredcameras are 24 widely used to measure temperaturedistribution on the surface of the machine tool. Body which has temperature nonzero emits IR radiations dpending upon its temprature.
This is calledcharacteristic radiation. This happens because of temperature of of body resultsin internal molecules motion. Since the molecule movement represents chargedisplacement, electromagnetic radiation (photon particles) is emitted. Thesephotons travels at the speed of light and behave according to the known opticalprinciples. They can be deflected,focused with a lens, or reflected from reflective surfacesNon-contact temperature measurements show ahigh dependency on the emission characteristics and the reflectivity of the analysed surface. The relationship betweenemission,reflection and transmission is given by 1.
1.3 ContactTemperatureContacttemperature sensors measure their own temperature. One concludes the temperatureof the body to which the sensor is in contact by supposing or knowing that thetwo are in thermal equilibrium, that is, there is no heat transfer betweenthem.Many possiblemeasurement error sources exist, when assumptions are made.It is very difficultmeasure the temprature of surface specially moving surfce by direct contact.
Itis wise to be cautuious when perfoming experiments of taking measuments.126.96.36.199 ThermocouplesThermocouples 20have two differentmetal at theirs sensing ends. Voltage is generated when temperature gradientoccurs between hot sensor element and cold reference junction.
Variation involtage notice as temperature through seebeck effect. The seebeck effect saysthat temperature gradient is linearly proportional to voltage and connected throughcoefficient of material used in Figure 2–7. Thermocouple construction 20 1.1.3.
2 Resistancetemperature detectorsResistance thermometers are also known as resistance temperaturedetectors, or RTDs. They are constructed usingone metal and material property of that material is function of temperature.The accuracy of resistance thermometers will be high if metal used inside has linearrelation with temperature, such as platinum. By using this linear relationship resistanceof material can be found out and temperature can be measured. 201.2 Computationof thermo-Mechanical errors of machine toolsThere are a lot of different methodologies have beendeveloped to the model thermo-elastic behaviour of the machine tool in order tocompensate thermal errors. In general, methodologies can be classified into twocategories 25i) Physical modelsii) Phenomenological models1.
2.1 Phenomenologicalmodels:-Phenomenological model constructs a relationship betweeninput parameters (e.g. Temperature) and an output value (e.g TCP displacement).Experiments are carried out at different loads and results with respect to timeare observed by regression model (RM). Other methods like neural networks (NN)and Fuzzy logic (FL) for compensation also listed in phenomenological models.
1.2.2 Physicalmodel:-Physical modeling approach simulates thermally induce errorsdistinguished, in temperature distribution and distortions, in order tocalculate TCP dislocation and enable real-time compensation. All considerationsare based on physical laws.
FEM models and FDM models approaches are part of physical modeling. 1.3 Reductionof Thermally induce errors:-A lot of people have presented different methods to reducethermal errors, put into net shell these methods can be classified into threecategories according to thesis5946,40i) Minimizing thetemperature fluctuations: for example by cooling or controlled environmentcondition as well as minimum heat generationii) Reducing thermal sensitivity: reducing the sensitivityof machine tool structural loop to temperature changesiii) Compensation of errors: for example by mean of mathematicalmodels1.3.1 Reducingthe temperature variations:-Temperature variations can be minimized by reducing themasses of machine tool structure41,thesis 594, applying cooling to a machine tool, use of oilshower, through air.By trying to create even temperature distribution thermalerror can be reduced of machine tool structure.
Much lower the temperaturedifference will be lower the thermal error present. The temperature gradient can be reduced by minimizing heatgenerated in elements of machine tool. P sekler et al 41 of thesis illustrate thermal error can be reduced by sizingdown the masses of machine tool structure. This usually applies to constructenergy efficient machine tools but also it also helps in reducing the lossesoccur in machine tool. With smaller masses less energy is required to move themresult in smaller losses and lower temperature on machine structure.The most common approach implemented widely in industry is toapply cooling to machine tool. Some approaches based on try to remove theexcess heat generated in machine tool elements. One of the approaches 40 isto design special cooling element for the spindle.
These cooling tubes try tomake us of Coanda-effect. Working principle of Coanda effect, fluid passage outfrom nozzle creating a primary stream. Temperature control of air in alithography application is shown in 42. Compensation using oil shower is used in 43 and 44.Another advantage of using oil shower is that insulation from fluctuations fromin room temperature. Various methodologies to reduce thermal errors that does notdirectly reduce the temperature gradient on machines but modifies it, is practiceof heating and cooling elements. It can be seen during application ofcompensation methods to machine (47,48,49).
