Vantage S6: Understanding Measurement Accuracy

Laser Alignment October 23, 2023

VANTAGE S6: Understanding Measurement Accuracy

For a successful measuring session, it’s crucial to assess the uncertainty linked to it by carefully gauging the influence of errors stemming from various identifiable sources. Nevertheless, some environmental factors, like excessive vibrations, mounting stability, and temperature-related effects such as gradients, air turbulence, or varying air temperatures along the laser beam’s path, prove challenging to precisely quantify. To adhere to sound metrology practices, it becomes imperative to minimize or eliminate errors from all sources.

Neglecting the control of environmental errors can lead to a substantial decline in measurement accuracy, to the extent that the entire measuring session might need to be discarded. To avoid such setbacks, it is advisable to conduct measurements in environments where environmental factors are rigorously controlled and maintained at a stable level whenever possible.

Effects of Atmospheric Conditions

Atmospheric conditions play a pivotal role in affecting Vantage measurements in various ways. Factors such as temperature gradients, air turbulence, or the presence of air pockets with varying temperatures along the laser beam’s path can cause deviations in both angular and radial distance measurements, resulting in reduced accuracy.

These conditions are inherently dynamic, meaning that the loss of accuracy can fluctuate in magnitude and direction. This variability not only compromises accuracy but also leads to poor repeatability in measurements. While increasing the number of samples per reading can enhance repeatability in such circumstances, it does not necessarily boost the accuracy of each individual measurement.

To mitigate these errors, it’s advisable to avoid measuring near sources of temperature fluctuations, such as heating and air conditioning ducts or frequently opening doors. It’s important to note that high airflow in an environment with uniform temperatures generally does not pose an issue; it’s the presence of differing air masses with distinct temperatures that causes the laser beam to deviate. One effective strategy for managing non-uniform temperature effects is to utilize a fan to ensure even air mixing, eliminating pockets of varying air temperatures within the measurement area.

The accuracy of radial distance measurements, a critical aspect of the Vantage’s ADM system, depends on knowing the air’s index of refraction. This is because these systems must convert light wave measurements into distance, and the speed of light is influenced by changing environmental conditions, including temperature, barometric pressure, and relative humidity. Accurate atmospheric condition data is essential for correctly calculating the index of refraction.

To address this, the Vantage system is equipped with weather sensors that measure the temperature, pressure, and humidity of the surrounding air every five seconds. Even small changes in these variables can result in a noticeable impact, with a 1 part per million change in the index of refraction occurring for a 3 mmHg change in pressure, a 1°C shift in temperature, or a 40% change in relative humidity at 40°C. Given the significant influence of air temperature on the index of refraction, it is crucial to place the Vantage’s external air temperature sensor in an environment matching the temperature of the laser beam’s path, avoiding locations like work carts, heating or air conditioning ducts, or any other heat source.

ENVIRONMENTAL FACTORS

In the realm of measurement precision, environmental factors like excessive vibration, the stability of your mounting setup, and temperature can play a significant role in influencing accuracy and repeatability. It’s crucial to minimize the impact of these external elements whenever feasible.

Avoid the movement of heavy objects near the part you’re measuring, both before and during a measurement session. This precautionary step is essential as the displacement of these objects can cause the workshop floor to shift or vibrate, potentially disrupting your measurement sessions. The degree of disruption hinges on the weight of the object in question and the stability of the workshop floor’s foundation.

Maintain consistency in how you support the part during measurements, aligning it with its intended function. This practice helps prevent distortions in the part that could result from differential loading when it’s put to use.

As a valuable recommendation, FARO suggests taking redundant readings for each measurement. This approach not only serves as a safety net for detecting significant errors or blunders but also helps mitigate any lingering environmental effects that couldn’t be entirely eliminated. Additionally, it provides a more robust statistical sampling. For instance, when measuring a planar surface, consider taking more than three readings to enhance the accuracy and reliability of your measurements.

Targets & Tooling

It’s essential to maintain the precision of your measurement setup by conducting regular inspections. Check the target nest and the SMR for any metal filings or debris that might obstruct the target’s proper placement within the nest. Similarly, closely examine your tooling for signs of wear, dings, or dents that could impede its contact with the part or the SMR at their intended positions.

