Cutting Edge Metrology

Lotus has released its most accurate car ever, thanks to cutting-edge metrology. We take a look at the CMM enabling high precision and real-time quality control at production-line speed

Since the first Lotus car was built in 1948, the company has amassed a reputation within the automotive industry for the design and manufacture of high-performance cars born out of success on the racetrack. The manufacturer has now unveiled its most accurate model ever, the Emira, made possible by an upgrade in quality control at its sports car factory in Norfolk, UK.

At the beginning of 2021, Lotus installed a 5-axis HC-90TR twin-arm coordinate measuring machine (CMM) from LK Metrology to enable fully non-contact dimensional inspection at production-line speed. The goal? Achieving real-time quality control of the Emira production line without comprising on precision.

“The Lotus Emira is our last new car to have an internal combustion engine, as we are going all-electric in the future,” says Tom Mackrill, Dimensional Quality Manager at Lotus’ Norfolk factory. “We are aiming the product at the premium sector of the global market, so a very stable production process is required to ensure high build quality.”

Rapid Automated Measuring

LK Metrology’s automated, multi-sensor inspection cell is designed to deliver fully non-contact dimensional inspection at production-line speed to enable real-time quality control. At Lotus’ factory, the CMM is set one metre into the floor on a special foundation that allows a car for the inspection carried on an automated guided vehicle (AGV)  to access the area without the need for lifting equipment.

Featuring a measuring volume of 6.3x1.6x2.5m, the machine is easily large enough to contain an entire car body, while advanced, triple-laser cross scanners are deployed on both arms to measure features automatically on both sides of the vehicle simultaneously. The LK HC-90 is reportedly the world’s most accurate range of horizontal-arm CMMs, able to measure to a volumetric accuracy of 10μm + L/200 and with 6μm repeatability. As a result, the machine combines the high performance of a bridge-type CMM with the flexibility of the horizontal-arm configuration widely used in car plants.

“Production rate of this car model in 2022 is substantially higher than last year, when we were ending production of the Elise, Exige and Evora, and is increasing strongly,” explains Mackrill. “So, we need a QC solution that not only supports high precision but is also fast enough to generate large amounts of data quickly to match the production rate.

“We reviewed other inspection technologies, including those involving various robot-mounted measuring heads,” he continues. “However, we concluded that robots, particularly if they are mobile, would introduce extra potential error in addition to that of the measuring devices they carry and would need a reference measuring system to calibrate and realign them. We, therefore, came to the conclusion that a static CMM was the way to go in the production of our new car, with a twin-arm configuration and laser cross scanners providing the required speed of metrology.”

Infinite Positioning Laser Scanners

The LK HC-90 is equipped with Renishaw PHS-2 two-axis CNC wrists that enable the laser scanners to rotate continuously and have infinite positioning, unlike conventional probe heads that only adjust in 7.5-degree steps. Each scanner can therefore be angled very precisely throughout an inspection cycle to achieve optimal positioning both inside and outside the chassis to rapidly collect freeform and geometrical data.

When inside the vehicle, the PHS-2 can rotate under program control to look outwards at the interior of the body work to take measurements, while the DMIS programs controlling each arm are essentially mirror images of each other, with additional movements inserted separately to address the minor differences unique to each side of the car.

Meanwhile, the CMM’s Nikon XC65DX-LS cross scanner with three laser lines captures in a single orientation and scans three times the amount of data typically acquired by a single-line laser scanner. This enables short inspection cycles and means that the sensor is unaffected by the colour or reflectivity of what is being measured. Additionally, automatic, real-time adjustment of the settings is delivered not only between successive laser stripes but also for individual points along each line.

Two-Stage Inspection

During the quality control process, AGVs deliver each Emira to the CMM from the production line for two stages of testing. The first stage occurs at the end of the framing line, while the second takes place when the car is fully assembled. The CMM is able to automatically verify the vehicle’s identification number, colour, right- or left-hand drive and which stage metrology program should be run.

After automatic calibration, the laser scanners take 130 measurements around the car over the two stages to assess the accuracy of the vehicle. Suspension and engine mounting points are critical areas for inspection, while the control of gap-and-flush is especially important due to the spacing between panels impacting directly on the perceived quality of the car on the forecourt.

Programs are created automatically from CAD models of the Emira in LK’s Camio software to ensure the laser scanners move along paths that mean the surfaces being measured are always within their field of view. The low-noise point clouds captured are then filtered to produce smooth and highly detailed meshes that can be aligned to enable comparison of the measured data with the CAD model. The CMM’s inspection tools then enable intelligent feature extraction, GD&T tolerancing and profile analysis, which are rapidly fed back to the production line.

According to Mackrill, the ability to feedback data to the production line to validate and control the manufacturing process is driving the reliability and repeatability of the Emira’s dimensional quality. As a result, the LK HC-90 is helping to ensure each vehicle is built right the first time around.

Not only has the machine improved the dimensional accuracy of the Emira, but it has also been instrumental in raising production efficiency and lowering the labour cost content of the car. As such, the cell is driving Lotus’ production strategy for the continuous improvement of individual parts and the whole process with the goal of zero-defect manufacture.

“The ceramic construction of the machine provides excellent stability combined with light weight for accuracy and speed,” Mackrill adds. “LK supplied a fully configured, multi-sensor, turnkey solution complete with first-draft measuring programs and ancillaries. Only the fixtures supporting the car body were provided by us.”