Thermal Effects in Measurement with CMMs

Thermal Effects in Measurement

Portable Coordinate Measurement Machines enable measurement outside of the CMM room. They are designed to work on the shop floor where they deal with vibration, variable temperatures, temperature gradients and obstructions. This solution deals with the topic of Thermal Effects in Measurement, and focuses on operating procedures for the FARO^® Laser Tracker to optimize accuracy in an environment with changing temperature.

All measurement devices have temperature sensitivity. Fixed CMMs perform best when placed in a thermally stable environment, but are still subject to local temperature variation within the machine. Portable CMMs must deal with a changing outside environment along with any internal temperature variation caused by varying duty cycles.

Temperature variation in the tracker’s working environment affects the scale of the laser wavelength, as well as the scale of the part, the floor, the stand, and the tracker itself. Temperature variation inside the tracker due to ambient temperature changes and tracker duty cycle can result in drift and changes in accuracy. In order to achieve the highest accuracy in an uncontrolled environment, it is important to follow proper measurement procedures to monitor the environment.

Laser Wavelength and Distance Accuracy
The FARO Laser Tracker has remote temperature sensor ports on the Master Control Unit (MCU). The external ports are used to read ambient air temperature in order to scale the laser wavelength of the Helium Neon and Infrared lasers. The temperature is measured every 5 seconds, and applied to the wavelength calculation. Placing the air sensor(s) close to the path of the laser beam maximizes the accuracy of the distance measurements. When more than one air sensor is used, the tracker uses the average reading.

Pointing Accuracy
When portable CMMs are subject to temperature change, they often require new compensation parameters to account for mechanical changes in the system. The FARO Laser Tracker is designed to reduce these affects through the use of fiber optics and a small optical platform. The optics are fixed onto a small platform, and the beam is steered by moving the entire platform under servo motor control. There are no optical elements used for beam steering.

To maximize pointing accuracy in an environment of changing temperature, backsight readings can be taken periodically to verify tracker accuracy. If the backsight results do not meet the measurement application requirements, Pointing CompIT can then be run to restore the tracker to maximum accuracy. Pointing CompIT takes about five minutes to run.

Drift
Drift is a term that refers to movement during a measurement session. This movement is actual physical movement of the part or measurement system due to vibration, physical contact or temperature change. For the purposes of this document, the discussion will focus on temperature change.

Changes in temperature have a direct affect on the size of the part. As the temperature rises, the part will grow, and the tracker will see this growth as movement of the part. Temperature changes will also affect the stand and the tracker itself.

To properly deal with drift, it is important to establish a good set of reference points at the beginning of a measurement session. These points are measured prior to any feature measurements, thus creating a baseline for scale, part position and tracker position. The reference points are then checked periodically to ensure that no drift has occurred. If drift is detected, these points are re-measured to re-transform the tracker back into the part. This process is equally effective for part drift as well as tracker movement due to changes in the stand or changes in the tracker itself.


Internal Temperature Monitoring of the FARO Laser Tracker
The FARO Laser Tracker has internal temperature sensors that continuously read internal temperature at critical locations inside the tracker.  These sensors are used to monitor the tracker's temperature during the initial warm up and for internal compensations of the measurements in real time using techniques patented by FARO.


Keywords:

thermal expansion, thermal, temperature, drift