Thin client HMIs in process automation

Virtualisation has become more and more popular in process automation and has opened up more possibilities for how HMI systems can be deployed.

Virtualisation is a technology that stems from the information technology (IT) industry and has become more and more popular in process automation. Virtualisation promises to ease software management while reducing costs. This trend also has an impact on the human machine interaction (HMI) in such systems. Especially in combination with thin client technology, virtualisation provides an easier and cost-efficient way to control process automation systems even in the harshest industrial environments.

What is a thin client?

Over the last decade, thin clients have become more and more popular in process automation systems and industrial applications. Especially with the trend to virtualised, centralised automation systems, thin clients represent a powerful and cost-efficient technology enabling the users to access the applications and information that run on centralised hosts (host servers).

In contrast to conventional, decentralised automation systems, where usually all data and applications run on powerful PC-based workstations, in centralised automation systems, the data and applications reside on the hosts which are usually servers. A thin client only runs the user interface that is required to access the applications on the host (see Figure 1).

Figure 1: Thin clients are the first choice of technology for centralised systems.

To do this, a thin client has a minimalistic, usually embedded, operating system (OS) and only provides drivers for the input and output devices (such as mouse, keyboard, touchscreen and monitor) that are connected to the thin client. Additionally, installed communication protocols enable the exchange of the system inputs and outputs between the thin client and host (see Figure 2).

All of these remote protocols rely on the same principle. The host generates the user interface (such as GUI and sounds), which is then compressed and sent via an Ethernet-based remote protocol to the thin client. The thin client receives the compressed data, decompresses them and displays them on the screen to the user.

User inputs (via keyboard, mouse, touchscreen, etc) are sent in the opposite direction. The thin client captures the physical user inputs and redirects them via the remote protocol to the host. The host decodes the user inputs and delegates them to the hosted operation system and applications. For the applications that run on the host this is transparent, which means that for the applications it looks like the user is interacting locally on the host. Due to today’s high-performance Ethernet infrastructures, the user experiences the interaction with a thin client like sitting directly on the host system.

Since thin clients work over Ethernet, they are also the first-choice technology for virtualised automation systems. Conventional technologies like keyboard-video-mouse (KVM) extenders are not suitable for virtualised systems. This is because one or many virtual machines (VMs) usually run on host server hardware, which has no dedicated physical interfaces to connect the KVMs to. VMs can be accessed only via Ethernet and the remote protocols.

Figure 2: A thin client only provides the user interface to the user. For a larger image click here.

Today, multiple communication protocols do exist, but there is only a very small set of protocols that are relevant to cover the majority of virtualised – and even conventional, non-virtualised – applications:

• Microsoft Remote Desktop Protocol (RDP): RDP is the most popular remote protocol for workstation-based and virtualised automation systems. While today’s most recent Microsoft OS can be accessed via an integrated RDP interface (such as for remote administration), professional set-ups with multiple users require a Windows Server OS. The server-based solution for multiuser access was formerly known as Microsoft Terminal Services and was introduced with Windows NT 4.0 Terminal Server Edition many years ago. With Microsoft’s strategy to virtualised infrastructures and the launch of Windows Server 2008 R2 in 2009, Terminal Services has been extended and renamed Remote Desktop Services (RDS).
• Virtual Network Computing (VNC): VNC is one of the older remote protocols, which is still quite popular. Especially in smaller, non-virtualised automation systems, this protocol is still used since several open source implementations do exist that allow the set-up of cost-efficient solutions.
• Citrix Independent Computing Architecture (ICA): ICA is a Citrix proprietary, platform-independent remote protocol that is used in large, professional and virtualised infrastructures with Citrix XenApp and XenDesktop.
• VMWare PC-over-IP (PCoIP): Originally introduced by Terradici, VMWare integrated this protocol in its virtualised server infrastructure. Besides PCoIP, VMWare also supports access to the hosted VMs via RDP.

