Coordinate Measuring Machines and Systems Flashcards
What is the primary function of a CMM
To measure the actual shape of a workpiece, compare it against the derired shape, and evaluate metrological information
Main components of a CMM
- mechanical frame with three axes and displacement transducers
- probe head
- control unit
- computer
Steps of a measurement with a CMM
- calibration of the stylus wrt the reference point of the head (center of the tip)
- determination of the Mathematical alignment: position and orientation of the workpiece reference frame wrt the machine reference frame
- measurement of the surface points on the workpiece
- evaluation of the geometric parameters of the workpiece
- representation or reporting of the results
Basic configurations of a CMM
- bridge (fixed or moving)
- cantilever
- horizontal arm
- gantry
- articulated arm
Moving bridge configuration: characteristics
- stationary table and moving bridge
- widely used
- reduced bending of the horizontal axis
- problem of yawing should be considered
Fixed bridge configuration: characteristics
- moving table and fixed bridge
- very rigid
- reduced speed with heavy loads
- no yawing problem
Cantilever configuration: characteristics
- fixed table, cantilever arm
- good accessibility
- bending of the cantilever reduces the accuracy
- suitable for long and thin parts
Horizontal arm configuration: characteristics
- ideal for measurements on car bodies
- moving long table
- possible dual arm configuration to reduce time
- excellent accessibility
Gantry configuration: characteristics
- suitable for very large parts
- fixed columns
- easy accessibility
Articulated arm configuration: characteristics
- robotic arm (non-cartesian CMM)
- probe in the end effector
- moved by an operator or by actuated joints
Hardware elements of a CMM
- structural elements
- bearing supports
- drive system
- displacement transducers
- probe head
- control system
ideal properties of the structural elements
- dimensional stability
- stiffness
- low weight
- high damping capacity
- low coefficient of Thermal expansion
- high thermal conductivity
Possible materials used
- Granite: very often used
- Aluminum: large thermal expansion compared to granite, but it has a very much higher thermal conductivity, so it is often used, since its deformation can be easily predicted and compensated
- composite: good properties, easy to manufacture, can be designed to obtain the requested properties; expensive
Bearing systems
- noncontact air bearings
- aerostatic air bearings use a thin film of air under pressure to provide load support
- accurate
- durable - mechanical contact bearings
- higher loads are supported
drive systems
- rack and pinion
- simple
- not very accurate for the backlash - belt drive
- simple
- not very accurate for its elasticity - friction drive
- simple, cheap
- low drive force - ball screw
- classical solution
- high stiffness - linear motor drive
- very high stiffness
- water cooling is required
- expensive
displacement transudcers
they have the role of determining the probe position when it touches the surface
- transmission scale
- made of glass
- scale with 50-100 lines per mm
- moving light source
- scanning reticle
- photocells measure fluctuation of the light, generating two sinusoidal signals phase-shifted by 90° - reflection scale
- made of steel
- alternate reflecting lines and diffusing gaps
- period signal produced by the photocell after reflection - interferential scale
- photoelectric head reads the interference fringes created by the light reflected from the scale - laser interferometer scale
- based on the light interference principle, discovered by Michelson and Morley
- one laser ray is directed to a fixed mirror as a reference
- one other laser ray is directed to a moving mirror
- the interference figures revealed in the two mirrors are different, and can be used to determine the displacement of the moving mirror
- much more accurate that optical systems, with resolution smaller that 1 nm
types of control strategies
- . Point-to-Point control
- continuous-path control
point to point control
- target position given as point coordinates
continuous path control
- precalculated scanning
- probe moves to the target position along a defined path
- used for known shape features
- force is not controlled
- force proportional to bending of the tip - adaptive scanning control
- adaptive control ensures that the tip Always maintains contact with the surface
- useful for unknown features
- actuators for each axis
- modulated contact force
fixturing
- it should be accurate and keep the part in position
- forces are very small so there are not many dynamics issues
- aluminum is usually used
types of probing systems
- contact probes
- non-contact probes
types of contact probes
in general, the information comes from the contact with the part
- hard probe: the operator detects the contact
- touch trigger probe: the machine detects the contact, then moves to another point
- measuring probe: the machine measures the displacement of the tip; the movement is with constant contact with the surface, while the measurements are taken with a certain frequency
types of touch-trigger probes
- kinematic resistive probe
- piezo-electric probe
- fiber probe (optical)
kinematic resistive probe: description
components:
- probing element to establish a mechanical interaction with the workpiece
- transmitting device (stylus stem)
- force generating element (spring) for producing a probing force
- sensor to evaluate contact information
- interface to CMM for transmitting the information
- there is a pretravel space from the contact to when the signal is triggered
- the pretravel, in a three point sensor probe, depends on the direction of approach like a triangle
piezo-electric probe: description
- smaller deformations can be detected
- pretravel is reduced
- fast movements of the probe may cause unwanted signals, so only signals followed by a mechanical contact are considered
types of measuring probes
- serial kinematics
- parallel kinematics
- image probing
- laser triangulation
- structured light
- focus variation principle
- computed tomography
serial kinematics contact measuring probe: description
- one independent axis for each degree of freedom
- the position is obtained by simply adding values for each axis
parallel kinematics contact measuring probe: description
- more independent axis than dof
- the position is a function of all the displacements of the axis
- more complex
why are the measures performed at low speed?
