High-performance metrology gives Lotus top line accuracy

A Lotus Emira being delivered by an AGV from the production line to the LK CMM to undergo stage 1 inspection
A Lotus Emira being delivered by an AGV from the production line to the LK CMM to undergo stage 1 inspection

The acquisition of LK Metrology’s latest CMM technology at renowned sportscar maker Lotus has enabled it to make the new Elmira the most accurate model it has ever made. PES reports.

Since the first Lotus car was built in 1948, the company has been a beacon in the automotive industry for the design and manufacture of high-performance cars born out of legendary success on the racetrack.

The latest sports car, the Emira, is by far the most accurate model ever built by Lotus thanks to an upgrade in quality control at the company's sports cars factory in Hethel, Norfolk. It includes investment in a 5-axis HC-90TR twin-arm CMM manufactured in Castle Donington by LK Metrology.

The automated, multi-sensor inspection cell, part of a multi-million pound investment delivers absolute, 100% non-contact dimensional inspection at production-line speed, enabling real-time quality control.

The CMM installed in Hethel is set 1m into the floor on a special foundation. It means that a car for inspection carried on an automated guided vehicle (AGV) is able to access the area without the need for any lifting equipment. The machine has a measuring volume of nominally 6.3 x 1.6 x 2.5m, easily large enough to contain an entire car body. Advanced, triple-laser cross scanners are deployed on both arms to measure features automatically on both sides of the Emira simultaneously.

The LK HC-90 is said to be one of the world's most accurate range of horizontal-arm CMMs, able to measure to a volumetric accuracy of 10.0μm + L/200 and with 6μm repeatability. The machine combines the high performance of a bridge-type CMM with the flexibility of the horizontal-arm configuration widely used in car plants.

The Emira chassis, which remains on the AGV, is lowered into position and aligned ready for inspection by laser scanning
The Emira chassis, which remains on the AGV, is lowered into position and aligned ready for inspection by laser scanning

The CMM completes automated measuring cycles so quickly that it is able to keep up with the quick TAKT time of the Emira production line. The fast throughput enables every car to be inspected, making this Lotus model the first whose manufacture is supported by such comprehensive quality control.

In two visits of the car at different stages of its build to the LK HC-90TR, a total of 130 measurements are made in scanning routines taking about 12 minutes and eight minutes respectively. The rapid inspection cycles leave time for an AGV that has delivered a chassis to the CMM to load it and subsequently return it to the line.

Multiple measuring

Before metrology starts, there is also 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. For speed and accuracy, the touch-probed sensors are exchanged automatically by the arms from a rack. The same sensor is used for probing some critical internal dimensions of the car, while the laser scanner performs high-speed scanning of the freeform surface areas.

Compared to traditional quality control practices underpinning the manufacture of these largely hand-built sports cars made from high tech, lightweight composite panels, the benefits of rapid point cloud acquisition and digitisation in the LK cell are enormous.

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 within the TAKT time of the Emira production line
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 within the TAKT time of the Emira production line

Formerly, a sample chassis would have undergone measurement line-side on a manual CMM having an arm equipped with a touch probe. Fewer features would have been checked in up to five hours on typically one in every couple of dozen chassis, which had to be taken out of production and reinserted afterwards.

This meant there was potential for error not only through human intervention but also due to a need to reposition the car during inspection to compensate for the limited axis travels of the old CMM. More seriously, any of the unchecked cars between the samples might be out of tolerance, leading to time-consuming, expensive rework.

Tom Mackrill, dimensional quality manager at the factory explains: “The Lotus Emira is our last new car to have an internal combustion engine. We are aiming the product at the premium segment of the global market, so a very stable production process is required to ensure build quality.

“Production rate of this model in 2022 was substantially higher than 2021, when we were ending production of the Elise, Exige and Evora, and it is increasing strongly, so we need a quality control 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 preferable in the productionisation of our new car, with a twin-arm configuration and laser cross scanners providing the required speed of metrology.”

He added that the reliability and repeatability are driving the dimensional quality of the Emira by feeding back data to the production line to validate and control the process, ensuring each vehicle is built right first time. Such feedback is particularly important to the composite panel framing facility due to the potential for variability in the adhesive bonds.

Infinite angles

The Renishaw PHS-2 2-axis CNC wrists that carry the laser scanners are able to rotate continuously and have infinite positioning. Each scanner can therefore be angled very precisely throughout an inspection cycle, allowing optimal positioning both inside and outside the chassis for acquisition of freeform and geometrical data.

Inside the vehicle, the PHS-2 is able to rotate under program control to look outwards at the interior of the bodywork to take measurements. 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.

The finished Lotus Emira
The finished Lotus Emira

The Nikon XC65DX-LS cross scanner with three laser lines captures in a single orientation and scan pass three times the amount of data that can be acquired by a single-line laser scanner. It enables short inspection cycles on the LK HC-90TR to support 100% quality control of the car.

The built-in technology means that the sensor is unaffected by the colour or reflectivity of what is being measured. It also provides automatic, real-time adjustment of the settings not only between successive laser stripes, but also for individual points along each line.

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.

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, geometric dimensioning and tolerancing (GD&T) and profile analysis, backed by rapid feedback of results to the production line.

Tom Mackrill, dimensional quality manager at Lotus Cars
Tom Mackrill, dimensional quality manager at Lotus Cars

Turnkey solution

The metrology cell was installed and commissioned by LK in the Hethel plant in January 2021, ahead of the start of Emira production. The automated equipment has been instrumental in raising production efficiency and lowering the labour cost content of the cars by ensuring that dimensional inspection controls the manufacturing process, rather than merely qualifying the compliance of the measured data.

It is driving production strategy for the continuous improvement of individual parts and the whole process with the goal of zero-defect manufacture, this being the whole purpose of Industry 4.0 with which the LK hardware and software are compatible.

Mr Mackrill concludes: “Once we had decided that the gold standard of static coordinate measurement was the correct metrology route for us to take, we reviewed a number of offerings from different potential suppliers.

“LK stood out as being the best in terms of high-end technology. The ceramic construction of the machine provides excellent stability combined with light weight for accuracy and speed. 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. LK has been very supportive to the extent that we regard the relationship between our respective engineering teams as a genuine partnership.”

The vehicle manufacturer has also invested in four further LK CMMs, all smaller bridge-type machines for its nearby LAS (Lotus Advanced Structures) division. Used for inspection of chassis components, there are three LK Altera M machines with measuring envelopes of nominally 3 x 2 x 1.5m, 2 x 1.5 x 1.2m, and 1.5 x 1.0 x 0.8m respectively and a larger, higher accuracy Ultima CMM with a 4 x 2 x 2m inspection volume.

LK Metrology
www.LKmetrology.com

Lotus Cars
www.lotuscars.com

Company

LK Metrology

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