Here is a sneak peek into four levels of industrial HMIs and design considerations for each one of them

BY
EDUARDO ESPARZA
Product
Marketing Engineer, Embedded Processors
Texas Instruments Inc.
www.ti.com

Every
controlled electrical or mechanical machine has a human-machine interface (HMI)
in the form of a pushbutton, lever, or touchscreen. At a high level, an HMI has
three basic elements: input, output, and something to handle the translation
between the two.

As
we move into the Industry 4.0 era, this model gets a little more complex.
Designers are adding graphical user interfaces (GUIs), moving from physical
buttons to virtual buttons on the GUI, increasing the number of tasks the HMI
can perform, and even displaying performance feedback in a closed-loop system.

0218_Feature_HMI_Fig-1

Fig. 1: An embedded processor is at the heart of the modern-day industrial HMI.

General HMI processor requirements
An
HMI can have many requirements for an embedded processor, depending on its
intended end application. There are four levels of HMI performance: entry
level, basic, mid-end, and high-end.

An
entry-level HMI has a very basic user interface. The output screen is usually
quarter video graphics array (QVGA), 320 x 240 at best, and has minimal 2D
graphics. These HMIs are geared toward cost-sensitive applications that require
only the bare essentials of a control interface. Designers may use a resistive
touchscreen here because it’s more economical than its capacitive-touch
counterpart.

Not
only is the resistive touchscreen more inexpensive than a capacitive
touchscreen, the BOM cost may also be lower because some processors can
natively support resistive touchscreens while capacitive touch sometimes
requires external components. This category of HMI is best for a
low-performance processor (

Basic
HMIs add improved display resolutions and a slightly better user interface than
entry-level HMIs. A basic HMI will have a touchscreen — usually resistive touch
— and up to extended graphics array (XGA) (1,024 x 768) display resolution to
improve the user experience. Depending on the required application processing
power, a processor for this category would be in the low- to mid-end
performance range (300 MHz to 800 MHz) and may benefit from a 2D graphics
accelerator.

0218_Feature_HMI_Fig-2

Fig. 2: Choosing the right processor for HMI requires careful design consideration.

A
mid-end HMI more closely reflects a typical GUI that a user might interact with
on a daily basis. Mid-end HMIs have 2D graphics, display resolutions up to XGA
(1,024 x 768), incorporate more control functionalities than the basic
category, and sometimes even introduce haptic or audible feedback. These
features greatly improve the user experience. For a mid-end HMI, the processor
has to include graphics acceleration, mid-range performance (600 MHz to 1 GHz),
and graphics libraries to help build the GUI.

High-end
HMIs are naturally multimedia-rich. They require a high-end SoC with
high-definition video support, 2D and 3D graphics accelerators, and
high-performance processors (multicore and >1 GHz). And this can greatly
benefit from on-chip DSPs to help accelerate audio and video processing.
Additionally, high-end HMIs usually require a processor that can handle
multiple high-resolution screen outputs and HTML5. One example is the Sitara
processor family based on Arm Cortex-A cores, which provides the scalability
needed to develop a single platform ranging from entry-level to high-end HMI
and supports industrial reliability.

You
can find HMIs in household appliances, vending machines, building automation
systems such as fire control panels or elevators, and electric vehicle charging
stations. However, one of the most prevalent uses of industrial HMIs is in the
factory automation space.

HMIs in factory automation systems
In
factory automation systems, an HMI connects the machine’s operator to a
controlling function, typically a programmable logic controller (PLC), which
controls the sensors, actuators, and machines on the factory floor. HMIs are
also more commonly included on machines and robots themselves and, in some
cases, manage some of the control function within the HMI. These applications
create a number of requirements for a processor in the HMI, including the need
for industrial communication capabilities, industrial-grade reliability, and
security features.

