Designing SoCs for Industrial HMI

The human-machine interface is a key element of industrial design, but designers need to reach the right balance of integration and flexibility to control costs.

By Matthew Knowles, Ph. D

Despite the current popularity of the X86 architecture in industrial systems, traditional CISC-based processors can no longer keep up with the demands of human-machine interfaces (HMI). HMI requirements now include increased processing needs and a wide range of network and peripheral interfaces, along with smaller sizes and tighter thermal budgets. Integrated systems on chip (SOCs) that include high performance RISC microprocessor along with a variety of standard interfaces and peripheral busses provide the flexibility needed to support diverse industrial automation environments, but they can be costly. The key to keeping these SOCs for industrial HMI cost effective is the right balance of integration and flexibility.

The factory worker is evolving into a computer operator. As volume manufacturing becomes more automated, factory workers interact less and less directly with the physical pieces of equipment. Instead, an operator will work with computer terminals or a user interface panel rather than work directly on the nuts and bolts of the equipment itself. Despite their popularity, however, standard general purpose computers, although flexible for a myriad of tasks, are not optimized for in-factory, human-to-machine control. This has resulted in the emergence of dedicated Human Machine Interface (HMI) compute systems. HMI is the convergence of visual computing with the reliability and integration requirements of today’s factory automation and control systems.

Although the details depend on specific industry and factory requirements, in general an HMI product must be able to control systems in a real time, deterministic manner with high reliability. At the same time it must provide a visual interface that is easy to read and shows meaningful real time data from diverse areas of the factory floor. Creating the right HMI system for such a complex environment requires an optimal combination of ingredients.

The heart of this critical part of the factory is the microprocessor that runs the HMI and the factory equipment. Many HMI solutions in the market today are X86 based, using processors from Intel and other manufacturers that provide performance and flexibility. Evolving HMI requirements, however, are increasing the need for processing performance and high throughput interfaces while requiring reduced heat generation and more compact size for the compute complex. Although X86 processors have recently made great strides in reducing form factor and thermal dissipation requirements, there are many areas of the factory where 32 bit, highly integrated RISC-based SoCs are becoming a more compelling solution.

Many such integrated RISC SOC designs are available that run operating systems (OSs) with rich graphical user interfaces (GUIs) that meet the needs of HMI. Unfortunately, each manufacturer integrates different functionality into their SoCs, making a straightforward apple-to-apples comparison more difficult than when making an X86 choice. In the X86 arena, there are nearly 20 options for microprocessor (models, not including speed grades) while in the SoC arena the number of choices exceeds 50 models. Each has a different mix of processor, I/O, and bus interfaces.

How Much is Enough?

Choosing the right balance between integration and modularity in the SoC selection is essential if designers wish to optimize an HMI platform design for multiple target market segments. One of the primary benefits of highly integrated RISC processors is the value of having multiple functions in one piece of silicon, saving board space and cost. Yet, as more features are integrated the platform becomes less flexible. Taking this to an extreme, if a designer creates an SoC with all the features for a high-end product, as shown in Figure 1, it becomes impractical to adapt. When the customer base demands a new feature or the product enters a new generation, the SoC will require a completely new silicon design, fabrication, and validation program. This is cost prohibitive for what are usually small volume products. A further drawback is that SoC will only meet the needs of one specific product segment and be unsuitable for use in lower-end designs.

Figure 1 - “All in one” custom designs that meet the requirements of high-end designs seem cost-effective, but in reality they can be too expensive to use across a product family. This forces individual designs for each product, which results in greater overall expense.

At the other extreme, where a separate core, chipset, I/O bus, memory, graphics, network are implemented, the product line becomes very flexible. Then, however, board space, software compatibility, and component cost become unacceptable. The key for the HMI designer is to strike the appropriate balance between integrated features and modularity for HMI. As diagrammed in Figure 2, a simpler SoC that integrates only the processor, memory controller, general-purpose I/O, and a peripheral bus, permits a modular approach that uses one SoC design to serve as the core for many systems.

Figure 2. A modular approach that uses a relatively simple integrated SoC with a common set of features can be used to meet requirements of several markets with a minimal set of support chips.

To understand the needs of industrial HMI, consider a typical example: automated electroplating. In this example, shown in Figure 3, the core computing element is an Intel® 80219 General Purpose PCI Processor based on Intel XScale® technology. The 80219 is a high performance, 32 bit SoC that has a 32-bit, 100MHz local bus combined with PCI/PCI-X connectivity.

Figure 3 - This representative modular HMI design, for electroplating process equipment, uses a processor integrated with I/O and buses that simplify the connection of elements such as the display device and analysis equipment.

The main process to control in the automated electroplater is the chemical bath. As a chemical reaction deposits metal from the chemical bath solution to the substrate (wafer or bumper), the concentration of the bath changes and needs to be restored. Monitoring the bath, running algorithms and implementing deterministic signaling to various subsystems requires a combination of different interfaces, I/O and powerful computing performance. For example, the process needs a high bandwidth interface to the instrument performing real time sampling of the chemical bath.

