Transmission rate and distance
The transfer rate is a key variable in system design and will determine many attributes of the overall system performance. The wireless transmission distance is determined by the receiver sensitivity and the transmitter output power. The difference between the two is called the link budget. The output power is limited by the standard specifications, so the distance is increased only by increasing the sensitivity, which is also very important due to the data rate. For all modulation methods, the lower the rate, the narrower the bandwidth of the receiver, and the higher the receiving sensitivity. The most widely used modulation method in today's cost-effective wireless transceivers is FSK or GFSK. To further reduce the receiver bandwidth of the FSK system, the only feasible way is to increase the accuracy of the reference crystal. Although untested but foreseeable, it is easy to produce a wider frequency deviation than the receiver bandwidth. Low-cost crystals typically have an accuracy of only 20 ppm, which limits the maximum data rate used at systems using a carrier frequency of 868 MHz or 915 MHz at 20 kbps with a sensitivity of -112 dBm. Higher sensitivity can be achieved with a temperature-compensated crystal, but the price of a temperature-compensated crystal will be three times that of a normal crystal.
Spread spectrum modulation has been used in Other fields for many years, but it has not been used in low-cost sensor network solutions until now. At an equivalent data rate, commercial low-cost spread-spectrum modulation can achieve 8-10 dB higher sensitivity than traditional FSK modulation. Semtech will introduce a new transceiver that integrates a spread-spectrum modulation called LoRa and traditional GFSK modulation. Figure 1 shows the sensitivity vs. data rate for both GFSK modulation and LoRa spread spectrum modulation systems.
Figure 1: Sensitivity versus data rate for both GFSK modulation and LoRa spread spectrum modulation systems.
Some spread-spectrum modulation methods are less sensitive to crystal-induced frequency deviations. These receivers achieve near-140 dBm sensitivity when using a low-cost 20 ppm crystal at a bandwidth of 125 kHz. Compared to the FSK system, this new spread spectrum method improves the sensitivity by 30 dB when using the same low-cost crystal, which is theoretically equivalent to an increase of 5 times the transmission distance. There is a conflict between achieving the maximum transmission distance by reducing the rate and requiring the longest battery life. The data rate determines the air transmission time, the higher the transmission rate, the less time it takes for the system to transmit or receive. A 100 kbps system requires only about half the transmission time of a 50 kbps system. A faster rate allows more nodes to coexist in the same area without contention, but this will reduce reception sensitivity and transmission distance. Each receiver provides multiple modes of operation and sleep, and the power consumption is different in different modes. The transmit and receive duty cycles of each node will determine which modes have the greatest impact on power consumption. For example, if a node is frequently in the receiving state, then receiving current is very important. Similarly, if a node transmits only once a day, sleep current is the most important factor.
Band selection
Considering a variety of 2.4GHz technology standards, including Bluetooth, Wi-Fi and Zigbee, many manufacturers believe that they have to use a standard protocol for design, so 2.4GHz actually becomes an option for the operating frequency of wireless transceivers. It is a fact that market-driven vendors have adopted existing standard protocols in many applications. For example, Wi-Fi provides a universal high-speed connectivity service, while Bluetooth provides compatible interconnect services for high-volume consumer markets such as mobile phones and computer peripherals. Despite this, many applications require only a fairly low rate and work in a closed wireless network environment. In these cases, proprietary protocols can significantly reduce system cost, minimize the resources of the processors used, and avoid the expense of specification compliance testing and logo authorization. Some markets have used different wireless protocols at the same time to meet system requirements. Taking automatic meter reading as an example, sub-GHz is used to realize long-distance transmission of backhaul data, and 2.4 GHz is used to complete indoor short-distance communication. The home security system can use Sub-GHz to transmit low-speed sensing data while using 2.4GHz for high-speed video transmission. Many market applications are still pending when it comes to what wireless transmission protocols are used. For example, existing home automation systems use Sub-GHz, but new 2.4 GHz devices are slowly entering the market. Whether to use Sub-GHz or 2.4GHz, there are many places to consider. The main advantage of 2.4GHz is that it is a universal frequency band and can use antennas of very small size. The 2.4 GHz antenna size is equivalent to one-third the size of the 900 MHz antenna, and because of the large number of applications for Bluetooth and Wi-Fi, the 2.4 GHz system can achieve lower cost. The main drawback of the 2.4 GHz system is that the communication distance is too short, which limits its application in wireless sensor networks; its space loss is about 9 dB higher than 900 MHz. In addition, the 2.4 GHz band is already very crowded and can be severely disrupted by devices such as Wi-Fi, Bluetooth and microwave ovens. Figure 2 summarizes the advantages and disadvantages of the 2.4 GHz and Sub-GHz systems.
The new spread-spectrum-based modulation technique will likely change the 2.4GHz and Sub-GHz characteristics shown in the above figure. The use of spread spectrum technology can offset the above 9dB free space loss, so that the 2.4GHz transmission distance can be comparable to today's Sub-GHz FSK system. Due to the performance of this modulation method, spread spectrum has better anti-interference robustness than ordinary FSK or GFSK modulation. The 30dB anti-co-channel interference and significantly improved selectivity/anti-blocking make the 2.4GHz system work more reliably. Since the different spreading sequences used by the spread spectrum system are mutually orthogonal, this means that they can be transmitted simultaneously in one channel. This performance can significantly improve network system capacity compared to existing FSK systems.
A Battery Charger supplies current to the base plate. Once the AGV is in charging position and the collector has made contact with the base plate, the AGV computer turns on the current.
The base plate has chamfered entry/exit ramps to facilitate smooth drive-on/drive-off of the spring loaded collector.
A battery charging contact consists of a base plate, which is installed on the floor or laterally at a bracket adjacent of the AGV runway, and a current collector which is installed on the vehicle.
Battery charging stations may be installed anywhere within the system where the production process allows the AGV to stop (staging areas, turn arounds, loading stops etc.).
Single Phase Charging Contacts
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