# An Action Classification Method for Forklift Monitoring in Industry 4.0 Scenarios
## 3 Materials and Methods
### The UHF-RFID Smart Gate  
Within the I-READ 4.0 system, the goal of the UHF-RFID Smart Gate is to monitor the crossing of goods at a point of interest within the warehouse. The gate must be able to completely identify the loaded pallets carried out by the forklift, to identify the forklift, and to understand its crossing direction. In fact, most gates can be crossed in both directions,  
Figure 4: RFID label printer at the end of the production line. Figure 3: (**a**) Column composed by two tagged pallets and (**b**) sketch of the tagged label applied on the pallet (the tag is on the label rear side). and it is important to correctly determine if a product enters or leaves a certain warehouse area. That is, the UHF-RFID Smart Gate must implement an action classification method to understand whether the forklift is entering (\(IN\)) or leaving (\(OUT\)) a certain zone. The UHF-RFID Smart Gate is composed of the following hardware:  
* An Impini Speedway Revolution R420 UHF-RFID reader;
* A circularly-polarized (CP) CAEN WANTENNAX019 antenna;
* A linearly-polarized (LP) CAEN WANTENNAX007 antenna;
* Two coaxial cables;
* An ethernet cable to connect the reader to the Event Server;
* A power supply for the reader. The circularly-polarized (CP) antenna CAEN WANTENNAX019 ([[https://www.caenrfid.com/it/products/wantenax019/](https://www.caenrfid.com/it/products/wantenax019/)]([https://www.caenrfid.com/it/products/wantenax019/](https://www.caenrfid.com/it/products/wantenax019/)), accessed on on 26 May 2021) was installed in the upper part of the gate, at a height of about 4.5 m. It has a gain of 8.5 dBc and a half power beam width of 65\({}^{\circ}\) on both planes (\(HPBW_{H}=HPBW_{V}=65^{\circ}\)). It was installed with a tilt angle of about 30\({}^{\circ}\) with respect to the horizontal plane to create an asymmetrical radiation-pattern footprint with respect to the gate. Thanks to this particular configuration characterized by an asymmetrical antenna deployment, the forklift crossing direction can be determined by using only a single antenna, as described later. Such CP antenna is mainly used to identify the forklift tags and to perform the action classification method. With the intention of increasing the reliability of the gate when detecting all the carried pallets, a second antenna was installed at the gate side. Since the RFID labels on the pallets are always applied at the same position and parallel to the ground, a linearly-polarized (LP) antenna was chosen to maximize the power radiated to the tag. The chosen model is the CAEN WANTENNAX007 ([[https://pdf.directhdustry.com/pdf/caen-rfid/wantennax007](https://pdf.directhdustry.com/pdf/caen-rfid/wantennax007)]([https://pdf.directhdustry.com/pdf/caen-rfid/wantennax007](https://pdf.directhdustry.com/pdf/caen-rfid/wantennax007)) /113435-366469.html, accessed on on 26 May 2021) with gain equal to \(8.0\pm 0.5\) dBi, and half power beam width equal to 65\({}^{\circ}\) on the horizontal plane (\(HPBW_{H}=65^{\circ}\)) and 68\({}^{\circ}\) on the vertical plane (\(HPBW_{V}=68^{\circ}\)). The antenna was fixed to the wall at a height of about 3 m from the ground and tilted to about 45\({}^{\circ}\) with respect to the horizontal plane. In Figure 5, two of the UHF-RFID Smart Gates installed at the warehouse entrance are shown. It must be highlighted that the gate infrastructure does not include additional invasive metallic structures as typical for tunnel gates [16]. The forklifts are equipped with two OMNI-ID EXO 2000 on-metal RFID tags ([[https://omni-id.com/datasheet/1373](https://omni-id.com/datasheet/1373)]([https://omni-id.com/datasheet/1373](https://omni-id.com/datasheet/1373)), accessed on 26 May 2021) to be identified by the gate. One tag is placed on the forklift upright at a height of 2.6 m (Figure 5(a)), while the second tag is placed on the forklift roof at a height of 2.2 m (Figure 5(b)) for redundancy purposes. Figure 5: RFID Smart Gates installed at two entrances of the _Tassignano_ warehouse.