Machine with temperature and structurally integrated sensors.

Machine tool deviations
mostly caused by thermal effects. Thermal induce displacement variations of
tool centre point (TCP) of machine tools
are vital problems that cause the
positioning errors.

To acquire the higher
accuracy of machine tools, it is necessary to find the effective methods for
reducing thermal errors.  Subproject has
been setup within TR96, to compensate the
thermal errors using hybrid intelligent method.
Machine portal is equipped with temperature and structurally integrated
sensors.  Deformation model use sensors
value to calculate on machine tool TCP dislocation. Afterwards, control system
will execute feed axis motion. Compensating deformation.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

   This thesis presents a Thermo-Mechanical analysis of DMG HS-55 machine
tool at IPT Fraunhofer in Aachen with the focus of
portal. HS-55 consist of large variety of
rectangular components that influence the thermal behaviour.  Portal is
characterized in detail regarding their position and orientation errors as function of underlying thermal loadings
contributing significantly to overall efficiency
of machine tool. Thesis deals with simulation of
Portal using finite element method (FEM) and qualitative structural deformation
have been observed. Whilst FEM has become
increasingly key tool in analysing the
static behaviour.

 

 

 

 

 

 

1

Introduction

 

Tool centre
point (TCP) dislocation due to thermal errors has
influenced the production of machines for a long
time. Due to the rise in demand for high precision workpieces, research
on thermal error compensation methods has been geared up in industries and research
institutes. Researchers highlighted thermal errors accounts for 70% of total
errors (tool wear, geometrical,
positional, kinematical errors) in a machine.
Through optimization of design and manufacturing technology, structural improvement of the machine tool can be possible which will minimize these errors.
However, the solely physical limitation
will not eliminate all thermal errors.

To have an idea about thermal characteristics is
complex. In general, there are six sources of thermal effect for machine tool system 1, (i) Heat produces in cutting
process; (ii) Heat produced by machine (iii) Heating or cooling effect due to cooling systems (iv) Heating or
cooling effect due to room (v) influence
of people (vi) Thermal memory from the last
environment.  These error sources reach
out each other through thermal phenomena (conduction, convection, radiation) result
in non-uniform temperature field which is main
source of thermal displacement of machine
tool. Providing temperature-controlled
environment requires high capital investments and running costs, which are good
to some extent but not in larger context. That’s why over the course of time
methodologies have been developed to compensate thermally induce errors.

1.1
Related work: –

Since the last decade’s various methodologies have been
presented and implemented, to identify, predict and compensate the overall
effect of thermal distribution in a machine tool. J.vryroubal 2 presented an approach which uses the multi-regression equation, comprises of standard
temperature measurement of machine tool and new temperature measurement of spindle
cooling liquid, to compensate thermal
errors. Another method was found by Marco Gherlone
3 along with four others by reconstruction of displacement field of a
structure from surface measured strains to predict, regulate and observe the
structure behaviour. Foss and Haugse 4 used the piecewise continuous function
for predicting deform shape. Instead of monitoring the reasons for deformation
(Temperature, loads, accelerations) and converting them into displacement
through a suitable model, the deformation-strain method was used.

Other
successes include adaptive neuro-fuzzy inference system to design two thermal
prediction models:by splitting the data space into rectangular sub-spaces (grid
model) and by exercising the fuzzy clustering method(FCM model) 5. A vector-angle-cosine
hybrid model which uses three models as reference multilinear regression model,a
natural exponential model and Finite element method 6. Compensation can also
be possible by using thermal error mode analysis for sensor location and vigorous
method, measuring noise and environmental changes7.

Micheal
Gebhardt classified compensation into physical and phenomenological model. physical
model basically discretization of machine structure into key elements. Elements
exhibit part of structure and physical properties like conduction, convection,
mass etc.Through Uniform temperature distribution deformation is to be
calculated. Edge of physical model are small modelling effort and few measurements
are required. Downside physical model are alignment of model and measurement
(model matching:- density, heat transfer). of  In phenomenological model,proportionality
constant and time constants of values from recorded test are used as model
parameters.Precedence of phenomenological model only measurement are needed and
no requirement of physical model. A hiccup, the experiment has to be performed numerous
time, which is time-consuming8.

 Significant 
factor influencing the accuracy in thermal error modelling is the
temperature
variables, which can be concluded by choosing suitable locations for the
temperature sensors.Fundamentally, stations and quantity of sensors are
determined based on engineering experience, more often sensors are placed right
next to heat source 9, or choosing them by using statistical approaches 10,and
decomposition methods 11. In general, it’s not practical to pre-select the
optimum locations for sensors and their required quantity because its a lot
depends upon which working environment machine is being used.