CMM can move quickly, but the measures must be performed at low speed, because high acceletaions give dynamic forces that impact on the performances of the system
probing mode
- for discrete point probing, the speed is constant, so that at the istant of contact the acceleration is zero, and there are no inertial forces
- for scanning mode, it is not possible to obtain zero acceleration, because the speed vector should at least change direction, so variable inertial forces are present and may be relevant. the speed must be low.
mechanical filtering effect of the tip ball: explanation
the diamater of the ball tip should be chosen considering the filtering effect it has on the measurement of a surface: small variation of the surface (roughness) are not detected by a big tip
effective tip ball diameter: explanation
w0: elastic deformation of the workpiece and tip at contact, due to Hertzian stress distribution
w1: elastic deformation of the sylus stem
- > the effective tip ball diameter is reduced due to the elastic deformation of the bodies
- > this part can be considered a rigid body
image probing: description
- a CCD camera takes a picture of the object
- a software extracts the element profile from the image
- illumination is a key factor: episcopic light in the same side of the camera; daiscopic light in the opposite side
- in general, 2D shapes can be measured, but the integation of auto-focus techniques can add the third dimension
laser triangulation: description
- a laser point or line is projected to the workpiece
- the point distance is evaluated by triangulation
structured light: description
- a series of lines of different dimension are projected on the surface
- from the deformation of the lines it is possible to obtain a three dimensional representation of the surface
fiber probe: description
- it’s a touch trigger probe
- the probing system is made of a optic fibre
- a CCD camera detects the motion of the light when it touches the piece
focus variation principle: description
- a microscope takes images of the sample at different heights
- at each height, only one section is focused, corresponding to the maximum of the function Fz
- from the multiple images, a 3D image is obtained
computer tomography: description
- X-rays can penetrate objects, being more absorbed depending on the material and shape encoutered
- a detector captures the x-rays escaping from the object (2D image)
- combining the images from many views of the object, a 3D voxel can be extracted
- from the voxel representation, the various features can be identified using suitable algorithms
- it is possible to see also defects of the material
multisensor systems
- use of different optical and mechanical sensors on the same CMM
- at the moment, only different separate measurements can be done
- it is a reasarch topic the way to integrate data from different sensors
types of software error compensation
- linear compensation: related to linear thermal expansion and linear scale error
- Computer Aided Accuracy
- volumetric compensation
- table bending - probe system compensation
- bending of the stylus
- deviation of measuring systems - Dynamic CAA
- dynamic deviations of the overall machine contruction due to accelerations
inspection planning: objectives
starting from 1. nominal part and the geometrical product specifications 2. coordinate measuring system available 3. knowledge of the machining process the inspection planning defines: 1. sampling strategies 2. fixture and probe configuration 3. probing path
definition of the sampling strategy
- choice of number and position of measurements
- different strategies give different errors
- a good strategy is better found having information about the process
- related to the cost strategy is the inspection cost
- the optimal strategy gives the lowest inspection cost
evaluation of the inspection cost
two main factors:
- inspection errors:
- false negative: cost of production of the part
- false positive: difficul to be evalueted, it depends on the behaviour of the customer - measurement cost:
- strategy dependent (dicrete or continuous measurements)
- strategy independent
definition of the probe configuration
- determine its orientation, structure, dimension
- accessibility constraints need to be considered
- it is important that the probe has the center of mass internal to its structure, to reduce bending
definition of the fixture configuration
- related to the probe configuration
- it defines the part orientation
path planning
- gieven the cloud of points to be measured, as defined in the sampling strategy, the best path should be found
- TSP algorithms cannot be applied because there is no a priori knowledge of the arc length, and so it is a NP-hard problem
- divide and conquer approach: visit problem separated from the computation of the length of the trajectories
- a Manhattan distance approach can be used to find a possible path with an estimation of the distances; then the TSP algorithm can be applied