Benefits

Using thin clients in a centralised automation system offers a large set of benefits. Many of the features, like centralised management, the need for less local computing resources and keeping the data on the host server, contribute to a reduced total cost of ownership compared to PC-based, decentralised infrastructures.

Reduced total cost of ownership

Since the applications reside on the host systems, thin clients have fewer hardware demands compared to workstation PCs. Low-power processors are sufficient to run the different remote protocols and to encode/decode the compressed data exchanged between the thin client and the host. This has an impact on the overall hardware costs, since the thin client components are significantly cheaper than the high-performance components needed for workstation PCs. To have the same performance in a thin client infrastructure does demand more powerful host servers, however. But since centralised infrastructures allow a more efficient use of the hardware resources (due to load management under virtualisation), the total hardware costs are lower, especially in mid-size to large applications.

Hardware and software longevity

Another benefit of thin clients is that they can have a longer lifetime than PCs. There are two reasons for this.

First, application software updates do not affect thin clients, since they only communicate with the host via a remote protocol. This allows thin clients to be used, even if the OS or applications on the host are updated.

The second reason is that the embedded OS that runs on thin clients is supported much longer than desktop operating systems (like Windows XP Professional or Windows 7 Professional) that usually run on PCs.

Reduced configuration effort

Thin clients are much simpler to configure. Instead of installing applications on several workstation PCs, thin clients only need to be configured. This is mostly limited to two steps: assign the thin client an IP address and specify the host server or VM name the thin client should connect to.

In large installations where multiple thin clients need to be configured, tools for centralised configuration and management help to maintain whole groups of thin clients with one mouse click. Due to the limited amount of settings that need to be made, this can be done even by personnel with limited IT knowledge.

Increased system availability

Especially in industrial environments, systems must run reliably not only for cost reasons, but to protect process equipment and personnel. With thin clients, the process reliability can be increased.

As pointed out before, thin clients have no locally stored data or applications and can be exchanged in a few minutes in case of a hardware defect. This does not affect the applications since they are running on the host. And since thin clients only need very limited computing power, industrial-grade components can be used for a lower price than a powerful workstation would cost. This has a positive effect on the robustness of the thin client and allows it to be used in harsh, industrial environments where hardware has to withstand heat, shock and vibration, dust, washdowns and explosive atmospheres.

In case of a host failure, backup hosts can be used. Modern thin clients also allow preconfigured connections to backup hosts to which the thin client can connect automatically as soon as a host failure is detected. With this feature, highly reliable process automation systems can be set up.

Increased flexibility

Thin clients use Ethernet technology to connect to their host systems. Therefore, thin clients can connect to any host system which is located in the LAN, WAN or even over the internet. This allows the implementation of sophisticated application scenarios, like connecting to backup hosts in case of a failure, to connect to, and to supervise, different machines in a plant or to access information from different system types like a distributed control system (DCS) and a manufacturing execution system (MES) that might run on two different hosts or networks.

Higher security

Centralised IT infrastructures also offer higher security since data and applications reside on the hosts in the data centre with centralised backups, redundant servers and other associated protections.

Thin clients in particular are further protected against manipulation, for example with tools like enhanced write filters and USB lockdowns that prevent users from installing software locally. This significantly reduces the threat of installing viruses.

Conclusion

Thin clients are a high-performance and low-cost solution for accessing applications and information in process automation applications. One of the key benefits of thin clients is that no data and applications are installed locally and therefore do not need to be maintained. Thin clients use the industry-standard Ethernet and remote protocols to access applications and data that are located on a host system, which can be a VM in a virtualised automation system or a conventional workstation-based set-up. This allows minimising the performance of the computing hardware on the thin client side and eases the system configuration.

Due to the use of standard technologies like Ethernet, end users can take advantage of readily available expertise to implement their automation systems. Software tools for centralised management of thin clients further help to ease the integration of thin clients, even for automation engineers without deeper IT background knowledge.

Image credit: ©stock.adobe.com/auremar