Richard Pardon
Starting in 1948, the Lotus brand has a long lineage of high performance cars. The Emira is the latest sports car to follow in this line and it’s the first to be subject to Lotus’ new quality control procedures. The upgrade comes thanks to the investment in a large coordinate measuring machine (CMM), which Lotus says has made the Emira the most accurate Lotus car yet. The CMM, manufactured in Castle Donington by LK Metrology, is a 5-axis HC-90TR twin-arm CMM.
Installed at the Lotus plant in Hethel, the CMM was built one metre into the floor on a special foundation. This means that the cars can be carried by an automated guided vehicle (AGV) onto the work surface of the CMM for inspection, without the need for lifting equipment. The machine has a measuring volume of nominally 6.3 x 1.6 x 2.5 m, so the entire car can be measured without the need for repositioning. On both arms, triple-laser cross scanners are deployed to measure features automatically on both sides of the Lotus Emira simultaneously. The LK HC-90TR boasts an accuracy of 10.0 μm + L/200 and with 6 μm repeatability, combining the high performance of a bridge-type CMM with the flexibility of the horizontal-arm configuration widely used in the automotive industry.
The fast TAKT time in the Emira production line requires a CMM that can work not only accurately, but also at high speeds. The Emira is inspected by the LK HC-90TR at two different stages in the production process, using a total of 130 measurements in scanning routines that take 12 minutes and 8 minutes respectively. Due to this speed, there is also enough time to verify the car’s alignment relative to the carline reference point system in the CMM, as well as to calibrate the laser scanners using dedicated calibration spheres.
Moving away from the former method of using a line-side manual CMM with a touch-probe on an arm brings many advantages with it. Using a manual CMM would mean that in a five-hour long check, fewer measurements would have been taken than the new automated system takes in a few minutes. The new system also takes human error out of the equation; the old method would have required the car to be repositioned during the checks because of the limited reach of the arm. Finally, due to its speed, the new system is able to measure every car, rather than just samples.
Tom Mackrill, Dimensional Quality Manager at the factory commented, "The Lotus Emira is our last new car to have an internal combustion engine, as we are going all-electric in future. We are aiming the product at the premium segment of the global market, so a very stable production process is required to ensure high build quality.
"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. 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. 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 productionisation of our new car, with a twin-arm configuration and laser cross scanners providing the required speed of metrology."
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Tom Mackrill, Dimensional Quality Manager, Lotus Cars
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The Emira chassis, which remains on the AGV, is lowered into position and aligned ready for inspection by laser scanning
Infinite positioning
Thanks to Renishaw’s PHS-2 two-axis CNC wrists that carry the laser scanners, the scanners can rotate fully and have infinite positioning, whereas conventional probe heads only adjust in 7.5 degree steps. This means that each scanner can easily be positioned to the optimal alignment for working both inside and outside the chassis of the car. When inside the car, the mobility of the PHS-2 is such that it can rotate to look outwards at the interior of the car to take measurements.
The scanners mounted on the arms are the Nikon XC65DX-LS, which has three laser lines capturing in a single orientation three times the amount of data that a single-line laser can, which is one of the major contributing factors to reducing the length of the inspection cycles. The scanner is unaffected by colour or reflectivity of what is being measured and provides automatic, real-time adjustment of the settings between successive laser stripes as well as for individual points along each line. The LS-suffix signifies a longer stand-off distance, meaning the scanner is accurate up to 170 mm away, increasing the machine’s overall ability to measure difficult-to-reach spots.
Measuring twice
The two-stage inspection routine is described as such:
“AGVs deliver every Emira to the CMM from the production line, both early on from the end of the framing line (stage 1) and when the car is fully assembled (stage 2). It is deposited without manual intervention in carline and a bar code reader, known as a scripting system, verifies the vehicle's identification number, colour, whether it is right or left hand drive and if a stage 1 or stage 2 metrology program should be run. The car in its stage 1 condition is not conventional body-in-white, as the body panels are already painted and the steering column and wiring looms are in place.
After automatic calibration of the laser scanners, they 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. Control of gap-and-flush is especially important, as the spacing between panels impacts directly on the perceived quality of the car on the forecourt.
Programs are created largely automatically from CAD models of the Emira in LK CAMIO software. It ensures the laser scanners move along paths such that the surfaces being measured are always within their field of view. The low-noise point clouds captured are filtered to produce smooth, highly detailed meshes that can be aligned to enable comparison of the measured data with the CAD model. Inspection tools are included for intelligent feature extraction, GD&T tolerancing and profile analysis, backed by rapid feedback of results to the production line and comprehensive reporting.
Lotus Cars elected to implement this two-stage, offline, in-process metrology solution as it is most expedient for the company's needs. However, in other scenarios one HC-90TR or more could be positioned within the production flow line itself, checking dimensions before the vehicle passes to the next build operation. Other offline applications include quality audits and verification checks.”
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A Lotus Emira being delivered by an automated guided vehicle from the production line to the LK CMM to undergo stage 1 inspection
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Close-up of the Renishaw PHS-2 wrist (left) that allows infinite positioning of the Nikon XC65DX-LS cross scanner, ensuring that the required inspection data is acquired well within the TAKT time of the Emira production line
Installation and satisfaction
The metrology cell was installed and commissioned by LK in the Hethel plant in January 2021, ahead of the start of Emira production. It’s a major step for Lotus into Industry 4.0, greatly increasing automation and aiming for zero-defect manufacturing.
Mr Mackrill noted that the ceramic construction of the machine, providing excellent stability combined with light weight for accuracy and speed, was one of the major reasons that LK stood out to them. LK supplied a completely configured, multi-sensor, turnkey solution which meant that only the fixtures supporting the car body were provided by Lotus.
The partnership has been so fruitful, in fact, that Lotus has ordered four further CMMs from LK. All four are of the smaller bridge-type machines and are destined for the LAS (Lotus Advanced Structures) division.