Basic requirements of the operators of general cargo distribution systems are high availability, maintenance-free and energy efficiency. Therefore, these applications offer linear motors as an alternative to conventional drives. The synchronous linear motor SLC from SEW-Eurodrive is not only suitable for long travel distances, but also helps to reduce the energy requirements of a sorter - often above 60%. This drive solution is now available as standard for sorter applications.
In intralogistics, distribution conveyors - also called sorters - play an important role in the transport of goods. They are used in a variety of industries: in postal and parcel delivery centers, in luggage conveyor systems for baggage and in mail-order sales in central distribution centers. The sorters can be classified according to the material handling principle, the assignment type and their structure. This is how a distinction is made between free space usage or single occupancy. In the latter case, the conveying means consist of coupled carriages which are guided as a continuous chain in a rail system. The goods receptacle is connected to a carriage, the Ausschleuse mechanism often with the delivery point. During discharge, a distinction is made between the intralogistic sorting and distribution systems for crossbelt sorters and tilt tray sorters. The tilting cups are mounted on the individual cars. Structurally, the sorter chain often runs in a circle.
Conventional drive principles
Some conventional propulsion systems include a transport chain, similar to a bicycle chain that connects the carriages together. It is driven by a geared motor with sprocket and pulled around in a circle. The sprocket does not have the shape of an ideal circle, but is a polygon. Therefore occur through the so-called polygonal effect of the sprocket periodic force peaks. Thus, the chain causes a noise load for people who work on the sorter.
In another drive principle, each car carries on the center line of the underside of a sword, which is compressed by two friction wheels and shot forward. These friction wheels have a high level of abrasion and slippage, especially in the case of unbalanced load, at each sword transition and during starting and stopping.
Because these two principles of action a rotary motion is converted into a linear movement, classic solutions with chain or friction wheel are subject to mechanical wear, the maintenance effort. The principle of transmission reduces the overall availability of the system and generates critical noises on the sorter.
Advantages of linear technology
An entirely different operating principle is the linear direct drive. Asynchronous linear motors for sorters have been around for some time, including SEW-Eurodrive. The principle of the linear motor can be easily derived from a conventional rotary asynchronous motor: one cuts open the motor and unrolls the stator with stator winding on a flat surface. Also, the short cage of the rotor is unrolled and flattened to a sword. Compared to the rotary technology, the asynchronous linear motor represents a significant advance. It is contact-free, has no mechanical wear and is quiet. The linear asynchronous motor is a double comb motor with an aluminum blade, which passes through under the car as in the friction wheel drive in the middle and applies the driving force. It corresponds to the cage rotor of the (rotary) asynchronous motor. To increase the continuous power of the engine, these engines are usually forced air. The load balancing is carried out automatically by the soft motor characteristic.
SEW-Eurodrive manufactures the asynchronous linear motor DLD as a standard product for the sorter market. The linear motor enables a flexible system design - the restrictions of the rotary drives for the overall construction are eliminated. There are cost savings on the one hand in construction and design, on the other hand in operation and maintenance. Linear group drives are easy to implement. An n-fold number of drives on one inverter gives you an n-fold driving force. A sorter can be a few 100 meters long. With 700 m length, for example, a sorter can be driven by twelve motors that are connected together to one inverter. Because of the asynchronous principle, no encoder information is necessary.
Synchronous linear motors
On the other hand, recent developments in sorter technology use synchronous linear motors developed by the Bruchsal drive automation system. These sorters exceed the previous solutions by a much higher efficiency. Replacing the aluminum blade with permanent magnets turns the asynchronous principle into a synchronous one - just like with rotary synchronous motors. The magnets form the movable secondary part. As a material ferrite magnets are used. They are available in large quantities, cost-effective and are not subject to export limits in the producing countries. In addition, ferrite magnets are absolutely rust-free even at 100% humidity and therefore do not need to be potted. The magnets are arranged alternately in opposite polarity (north - south - north - south ...) on a carrier plate. This creates a long and narrow runner. The customer provides this construction with a cover and attaches it to the underbody of the car. The generated force of the motor is proportional to the area covered by the magnets.
For sorter applications, it's not all about dynamics. Rather, these drives focus on energy efficiency. Finally, a sorter usually operates in one or two layers in S1 mode. A major advantage of the synchronous linear motor SLC is its significantly higher efficiency compared to an asynchronous linear motor. In addition, this engine is typically operated without a fan.
Wanderfeld moves magnets
Instead of the double-chamber arrangement, the synchronous linear motor also uses a stationary primary part which carries the winding. It is connected to a frequency converter that generates the rotating field. This creates a traveling field in the stator of the linear motor that moves the magnets. The inverter used is the Movidrive series from SEW-Eurodrive. The sorter manufacturers use them in small, decentralized control cabinets. Two to four primary parts are connected in parallel to one inverter. The commutation information comes from a sensor that also sits in this drive station. Approximately all 70 m is provided such a station. The total length is typically several 100 m, but can also reach 2 km.
The linear motor SLC was developed for an air gap with 4,5 mm nominal distance. This dimension is ideal for the steel tolerances occurring. After all, the sorters run all year round - with solar radiation in summer as well as open hall doors in winter. The machines are usually not tempered. Nevertheless, no components may grind on each other.
Reliable encoder information
In order for the current in the linear motor to be properly commutated, the converter must know the position of the magnets in synchronous drive principles. The sensor box NL16 is used for this purpose.Frequency inverter Movidrive evaluates. The special feature is that it also works in the coupling area between the carriages in which no magnets are present. Up to two magnetic gaps can bridge the box and cover it with valid encoder information. This product has been patent pending.
Incidentally, the engine basically also enables new developments in positioning applications. As a technical solution, a crane bridge or a small stacker crane are conceivable. The motor itself is interesting wherever you want to realize a linear direct drive with a relatively low power density and the travel lengths and air gaps for linear motors with rare-earth magnets are unprofitable.
The car is Lothar Berger, Engineering Servo Drive Technology at the SEW Eurodrive in Bruchsal
Logimat, hall 1, booth 940