Bus Connections Add Flexibility

In this example, a proprietary analyzer system that does not have a standard industrial or PC-type interface is monitoring the bath. The analyzer samples the chemical composition of the bath and provides a data stream on a proprietary bus to an FPGA for data processing. Having a high bandwidth peripheral or local bus on the SoC provides an easy way to attach the FPGA. Without an integrated local bus with enough bandwidth, a separate bridge-type device would be required to convert the FPGA’s signals to PCI or some other interface standard.

The peripheral or local bus also provides the flexibility needed to attach to the SoC the necessary industrial control protocol controllers (CAN / Profibus / Fieldbus) that send commands to the electroplating system. Attaching the appropriate ASIC for each vertical application or market segment provides the HMI design with a modular structure that meets a wide variety of industrial market needs with a maximum of reuse. Integrating the protocol controller would be too restrictive unless all of a customer’s applications use only one specific control bus.

In a complex piece of processing equipment such as an automated electroplater, there are also a number of simple sensors that monitor material position within the process flow, control interlocks and turn valves on and off. Having integrated GPIOs (General Purpose Input Outputs) on the SoC to control these simple sensors and valves saves board space and design complexity within the HMI. In this example, although the analyzer requires a sophisticated high bandwidth interface, the actual dispense subsystem only needs low level GPIOs to control open/closed states of the valves.

Include Physical Design Considerations

Due to increased cost of automated factor floor space and reduced power budgets, industrial equipment needs to occupy smaller footprints on the factory floor while consuming less power. If an equipment manufacturer can reduce both the power consumption and footprint of their product, this reduces the factory’s total cost of ownership for the equipment and makes the product more competitive. The limit of this reduction, as the control subsystem is shrinks in size, is the size of the display panel. The compute complex of HMI must, thus, essentially fit in the form factor defined by the flat panel display.

From a thermal perspective, this size precludes the use of large heat sinks or fans in HMI designs. This, in turn imposes low power as a strict requirement. Typically, the compute complex must be a fanless design with (at most) minimal convection cooling. The processor needs to be under ~10W; optimally the entire compute complex should meet this requirement as well. Fortunately, most 32 bit RISC SoCs meet this requirement.

The ability of the HMI to adequately convey information to the operator is, of course, also a critical design consideration. As complexity of a particular piece of processing equipment increases, or as the factory automation topology complexity increases, more and more information needs to be conveyed in real time through the primary visual interface. This is why a key product family differentiator for HMI is screen size and resolution. Therefore, choosing an SoC with an integrated graphics controller could be limiting. On the other hand, given the availability of PCI and PCI Express based graphics controllers from the high volume PC market, a modular graphics approach provides both flexibility and low cost. In the example, a graphics controller with the desired resolution can readily attach to the 80219 PCI bus.

Keep Operating System Needs In Mind

The right software is also essential to the HMI design. The timing of control signals for the electroplater makes the difference between creating a saleable product and generating scrap material. Providing the right timing requires the system to run an operating system that offers true deterministic control. There are a number of such RTOSs (Real Time Operating Systems) available commercially, including Windriver’s VxWorks, Mentor Graphics’ Nucleus, and various versions of embedded Linux.

The system also needs a Graphical User Interface (GUI) for ease of use. Microsoft Windows CE 5.0 can be used for its rich GUI and its real time control. In this example, applications can be optimized to run on the .NET Compact Framework under Microsoft Windows CE 5.0 to obtain the real time control needed. The rich CE GUI then provides the environment for designers to produce compelling, meaningful interaction models for the factory operator.

As HMI requirements continue to evolve, this dual need for graphics and real-time control may mean that multicore SoCs are on the horizon for HMI designs. Developers are already investigating new ways to exploit the added performance, flexibility and reliability of multicore processors. On the reliability front, for example, to meet the needs of mission-critical industrial application it would be useful to be able to isolate the RTOS from the OS running the GUI and other user controls, a task easily accomplished with a dual core integrated SOC as shown in Figure 4.

Figure 4 - SoCs are evolving to dual cores to add reliability and performance. Such devices let two different operating systems run simultaneously, allowing designers to use one OS for graphics/interface and another OS for real time control.

Dual core integrated SoCs exist today, but they are targeted at high end networking applications and priced beyond the range of most HMI designs. That situation is bound to change as cores and smaller process geometries become more available. Expect to see more cost effective dual core SoCs in the future that will allow a graphical and a real-time OS to run independently.

The advantage of the dual-core approach shows that high levels of integration are useful in industrial HMI systems, but the elements to be integrated need to be chosen carefully. Placing too many dedicated functions into an SoC can ultimately prove to be just as costly as not including enough. The key is to maintain flexibility where it allows product differentiation and integrate the features that can serve as a broad common base.

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Matthew Knowles is Strategic and Product Marketing for Intel’s Storage Components Division, headquartered in Chandler, AZ.