 

Filipos
last paragraph, thermal issue se fem beahviour wala paraagraph

1.1   
Layout of report:-

Second
chapter descibes the basic concept of thermal deformation, sources of heat,methods
for measuring deformattion. Deficiencies in state of art.

Third
chapter explains experimental setup, measuring the seformation using
temperature sensors and integradted deformation sensors.

The
chapter four covers the thermomechanical FEM equations, analysis layout,boundy
conditions,Meshing.

Chapter
five explains analysis,results of portal beam deformations.

The last
chapter give the summry of above work and tell what improvents can be made.

 

 

 

 

 

 

 

 

 

 

2

State of art

Heat sources in machine tools:-

There
are internal and external heat sources that have an impact on machine tool
performance.

External
heat sources:-

The
significant external heat source is environmental temperature but there is also
an influence of solar radiation or forced convection. Environmental temperature
fluctuations during day and night but also over the different seasons of the year.

 

Internal heat sources.-

Every
heat source connected to machine tool structure directly named as an internal
heat source. Machine elements as bearings, spindle, feed drive as well as additional
units like chillers, coolers etc, as long as come into direct contact, act as
an internal heat source. Heat produce in these elements result from local power
loses due frictional and electrical effects. The ultimate consequence of this
heat is inhomogeneous temperature field in machine tool structure which is root
cause of thermal elastic deformation, reason for TCP displacement.( master
thesis)

Sources of heat generation in HS-55:-

The
major source of heat generation will be the heat generated by the process,
which will be
transferred through the Spindle stock into the machine structure. The
frictional heat generated
through the movement of the Z-axis (horizontal movement of the Spindle stock),
and the heat
generated by the motor will also account for the deformation.
1) Heat generated by the linear guides in the cross-directional movement
(X-direction in fig)
2) Linear motors in cross-directional and 2 motors for movement of table
3) Frictional heat generated by the linear guides in the direction of the table
movement
4) Process heat generation (omkar)

Fundamental of Heat transfer:-

Thermodynamics
illustrate the fundamental behaviour of heat and temperature and encompass the
three laws of thermodynamics. Heat transfer goes explains the mechanisms of
heat exchange and the rate at which heat ?ows, giving us a way to calculate
heat ?ow within, to and from objects or the environment.
There are three modes of heat transfer: conduction, convection and radiation.

Conduction:-

Conduction
is the first mode of heat transfer. Conduction is the type of heat transfer in which heat is passed
from hot to cold directly through a material by way of molecule-to-molecule
interactions. Conduction occurs by collisions between the particles inside the
body that is being heated. Materials that easily allow energy to pass through
them are called conductors. Materials that do not allow energy to easily pass through
them are called insulators.

The Fourier’s law of heat
conduction is a basic law describing a linear conductive heat flux (heat flow
per unit area W/m2) through a material. Heat flows in the direction opposite
to the thermal gradient, thus, from hot to cold. The rate at which this happens
is determined by the temperature difference (K) and the thermal conductivity
(W/(m K)) of the material.

q=(K/L)

    q= heat flux (w/

                 k=thermal
conductivity ( w/m k)

Convection:-

Sometimes heat
is transferred through bulk movement of a substance. This type of heat transfer
is the main way in which heat is transferred through a fluid. The hot fluids
have a smaller density and rise to due to buoyant force. We call this type of
heat transfer convection. Convection is the process in which heat is
transferred from place to place by the bulk movement of a fluid.

                                                              
q=h

q=heat flux
(w/

h=heat
transfer coefficient

 =temperature difference surface and fluid

                      

There are
two general types of convective heat transfer:
1. Natural: a heated fluid is usually lighter than colder fluid around it. The
difference in density results in a fluid flow upward. As the fluid takes its
thermal energy along, the heat is transferred.
2. Forced: a fluid can be displaced actively, e.g., by blowing or pumping, to
transfer heat.

 

 

Radiation:-

It is
possible to transfer heat without any material being present. Heat that is transferred without the need of any material (or medium) is called
radiation. Radiation is the process in which heat is transferred by way of
electromagnetic radiation. The energy radiated per time by an object–that is,
the radiated power, P–is proportional to the surface area, A, over which the
radiation occurs. It also depends on the temperature of the object.