For optimal outcomes, it’s imperative to integrate all tooling and SMRs into your gage calibration system and verify their adherence to the correct dimensional values. Even minuscule damage to your tooling can significantly surpass the accuracy specifications of the Vantage system, potentially yielding inaccurate measurement results.

One common source of measurement inaccuracies stems from dimensional deviations in target offsets. Before commencing measurements, take a moment to review the Probe settings within your measurement software. Ensure that you’ve selected any additional tooling necessary to guarantee the correct offset is applied to your measurements. This meticulous attention to detail can make a substantial difference in the precision of your measurement session.

Physical Changes in the Part or Stand

It’s crucial to regularly assess the stability of both the instrument stand and the part during your measuring sessions, as any alterations to their positions can lead to a decline in measurement accuracy. Whenever feasible, aim to position the instrument stand on the same section of flooring as the part. Avoid spanning the stand’s legs across two or more flooring sections, as this can compromise stability.

An often underestimated factor affecting measurement precision is the impact of temperature variations on the instrument stand. If the stand was stored in an environment with a different ambient temperature than the one where you’re currently conducting measurements, it may undergo movement as it adapts to its new environmental conditions.

Temperature fluctuations in the measuring environment can also undermine accuracy by inducing thermal expansion or contraction in the part. The extent of this effect is contingent on factors such as the material and size of the part, the magnitude of temperature change, and the rate at which the temperature changes. To monitor this, regularly check reference nests placed on the part. Utilize tools like FARO Tracker Utilities and most other inspection software, which offer methods to re-measure reference nests and realign them while applying a scaling adjustment to compensate for uniform thermal expansion or contraction in subsequent measurements. In certain scenarios, these environmental shifts may even impact the Vantage’s angular accuracy, necessitating an Angular Accuracy check before realigning with the part.

Whenever possible, take precautions to shield both the Vantage system and the part from external heat sources. Radiant energy from the sun, intense lighting, or space heaters during measurements can introduce non-uniform expansion in the measuring equipment or the part, ultimately undermining measurement accuracy.

Angular Accuracy Checks

The Vantage system records azimuth and zenith angles, as well as the distance to the target for each measurement. To refine these readings, a kinematic model comes into play, featuring parameters that account for the laser beams’ four degrees of freedom (two rotational and two translational), as well as two parameters for the gimbal, specifically axis offset and axis non-squareness.

To ensure the system’s accuracy, it’s advisable to conduct Angular Accuracy Checks. These checks involve comparing a point reading taken in frontsight mode with one taken in backsight mode. The resulting deviation reveals twice the worst-case error for a point measured at the same range and position as the backsight reading.

Despite the kinematic model’s effectiveness in reducing measurement errors, it’s important to note that there are several factors it doesn’t address. These include errors stemming from target quality, atmospheric influences, and thermal expansion, all of which can have an impact on measurement accuracy.

Positioning of the Vantage

In all Vantage measurements, you’ll find two angular measurements and one radial distance measurement. Both the angular measurement system and the radial measurement system come with specified maximum error values. Additionally, there’s another potential source of error known as the R0 parameter, which signifies a radial offset. This reflects the error in the known distance from the Vantage’s origin to the SMR while it’s in the Tracker Mounted Reset (TMR), also known as the Home position. You can find the Maximum Permissible Error (MPE) for these elements and equations for calculating the Maximum Permissible Error for the distance between two points in the Product Specifications.

It’s important to acknowledge that all Vantage measurements encompass these sources of error. However, the data in the Product Specifications indicates that the accuracy of measuring the distance between two points can vary depending on the Vantage’s position. For instance, when a Vantage is tasked with measuring the length of a scale bar aligned directly with the laser beam, it tends to yield more accurate results than if the same scale bar is positioned horizontally to the Vantage. Several factors contribute to this phenomenon:

The maximum permissible error of the Vantage’s radial distance measurement system is lower than that of the angular measurement system.
The further you move from the Vantage, both the maximum permissible error of its angular measurement system and radial distance measurement system increase.
The R0 error contribution when measuring the distance between two points varies between different Vantage positions.

