The basics of coordinate measurement machines

1. mechanical Structure of high precision: a unit of operation that controls of automatic or digital form. This unit can situate the sensory element in any point inside his volume of work of a way repetible.
2. Handle of data and system of control: computer system, usually with an architecture distributed to control the dynamic activities of the CMM and for the taking of data
3. Software CMM: it is the operating system of the CMM, allows to control the dynamics, the programming and the communication between the CMM and the outside. To a CMM can him enter packages of applications of specific software that together with the software of the CMM can llevar specific applications as it is the measurement of gears, praise of turbine, etc.
4. Sensors: Sophisticated mechanical elements-electronic or optical-electronic that register the coordinates of the points of the surface of the piece that has to measure . The sensor can go in in contact with the piece (palpador) or no (sensor of measurement without contact).

The capacity to understand and evaluate the four main elements of the machine allows to configure a system centred in the problems metrológicos that have to resolve . Besides, have a basic knowledge of these elements allows to know if the system needs an update and like this keeps in a maximum functional efficiency.

Although we will treat the operation of these four elements and his interaction in an article devoted to this, will do a general introduction indicating the main function of the elements in a cycle of simple measurement.

The mechanical structure of a machine of measurement by coordinates is, in the majority of cases, the physical representation of a “System of Reference Cartesiana” in which each one of the axles represents one of the axles X,And and Z of the own system of reference. Each axis can move in regard to the others and centre in a rule so that anytime it can observe his position with regard to the origin of the system of reference. If the origin is the same for the three axles and a point describes according to his origin, the position of this point in the space⁽¹⁾ can know in real time according to the value of the three coordinates X, And and Z indicated by the rules of the three axles of the machine. Besides, if the point corresponds to a characteristic point of the sensor, for example, the centre of the ball of the palpador, then can know the position of the sensor in the space and can register anytime with the expressesion X(value), And (value) and Z (value).

In a tactile sensor the tip is usually formed by a sphere of hard material with a very low level of error. In the tactile measurements⁽²⁾ the tip of the sensor goes in in contact with the piece that has to measure to detect the position of the point that has taken ⁽³⁾.

If it knows the position in the space of the centre of the tip as well as his dynamic diameter⁽⁴⁾, then can calculate the coordinates of the points that have taken .

To understand better the basic principle, the example that shows in the figure “1” represents the compensation in one of the axles of the piece, this compensation works in pieces designated of “simple geometry” by his form. In the case of pieces of “complex geometry”, the compensation will have to do in the perpendicular point and tangente to the point that has taken , however, the principle of reading is always the same.

• Dimensions: derived from the length of the Cartesian axes, dimensions determine the volume of measurement of the structure. The dimensions can range from less than half a metre to many metres, it depends on the measurement volumes required for the type of pieces that have to measure, from a motor to a body.

The dimensions of the mechanical structure have determining influence on the characteristics of the CMM, such as the "architecture" "measurement uncertainty" the reaction of the structure to "thermal gradients", etc.

• Architecture: architectures that can have a CMM have been consolidated over the years. It depends on the size of the mechanical structure and what you want to maximize:

• The "dynamism" (speed and acceleration of the CMM)
• Metrological of the CMM
• Accessibility to the piece which has to be measured

Architectures that are used are as follows:

• Cantilever with fixed table
• Movable bridge
• Gantry
• Bridge in the form of "L"
• Fixed bridge
• Cantilever with mobile table
• Column
• Mobile arm, horizontal arm
• Table sets horizontal arm
• Jointed-arm robots.

1. Space means the volume of the CMM measurement
2. The most widely used currently
3. A sign that allows to capture the value of the three coordinates of the center of the tip at this very moment is generated with a transducer, when the tip comes into contact with the piece.
4. To obtain accurate readings the diameter of the sphere of the tip has to determine using the JMC with the same dynamic parameters that will be used during normal operations to calibrate.