In order to reduce tool center pointdisplacement key elements of machine tool either can be heated or cooled. Forspecial cases feed drives are used to for reduction of angular errors on threeaxis machine.1.3.2 Reductionof thermal sensitivity:- Other than temperature gradients approach thermal error ofmachine tool can also be reduced by minimizing the sensitivity of elements totemperature changes.
Meaning of this machine tool design in this way that largedeformations do not occur. This can be achieved by applying thermo-symmetrical design to machine tool. In50, thesis 5946 boundary conditions are applied to headstockof lathe in such a way that center of axis does not move during the thermal expansion.Thermal deformation on machine tools 51 present a methodology according to that non-sensitive machine can be design in such way that specific directionalthermal expansion do not affect that workpiece accuracy.
188.8.131.52 Advancematerial for compensation of thermal displacement:- (Thermal issues page 782)Material optimization can be effective in reducing thethermal errors in machine tool.
Alternative materials like carbon fiberreinforced plastic (CFRP) has negative linear expansion coefficient can be usedto compensate thermal displacement of machine component which have positivelinear expansion coefficient such as aluminum.Another example thermal distortions due to local temperaturegradients can be reduced by using polymer concrete in machine tool bed. Achieveablereduction is upto 30%.
(thermalisuue page 783) 184.108.40.206 Active compensation using adaptronicdevices:- (thermal issue 783)CFRP structure are used for active compensation of angulardisplacement of main spindle of housings and heating of unidirectional carbon fibrereinforced laminate.In an adaptronic system, negative thermal expansion of CFRP-structure compensate the thermal displacement. Thermal sensors, controllers,and CFRP actuators make possible controlled heating of CFRP laminate by heating filaments andPeltier elements.
1.3.3 Compensation:- thermal issue In general, Thermal displacement can be estimated in twoclasses of methods: Direct compensation and indirect compensation. Processchain of thermal deformation Direct method uses touch probes to compensate error, for thatmachine has to be stopped during an operation to take measurements, the bigdrawback of direct methodology, ultimately productivity reduce. On other hand indirect measurement reduce downtime by activecompensation.
The indirect approach uses temperature measurement to calculateTCP displacement with help of mathematical models. The most common model used for are describedbelow:-220.127.116.11 Methodof thermal error compensation based on linear and nonlinear regression:- Regression model is applied for error compensation, it definesa relationship between dependent and several independents values. In case ofthermal errors temperature in specific machine tool points are independentvariable and dependent variable are TCP displacement. It is active compensationmethod which means without disturbing the machine process errors can becalculated. The hindrance with indirect compensation is the installation of measuringsystem is very costly.
The drawback of RA is selecting positions for temperaturesensors if too many positions are taken it will increase the cost if few thanthe accuracy of the solution will be compromised.1.3.
3.2 Compensationbased on neural networks:- 8thesisUsing Neural network approach for thermal error compensationis a common practice. Feedforward networks are used for thermal errorcompensation, temperature probes act as input. Neural network approximates theTCP dislocation relying on the temperature of machine tool. Input and outputlayers act as input and output buffer for temperature measurement of machineand machine thermal errors respectively. Layers in between them are calledhidden layers.
The working principle of these to suppress the noise. Each input is multiplied by the interrelated weight. All ofthese weighted inputs are summed up and combined with a threshold to find outactivation level of the neuron.1.
3.3.3 Physicalmodels:-Compare to ANN and RM a lot of others models are in practice.In 7, thesis 5946 lumped capacitance method is used tocalculate the temperature distribution of the machine tool.
To do that thermalbehavior knowledge will be needed because one must know which parts of machinecan be lumped and how to apply proper boundary conditions for lumped bodies. Aseries of temperature is used and TCP dislocation is computed by stress-freetheory and rigid body kinematics.With advancement in computer field and accessibility ofmodels, e.
g FEM models, new reduction procedure can be developed. Denkena etal, 42 thermal issues 785applied FEM to calculate thermal deformation of machine tool in steady stateversus load profile. During operation TCP displacement are compensated with a linear model comparing temperaturemeasured on machine tool structure with those computed n steady state. FDEMapproach is endorsed fot real-time compensation of machine tools 141 thermal issues 785.Unknown boundary condition, simulation-based model, Volumetric TCPdisplacement, use of thermal location and components errors as correctionvalues.
A mixture of FEM and FDM used in 76 a transient thermal analysis has beenperformed using FDM and TCP displacements has been compensated using FEM.