Contactless encoder in potato processing

At the end of the 16th century, the potato gradually spread to Europe. Spanish conquerors brought them from the new world to the Iberian Peninsula and from there to the then Spanish Netherlands. In the beginning, the plant was passed around Europe's farms more as a botanical rarity than as an agricultural miracle. In the beginning, it was also difficult to grow the potato with a high yield and a good taste. But once the cultivation started to roll, the tuber secured the population growth of Europe during this time. American historian William McNeill even sees the potato in the Social Research journal as a key factor in the surprising rise of the West.

The current problems in potato production seem relatively small compared to the initial difficulties at that time. Today it is less about life and death than about agricultural efficiency. These large potato processors, such as Schaap Holland B.V. in Biddinghuizen, Netherlands, try to achieve agricultural efficiency by processing their potatoes in a way that is as automated and food-friendly as possible.

Around 300 farmers supply the business, enabling Schaap to deliver around 45,000 tons of potatoes to trade and restaurants every year. The company offers potatoes in two forms of processing: cleaned potatoes with skin or peeled, chilled potatoes for immediate processing. Both product groups are again divided into different potato types, shapes and packaging sizes. Accordingly, the operation in Biddinghuizen is divided into two main areas. A hygienic area for the peeled potatoes and an area in which the unpeeled potatoes are washed, sorted and packed.

Fully coupled process

Most of the potato sorting, washing, and peeling is done automatically. In a continuous processing line, the potatoes are transported from washing to the cooling tunnel. However, the complete coupling of the entire process also presents challenges for the automation engineers of the company Schaap. For example, if a machine stop is caused in the packaging machine at the end of production, the entire line is stopped. To prevent this, Schaap uses buffer belts in front of the packaging scales.

If the packaging material needs to be added there, the entire process no longer stops because the buffer conveyor reduces the speed and absorbs the delay. As of late, the motion of the motor that drives the belt has been detected by a contactless inductive encoder, the QR24 from TURCK. This means that up to 500 kg of potatoes can be buffered in the process. "We use laser sensors to measure the height of the potatoes on the belt so that we know how many potatoes are on them. We need an impulse from the encoder every centimeter to be able to adjust the speed," says Henk van Raalte, Head of Technology and Maintenance at Schaap.

High mechanical stress

The motor shaft rotates slowly. The QR24 has therefore been parameterized to twelve pulses per revolution. The linear movement of the cooling belt only needs to be recorded with one pulse per five centimeters. However, the mechanical challenge is difficult. The previously used incremental encoder with an optical measuring principle had to be mounted with a spring. It was mounted on the housing around the shaft with two small spring plates. "The stability and precision of these encoders was not an issue, but mechanical problems were caused by the previous solution," explains van Raalte. "The spring always moves due to the vibration of the motor, so it no longer worked after two years."

This is no longer a problem with the non-contact TURCK encoder. Spring couplings are not required since there is no mechanical connection between the shaft and the sensor unit. Only the position element is attached directly to the shaft. "We have been looking for an encoder like this for years," says van Raalte. "When I saw the device on the front page of the customer magazine, I knew we needed something like that."

No need for bearings or spring couplings

All QR24 models feature a fully encapsulated sensor and position marker, designed as two independent, completely sealed units that are impervious to shaft vibrations or impacts. Wear-prone ball bearings or seals that could cause machine downtime or lengthy maintenance are not needed. This makes the QR24 series superior to both optical and magnetic encoders.

Van Raalte connects the output signal of the QR24 to a standard input on a Siemens S7 1500 PLC. The rotary encoder is configured using the PACTware parameterization software on the PC. While TURCK's Easy-Teach adapter allows nine preset values to be selected on the system – typically between 360 and 5000 pulses per revolution – PACTware enables free selection from 1 to 5000 pulses per revolution. Schaap chose twelve pulses per revolution because the subsequent conveyor belt is monitored at six pulses per revolution, making conversion easier. The TURCK encoder is mounted under a metal protective cover on the motor. With the QR24 in stainless steel, TURCK now offers an especially robust solution that can be installed in the food industry without additional protective measures. In potato production, Schaap employees spray the entire system once a week with cleaning foam, which is rinsed off at 15-bar pressure after 20 minutes, followed by additional disinfection.

Avoiding encoder wear

Encoders are also used in the area where unpeeled potatoes are processed. Vibrations issues and complex mounting setups often take up a lot of time for Schaap's technicians. For example, on a roller dryer, an optical encoder with 4096 pulses monitors the linear movement of the belt. When it needs to be emptied for batch change, a rake moves across the belt to push all the potatoes down. The encoder here is mounted with wear-prone spring elements and a coupling, meaning it must be regularly replaced or repaired due to mechanical defects.

On another highly vibrating conveyor, an optical incremental encoder currently monitors belt movement. To minimize vibrations, the technicians have installed a shaft on two bearings, a time-consuming solution that will soon be unnecessary when the QR24 is deployed.

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