Smart motors keep conveyors moving

Motors1.jpg

With Swire Cold Storage's Cannon Hill operation handling over 30,000 boxes of chilled product every day, it is not surprising that the site's vast conveyor system plays a key role in the company's temperature controlled warehousing and distribution operations.

However, as production levels have continued to rise over the years at the Brisbane site, so has the need to improve reliability of the equipment due to an ageing conveyor system and an ever-increasing volume of product.

Colin Carter, Swire Cold Storage's Engineering Manager for Queensland, explained that the problems mostly occurred on the main carton conveyor line, which carries a variety of chilled boxed products, where it splits into five separate distribution lines.

"Because of the high volume of boxes coming down the main conveyor line, the chain drive pushers just couldn't cope at peak times, with the boxes getting caught up and forcing us to stop the whole conveyor line. Any downtime has a major impact on our customers, something we work hard to keep to a minimum," Carter said.

Swire Cold Storage is Australia's largest cold-chain logistics service provider with a network of 17 facilities nationwide.

"The issue had been with us for quite a long time, but only at peak load times did it become critical. "However, it became more of a point of focus as our volumes increased. We had tried a number of things to try to fix the problem, but with no success," Carter explained.

Low cost project

In the end, the company managed to fix the problem for far less money than they initially thought. "In fact it was a very low cost project for such a big improvement to our production efficiency," remarked Carter. 

Shahry Zand, applications engineer with SEW-Eurodrive, explained that the problem was with the pushers, "they just couldn't keep pace with the main conveyor line".

"The main conveyor line travels around one metre per second with the gap between boxes set at just one metre at peak load times. This means the pusher must finish its pushing operation and be ready in its home position for the next push in less than 0.7 seconds.

"The boxes are mainly 0.5 metre x 0.5 metre with varying heights, so basically they have the same footprint." 

The chain drive pushers were driven by an older SEW-Eurodrive Movimot geared motor controlled by a PLC via a MFD DeviceNet module.

"The positioning was being done in the PLC based on a home Proximity switch. However, due to the DeviceNet/PLC delays, the 0.7s total pushing time wasn't quite achievable and the motor wasn't able to stop at the home Prox all the time.

"As a result the next box was crashing into the pusher and the whole main conveyor had to be stopped to clear and home the pusher," he explained.

Swire had tried to modify the PLC program by changing the motor speed, ramp times and delay times, but with no success.

Replace cabling

In a perfect world, the high dynamic nature of the application really called for a servo drive with a high-resolution encoder.

"However that option would work out to be quite expensive, with each pusher costing around $10,000," explained Zand. Plus as the cabling would need to be replaced by shielded cabling and the inverters installed in a control cabinet, the total cost would have been over $100,000. 

"Overall, the servo drive option involved a lot of changes and considerable disruption to the building which the customer didn't want to do. Instead we were able to fix the problem on the high-volume Queensland line, the most problematic, for less than $2200," Zand noted.

As well, SEW-Eurodrive was able to commission the conveyor in just one day, on the weekend, with no disruption to production at all. 

"In order to eliminate the DeviceNet/PLC delays, we proposed that the positioning be done by an intelligent SEW-Eurodrive MQD DeviceNet module coupled close to the Movimot. Based on the required positioning accuracy a simple 24 pulse/rev built-in motor encoder was selected," Zand said.

SEW-Eurodrive's Movimot gearmotors have IP55 enclosure as standard and are also available with IP65 and IP66 enclosures on request.The six-year-old SEW-Eurodrive motor, which was still in a good working condition, was replaced with a SEW-Eurodrive DRE high efficiency motor. 

"We also replaced the original MFD DeviceNet module, which is basically a gateway just for communicating with the PLC, with a MQD DeviceNet module with internal positioning and sequence control (IPOS) capabilities.

"We needed IPOS to directly process the encoder signal and to program it to do the positioning independent of the main PLC. The main PLC would just provide a go command and the rest of the positioning and control of the pusher would be handled by the MQD and Movimot," Zand said.

To achieve the total pushing cycle of less than 0.5s, a speed profile was programmed in IPOS based on the pusher position. The challenge was to prevent damaging the boxes by hitting them at high speed and also be able to stop the pusher at the home prox within the required accuracy. 

"As the required ramp up/down time of 0.15s was really pushing the limits of an induction motor, we decided to add another feature in the IPOS program to make it even more reliable," said Zand. 

"If for any reason the pusher stops after the Home Prox, it is programmed to come back quickly to the Home Prox before the next box crashes into it. 

"Thanks to the new built-in encoder, the pusher hasn't missed the Home Prox even once and it's pushing the boxes quicker and smoother than ever before. 

"At this stage we have only replaced the problem line, the busiest line, mainly to prove that our engineering works," Zand said. 

Standardise motors

Carter was impressed with the improvements to the conveyor lines. "Since putting in the new drive our downtime has decreased considerably," he said.

He explained that the one remaining pusher operates on a very slow moving line, "so we don't need to upgrade that at this point in time. However eventually we probably will, just to standardise the motors on that conveyor line.

"We only put one in at the beginning to see how it worked, but within the first week we could clearly see the problem had been fixed," Carter said.

"Our customer sends the boxed product to us, where we basically sort it in our temperature controlled warehousing and distribution centre, which is set at two degrees Centigrade. The company is presently running two shifts each working day, starting at six in the morning.

"However, the flow of product is not always constant, with the peak in the morning," he noted. "We probably have close to 1800 metres of conveyor lines here.

"The tunnel from the meatworks is over 250 metres long alone, with three conveyors, plus a return pallet conveyor in the tunnel, plus there are all the conveyors around the site. It's quite an impressive operation." We have over 150 of their drives on site and over 200 SEW-Eurodrive motors and gearboxes," Carter said.

Proximity switches

The relatively new built-in encoder option with up to 24 pulses per motor revolution is a suitable solution for simple positioning and sequence control applications.

"For example it can replace proximity switches on older conveyor lines to improve speed, accuracy and reliability without spending big dollars," said Zand.

"Another advantage of these built-in encoders is their ability to be retrofitted to existing DR Series SEW-Eurodrive motors, without adding any length to the motor.  What many companies don't realise is the high number of different products and options we have.

"Even in the decentralised range of Variable Frequency Drives (VSD), which includes Movimot, we have a lot of options, which makes them very flexible," Zand added.

For this application, Swire did not want to make major changes and they wanted to keep the costs down. Zand and his team were able to implement a solution with minimal cost and exactly the accuracy required.

SEW-Eurodrive's Movimot gearmotor, with an integrated frequency inverter, is a combination of gearmotor and frequency inverter in the 0.37kW to 4.0kW power range.

"Despite the integrated frequency inverter, Movimot requires only slightly more installation space than standard gearmotors and can be supplied in all standard versions and mounting positions, with and without a brake, for three phase supply voltages of 380 Volts to 500 Volts and 200 Volts to 240 Volts," concluded Zand.

SEW-Eurodrive's Movimot gearmotors have IP55 enclosure as standard and are also available with IP65 and IP66 enclosures on request.