Industrial communications
Standard
Ethernet doesn’t have the deterministic functionality required for industrial
automation. And that’s where protocols designed for industrial communication
come into play. Industrial Ethernet protocols achieve the real-time,
deterministic communication needed between different types of end equipment in
a control system.

There
are currently over a dozen different protocols created for industrial Ethernet.
A processor, FPGA, or ASIC is required to handle these protocols in an HMI. In
many cases, an HMI will have a host processor and a separate ASIC or FPGA to
run a single protocol.

As
an alternative to the FPGA or ASIC, integrated solutions exist that can serve
as both the application processor and communications engine for industrial
Ethernet; these solutions may even extend functionality to support multiple
protocols.

Multi-protocol
support in an HMI adds a much-needed level of flexibility for Industry 4.0
because control systems in a smart factory are often pieced together from
different solutions running different protocols. With multi-protocol support,
an HMI can act as a gateway between different protocols.

 

Industrial-grade quality
Factories
operate 24/7 year-round in most cases. And conditions can vary from
sub-freezing to boiling-hot temperatures, depending on what the factory is
producing. An HMI inside a factory must be able to withstand these conditions,
and so must the processor inside it. It brings up the need for industrial-grade
processors in factory automation HMIs.

0218_Feature_HMI_Fig-3

Fig. 3: HMI requirements significantly increase in factory automation.

An
industrial-grade processor must be able to withstand a wide range of
temperatures, typically –40°C to 105°C. Additionally, because factory equipment
operates for long periods of time, extensive device longevity testing is
required. One metric used for a device’s longevity is its power-on hours (POH),
which is the number of hours that it can be powered and operate properly. A
processor with a wide temperature range and POH exceeding 88,000 would
essentially be able to run for over 10 years. Most industrial HMIs are required
to meet at least 100,000 POH.

Security
Although
the HMI and the rest of the control network are usually configured on an
internal Ethernet network isolated from the main internet, there still exists
the possibility of a malicious party eavesdropping or altering communications
between the HMI and other parts of the system. To help deter unwanted
interference, embedded processors often integrate cryptographic accelerators to
encrypt data. Secure boot is another popular security option available to help
protect an HMI manufacturer’s intellectual property.

Other HMI aspects
Because
an HMI is primarily a user interface, it requires the use of a high-level
operating system (OS). Popular OSes for HMI include Windows CE, Android, and
Linux. Windows CE has been popular in HMIs for many years, especially in
factory automation, but Android and Linux are gaining traction for a couple of
reasons.

First,
Android and Linux are open-source OSes, which means that they can be free to
implement. Additionally, because they are open-source, there is a large
community supporting software and providing example code for each OS.

Android
is popular in systems in which a large variety of users will interact with the
HMI, such as in a vending machine or appliance. Android is already popular in
the handheld device market, so the learning curve for someone new to the HMI is
minimized because they may already be familiar with the OS.

In
factory automation, Linux has become the likely choice because it is widely
considered to be stable, reliable, and secure. Many industrial HMIs don’t
require all of the features that come with Android. On the other hand, Linux
also supports frameworks like Qt and the Open Graphics Library (OpenGL), which
help build an effective GUI.

Another
feature that is gaining popularity in HMI is virtualization. As previously
mentioned, HMIs are popularly integrated with other end equipment such as PLCs,
industrial robots, and CNC machines. One method of integration would be to have
separate processors for the HMI and other application, but this can be costly
and require additional board space.

The
alternative method is to have a single, multi-core processor with a core
dedicated to the HMI and another core dedicated to the application. The cores
can run different OSes such as an RTOS and Linux, depending on whether a
real-time operation is needed.

Summary

HMIs cover a broad array of end applications at
a wide range of performance levels but share a few common features, including
GUIs, connection to the control system, and touch-based controls. At the bare
minimum, a processor must be able to support these entry-level HMI
requirements. These features can be taken further by basic, mid-end, and
high-end HMIs to include HD graphics, web browsing, video, and multiple-screen
support.