While each measurement session and geometry is distinct, you can sometimes enhance the overall accuracy of your measurements by minimizing angular movement in comparison to the dimension you’re interested in measuring. For instance, if the straightness of a rail is more critical than its height or length, positioning the Vantage alongside the rail, so the radial measurement system primarily gauges straightness, generally results in more accurate measurements. Conversely, if the rail’s length takes precedence over its straightness, placing the Vantage at the rail’s end to predominantly measure its length would typically yield more precise measurements. Of course, this assumes that placing the Vantage in these positions doesn’t introduce other sources of error like floor vibrations or air turbulence due to temperature gradients, and so on.

Recommendations for Optimal Results

Measurement equipment often faces challenges linked to environmental conditions, target elements, target tooling, movements in the setup, and other factors. Nevertheless, there are specific guidelines tailored to the Vantage system that, when adhered to, enhance measurement accuracy.

For optimal results, it’s advisable to conduct measurements in an environment characterized by minimal temperature fluctuations, temperature gradients, air disturbances, and floor vibrations. Take proactive measures to minimize these environmental factors before commencing your measurements.
Ensure that the Vantage system fully completes its Thermal Stabilization during the Startup Checks. Skipping this step can lead to a reduction in the Vantage’s accuracy. It’s a crucial phase not to be overlooked for optimal performance.

Before embarking on your measurements, it’s crucial to ensure the Vantage’s accuracy by initiating the Quick Compensation routine. If the Angular Accuracy Checks fail and CompIT recommends it, also perform the Angular Accuracy Checks and Pointing Compensation routines.

While Quick Compensation offers a swift compensation process and is suitable for cases where the utmost accuracy isn’t necessary, the Pointing Compensation routine delivers superior results, reducing backsight errors and enhancing Angular Accuracy.
When aiming for the highest accuracy, particularly over extended distances, it becomes essential to compute the most precise kinematic parameters. To achieve top-tier accuracy in your measurements, especially for longer-distance applications, it’s imperative to have a well-warmed Vantage system and execute the Pointing Compensation routine.
Regularly assess potential shifts between the part and the Vantage by utilizing common reference points attached to the part. If significant displacement is observed or if there has been a notable temperature change of approximately 2.8 degrees Celsius (5 degrees Fahrenheit), it’s advisable to employ the Move Device command within FARO CAM2 2020 or the equivalent in your chosen software to realign the system with the part in case of any movement. This check remains essential, even in environments where temperature variations and vibrations are kept to a minimum.
For exceptionally precise measurements, it’s important to periodically perform a reset of the SMR to the Home position. This procedure, also known as Tracker Mounted Reset (TMR), restores the Absolute Distance Measurement (ADM) distance to the known value, which corresponds to the center of the SMR in the Home position. This action helps eliminate very subtle variations that can accumulate over time or due to temperature fluctuations in radial measurements. While these minute changes typically do not affect most applications, they can have an impact on measurements that require the utmost precision.
As a default, individual measurements use a sampling rate of 1000 samples per reading, resulting in a total measurement time of 1 second. However, for scanning or dynamic measurement methods, you can enhance efficiency by reducing the total number of samples per reading. By adjusting this setting, you can significantly shorten the measurement time. For instance, when scanning a curved surface, lowering the samples per reading to 5 results in individual measurements taking just 5 milliseconds. This setting allows you to obtain a series of readings that accurately trace the path of the SMR over the curve. You can modify this configuration using the Device Center command in your measurement software.
Avoid turning off the Vantage’s motors during breaks or shift changes, as powering them off for extended periods may lead to a slight deviation in the Vantage’s internal operating temperature away from its stabilized state. This variation can negatively impact measurement accuracy in the readings taken immediately after restarting. Keeping the motors running will prevent this issue without causing harm to the Vantage.
Refrain from covering the Vantage while it is powered on. Instead, use the Vantage’s dust cover when it is turned off. Covering the system while it’s operational prevents the Vantage from adequately adjusting to the current ambient temperature, potentially affecting measurement accuracy in subsequent readings. If your environment is particularly dusty, and you’re concerned about dust settling on the embedded targets, you can move the Vantage to the left or right side to rotate the Azimuth axis and safeguard these targets.