This behaviour is described in the Stefan-Boltzmann
Law:

P=e

A

The
constant

 in this expression in this expression is a
fundamental physics constant, Stefan-Boltzmann constant

= 5.67*

 w/

  

 

The other
constant is the emissivity, e. It is a dimensionless number between 0 and 1
that indicates how effective the object is in radiating energy. A value of 1
means that the object is a perfect radiator. Experiments show that objects absorb radiation from their
surroundings according to the same law, the Stefan-Boltzmann Law, by which they
emit radiation. Thus, if the temperature of an object is T and its surroundings
are at the temperature Ts, the net power radiated by the object is

 

 

P=e

A(T

 

 

 

 

Refrence dspe.nl and lesson 6.03

 

 

Measuring
the Thermal deformation: –

Nowadays
various methodologies exist to measure the displacement of machine tool
components. Some standards have been developed
to which proved to be a good guideline 
as well as some other measuring techniques (eth 594602)

Standards:-

Selecting
a best-fit measuring system depends on errors sources. Temperature change is a gradual
due environmental effect, result in volumetric accuracy. Internal heat sources
lead to local deformation of machine tool structure and affect volumetric
performance j. may et al.Thermal
issue in machine tool. ISO 230 is key relevant series on the measuring machine
tool. This series deals with different parts of machine tools. ISO 230-3 Test
code machine tools-part 3: Determination of thermal effects.2007

Environmental
temperature variation error (ETVE)

Thermal
deformation due to rotating spindle

Thermal
deformation by linear motion of axis eth 594602

Temperature measurement:-

Measurement
of temperature has a very important role in machine tool behaviour. Temperature
is the physical property measured by sensing technology.Measurement system
principle can be placed into two categories contact measurement and non-contact
measurement.

Non-contact Temperature measurement:-

Infrared cameras are widely used to measure temperature
distribution on the surface of the machine tool. Every form of matter
with a temperature (T) above absolute zero emits infrared radiation according to its temperature. This is
called characteristic radiation. The cause of this is the internal mechanical
movement of molecules. The intensity of this movement depends on the
temperature of the object. Since the molecule movement represents charge displacement,
electromagnetic radiation (photon particles) is emitted. These photons move at
the speed of light and behave according to the known optical principles. They can be deflected, focused with a lens, or
reflected from reflective surfaces

 

A further
reason for having devices for different wavelength ranges is the emissivity
pattern of some materials known as non-grey
bodies (glass, metals, and plastic films). Many bodies, however, emit less
radiation at the same temperature. The relation between the real emissive power
and that of a blackbody is known as emissivity ? (epsilon) and can be a maximum
of 1 (body corresponds to the ideal blackbody) and a minimum of 0. Bodies with
an emissivity less than 1 are called grey bodies. Bodies, where emissivity is also
dependent on temperature and wavelength, are
called non-grey bodies.

 

Non-contact temperature measurements show a high dependency on the
emission characteristics and the reflectivity of the analysed surface. The
relationship between emission

, reflection

 and transmission

 is given by thermal
issue,pri nciple non contact temp
measument

 

 

 

Contact Temperature:-

Contact
temperature sensors measure their own temperature. One infers the temperature
of the object to which the sensor is in contact by assuming or knowing that the
two are in thermal equilibrium, that is, there is no heat flow between them.

Many
potential measurement error sources exist, as you can appreciate, especially
from too many unverified assumptions. Temperatures of surfaces are especially
tricky to measure by contact means and very difficult if the surface is moving.
It is wise to be very careful when using such sensors on new applications.

Thermocouples:-13

Thermocouples are sensors composed of two
different metals at their sensing end. A
voltage is created when there is a temperature gradient between the hot sensor
element
and the cold reference junction. The change in voltage can be reported as a
temperature through the Seebeck effect (Love 2007). The Seebeck effect says that the change
in voltage is linearly proportional to the change in temperature and the two
variables are
related to each other through a coefficient
that is determined by the materials used in the
thermocouple (Janata 2009).
Figure depicts the construction of a thermocouple

 

Resistance temperature detectors:-13

Resistance thermometers are also known
as resistance temperature detectors, or RTDs.
They are typically made of a single pure metal (Dames 2008). Each metal has a material
property of electrical resistance that is a function of temperature. The most
accurate resistance thermometers are ones that use metals that have a very
linear relationship with temperature, such as platinum. By using the
relationship curves between electrical resistance and temperature, when the
resistance of the metal is measured, a temperature can be calculated (Dames 2008)