Dynamics of coordinate measuring machines

These issues continue to be of vital importance. However, the copy has changed the rules of the game by introducing another factor: the forces of inertia.

By measuring the differential points, these forces of inertia are usually negligible. During copying, acceleration, and therefore inertia loads are always present. As the speed increases, increases acceleration. In fact, acceleration is increasing much more rapidly, and varies with the square of the speed of copying above a normal curve trajectory.

Low speed inertia forces are negligible, therefore, it is in this area which should operate systems of copied conventional, that they do not have any kind of dynamic compensation. As the speed increases, dynamic forces dominate quickly this system measurement performance. However, most of the MMC are used in a production environment in which the cycle times are important. This is a great advantage to be taken into account if you want a faster measurements.

An example: the layout shows data measuring a same surface at two different speeds. Illustrates how the dynamic forces accumulate as increases speed, resulting in an increase in errors in the form.

The red line shows the readings at low speed (10 mm/s / 0.4 in./s), which provides a precise calculation of the surface. The blue trace shows the readings at high speed without correcting (150 mm/s / 6 in./s), and shows how the measured size of the hole decreases to accumulate centripetal in cuff forces as the machine moves over the surface. At these speeds, the inertia forces are up 225 times higher than the achieved during a copy at low speed.

The modern MMC can move at high speeds - several hundred mm/s. However, the copied normal can become much more low-speed if you need some precision. To obtain an acceptable accuracy in parts of little tolerance, systems of copied conventional conducted measurements at low speed – generally, less than 20 mm/s (0.8 in./s).

Obviously there is the possibility to copy at a much higher speed, provided that the dynamic errors induced by the deflection of the structure of the machine can be overcome.

Errors with speed graphic shows the increase in errors at a rate of speed, the inertia forces are related to the acceleration, in turn related to the square of velocity.

Emax is the maximum error permitted in a concrete surface measuring. As a general rule, it should be approximately 10% of the tolerance of the surface. To draw this error in the chart we can see the maximum speed, S1, which you must copy the surface.

We need to find how to change the relationship between speed and accuracy, so that it is possible to achieve higher speeds while maintaining accuracy. Renishaw succeeds through Renscan DC, a patented technique for measurement of dynamic compensation.

The factors that determine the profile of acceleration of the machine during the cycle of copied include:

The surface area configuration: size, shape and orientation, that define the route of the movement of the machine.

Speed of copied: defines the speed of target to be achieved during the copy (considering that most of the copied start at zero speed, so the speed is not maintained during the copy).

Performance of the engine and the servo of the machine: how much can accelerate to the speed of copied destination?

For each specific profile of acceleration, there are factors that increase the repetition of dynamic errors (deflection of inertia), and may be offset by Renscan DC:

Location of the 'Macro': the rigidity of the structure of the machine and, consequently, dynamic performance, varies during the whole operating range. The position of the surface to be measured is therefore a significant factor.

Status of the machine: the State of bearings and motor system, which may be the origin of an "inclination" during copying.

Although no one can predict, these dynamic errors are repetitive. This means that it is possible to compensate for them, provided that certain conditions are met.

There are also other factors that create minor non repetitive dynamic errors ('noise' of the system) in a given profile of acceleration, and not are compensated by Renscan DC:

Condition of the surface: the uneven surfaces cause vibrations and certain errors in the measurement.

Noise of the servo / stability: the capacity of the machine to maintain a scheduled speed depends on the parameters of control and electro-mechanical performance of the motor system.

These dynamic errors are more random in nature and, therefore, it is not possible to compensate for them. However, they represent only a fraction of the total number of dynamic errors.

Finally, there are factors that do not affect the dynamic error:

'Micro' position: small variations on the nominal position, the shape and size of the surface in a manufacturing and normal component tolerances.

Temperature: dynamic errors are immune to temperature. However, variations in temperature can have an impact on static accuracy and, consequently, in the performance of the measurement.