Whitepaper: Modern CMM Design Concepts - Repeatable Accuracy in Changing Conditions

Global CMMs use both static geometric and thermal error compensation techniques in order to achieve high accuracy and environmental flexibility. The method for compensation of static geometric errors consists of four main steps:  

• Measurement of errors on each individual Global CMM (data collection)
• Calculation of correction values and storage in a correction table (the error map)
• Application of error correction values by the controller software during CMM operation
• Verification of CMM performance

The Global error map is based on a general kinematic model of three nominally perpendicular axes. The purpose of this model is to calculate the combined effect of all the geometric errors of all axes on the position of the probe tip. This combined effect is the error correction that is added to the scale readings.

The corrections for granite bending and scale expansion and contraction described above are examples of thermal error compensation used in the Global design. Recall that thermal errors are changes in machine geometry caused by changes in the thermal environment. The effects of thermal errors on the location of the probe tip are identical to the effect of static geometric errors. The main difference is their root cause. Static geometric errors are a property of the completely assembled machine only. In principle they do not change over time. Therefore they can be captured at one point in time and stored in a data file.

In contrast, thermal errors are a function of both the thermal environment and the machine's thermal response. These errors change constantly unless the machine is located in a tightly temperature controlled environment. In order to calculate the effects of the thermal errors on the location of the probe tip we need a temperature model that characterizes the machine thermal behavior as well as input values that characterize the thermal environment. The input values are machine surface temperatures measured by temperature sensors at various locations on the machine.

Although some models use more sensors, most Global models use eleven temperature sensors placed throughout the structure: two sensors per axis to compensate for linear expansion and contraction of the scales, four sensors on the granite (two on the top and two on the bottom) to compensate for granite bending, and one on the part being measured.

In summary, the design of the modern CMM includes tradeoffs involving accuracy, throughput, and environmental flexibility to name a few of the most important parameters. By making intelligent design decisions involving the selection and application of the appropriate materials and taking full advantage of software error compensation, today's CMM designer can balance CMM design parameters to provide real-world metrology solutions.