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Ensuring uninterrupted wireless connectivity for AMH systems

30 August 2016

Jeffrey Ke looks at the challenges of developing wireless communication solutions for automated materials handling systems.

Automated Materials Handling (AMH) refers to the automation of the process of loading, moving, and unloading of materials in a production environment. The main objectives of an AMH system is to reduce the cost of operation, minimise human error and meet safety requirements. 

Because of their potential role within the Industrial Internet of Things (IIoT) there has been a recent been a resurgence in interest in the AMH market. Indeed, the AMH market in manufacturing has been predicted to grow at a rate of between 5% and 6% every year up to 2020. Currently, adoption of the IIoT is mostly limited to achieving operational efficiency, so its full potential has yet to be tapped. However, as the standards for the IIoT evolve and as we move towards greater adoption of open standards, more devices and equipment will be able to communicate directly with each other on the network. The IIoT is also expected to drive the communication interfaces for conveyors, automated storage and retrieval systems (AS/RS), forklifts, and other AMH-related systems. The centralisation of the data collected from these systems can provide useful information to aid decision makers and the AMH industry is currently using big data applications to help address maintenance and service delivery issues and to manage and streamline processes.

Wifi-based technology is the most commonly used communication method in AS/RS and automated guided vehicles (AGV) systems because it offers flexibility, allowing multiple access points to be deployed to support the roaming of clients installed on moving platforms.
 
It is also easy to implement Wifi-based devices on moving vehicles giving them the ability to communicate without the need for a physical connection. The standardisation of the 802.11 Wifi-based technologies also makes interoperability between wireless devices from different vendors much easier when compared to non-standard technologies, so expanding your system or network as well as finding replacements for the wireless devices deployed in the system becomes easier. 

Although Wifi-based technology has advantages over other communication solutions, there are some areas to which system integrators need to pay special attention when looking to deploy wireless solutions in AS/RSs and AGVs. These include: 

Constantly moving platforms: Ensuring reliable communication while on the move is a key concern for AS/RS and AGV systems. Seamless roaming and anti-vibration protection of the client devices are also important considerations when choosing a suitable solution. 

Wifi networks have limited signal coverage, which means multiple access points need to be installed to achieve full signal coverage in a warehouse. It is critical to ensure that the clients are able to roam smoothly between these access points with minimal handover time. It is common for Wifi devices based on older technology to experience a three to five-second disconnection time as they move between access points, causing disruption to real-time operations. 

The basic requirement of a reliable wireless roaming technology is to pre-emptively seek out neighboring access points with strong signals and prepare to connect to new access points as the client moves closer to them to eliminate any downtime. 

The following advanced wireless roaming features can also increase the efficiency of the client devices: 

Reliable roaming performance with multiple channels:  Due to the limited bandwidth available on each channel frequency, system integrators must use multiple frequency channels to avoid channel congestion. The wireless-roaming technology should provide smooth roaming between different access points (APs) using different channels. 

Adjustable roaming parameters: The roaming function must allow users to adjust the roaming parameters to adapt to different venues and site environments. The roaming parameters are configured to allow location-based load balancing to ensure that wireless clients are connected to the closest access point to avoid network traffic congestion. 

Best wireless encryption without affecting roaming performance: Wireless signals are transmitted through the air and are, therefore, vulnerable to hacking. Encryption protocols that provide high data security are complex to implement and can affect roaming performance. An advanced wireless roaming function that can provide the most secure data encryption possible and millisecond-level handoff times for clients is required to secure the network. 

Shock/vibration protection 
Wireless devices mounted on systems prone to vibration and shock can cause electrical shorts, broken solder joints, loose PCB components, PCB delamination, and cracked device housings. Shock and vibration can also disable a wireless device by shaking loose wires for power, data, and redundancy. The IEC 60068-2-6 standard specifies the guidelines that wireless devices must abide by to ensure protection against high vibration and shock. Devices must be tested against these standards and need to pass the criteria set by the standards to be able to provide reliable performance in highly mobile environments. 

The main concern for most system integrators when choosing wireless devices for compact AGV systems is their ability to withstand electrical interference.

Electrical disturbance can interrupt wireless transmission by entering through the power inputs and antenna ports. To utilise the limited space on AGV systems, all the onboard devices usually share the same power source, including wireless devices and motors. When the motor is turned on, it could generate inrush current that can damage the wireless device through the power port and in extreme cases can stop the wireless communications. Antenna extensions are usually mounted on the metal casing of the AGV or the shuttles used by the AS/RS to achieve better signal strength, which can lead to airborne electrostatic charges damaging the wireless components through the antennas and antenna cables. Typically extra power and antenna-isolator accessories are required to strengthen the system, but this method can increase system cost and requires extra installation space. Wireless devices should be provided with built-in isolation to protect them from electrical disturbance.

Wireless installations
It is important to set up the wireless network correctly to avoid operational glitches. In wireless communications, multipath is a propagation phenomenon that results in radio signals reaching the receiving antenna via two or more paths. In a typical warehouse environment, metal frames and shelves can cause reflection and refraction of the radiated signals resulting in multipath fading. The fact that reflected/refracted signals traverse a longer distance than direct line-of-sight transmissions could cause the signals to arrive out of phase, leading to signal degradation at the receiver’s end. However, 802.11n multiple-input-and-multiple-output (MIMO) technology can be utilised here, by reconstructing a strong signal at the receiver through the application of error-correction techniques on the weak reflected/refracted signals. 

MIMO provides a way to utilise the signals between a transmitter and a receiver to improve the data throughput available on a given channel. By using multiple antennas at the transmitter and receiver and by applying some complex digital signal processing, MIMO technology enables the wireless devices to set up multiple data streams on the same channel, increasing data capacity of a channel. In addition to overcoming the effects of multipath propagation, MIMO radios can also be used to increase the Wifi coverage and improve signal reception on the wireless devices. 

Configuration and maintenance 
Although wireless devices are usually easy to install, industrial operators still need to deal with complex installation processes and the sheer number of wireless devices that need to be set up before enabling the entire wireless network makes the task even more daunting. The wireless clients deployed in AS/RS or AGV systems have to be set up one-by-one to ensure proper connection with the APs. When device errors occur, the mobile systems need to be halted. Restarting or rebooting can be time-consuming and disruptive for operators. Although the demands on APs are far less than for wireless clients, making manual setup easier, access points installed on the top of storage shelves to provide greater coverage make device maintenance difficult. Wireless devices that are easy to configure and maintain can make installation and troubleshooting easier in the warehouse environment as well as increasing efficiency and productivity. 

A smart setup function can simplify setup, configuration, and maintenance processes for operators. This function should be able to detect the role of each wireless device inside a warehouse and automatically choose suitable setup options to connect APs and clients.

Moxa believes that its AWK-A series devices are able to provide all the basic building blocks to enable the creation of reliable, high-performance mobile Wifi networks, offering a tailor-made solution for system integrators in AS/RS and AGV markets.

A whitepaper – Five critical elements of uninterrupted wireless connectivity for AS/RS and AGV systems  – can be downloaded from the Whitepaper section of the Control Engineering Europe website.www.controlengeurope.com

Jeffrey Ke is product manager at Moxa.


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