Human-machine interface (HMI) displays can convey all types of information and allow operators to select recipes and control machine production. But some functions need more than a digital touchscreen display. That’s where physical buttons might be a better choice. Our panel of experts advises which functions are better-suited for buttons.
A digital human-machine interface (HMI) display can provide lots of information, as well as touchscreen interaction. However, some functions need physical buttons instead of digital screens. Which ones, and what types of physical buttons and switches should machine builders use as part of the operator interface?
While the available number of standard button configurations and options can address almost any potential user need, volume buyers of control panels and panel PCs can completely customize the panel design to the point where it doesn’t look anything like a standard offering. Totally unique buttons and switches can be sourced for custom panel design and the overall design. These can be delivered in the form of passive panels that connect to a separate hardware controller or as a panel PC that serves as the all-in-one HMI hardware and machine controller through automation software that runs on the panel’s integrated CPU. So, whether the device is selected off the shelf or developed for a proprietary machine design, there are panel displays with the exact button configurations available today for any corporate, industry and application need (Figure 1).
Even as HMI technology evolves, physical buttons absolutely have a place as part of operator interface. To increase productivity, physical buttons can be placed directly in the operator workstation for functions like cycle start on a machine or two-hand control on a press.
Select buttons are rugged enough to survive impact and grime, and they can be mounted directly to standard brackets without any box or panel. Reset and emergency-stop buttons for safety functions must be located strategically, and often the best place is not where the HMI is located.
HMIs do provide information to the operator, but this can also be accomplished by illuminated physical buttons with configurable color outputs. A growing application for illuminated touch buttons is in collaborative robotics; the robot communicates to the operator via colors on the button, and the operator responds via touch. Small, lightweight buttons go where HMIs can’t.
One of the most important physical buttons on any machine is an emergency stop. When a critical error occurs, oftentimes the machine needs to be stopped immediately to minimize the impact of the error. As multi-functional as HMI displays are, it takes time to navigate through the menus to address different alarms. Push buttons can be hit at a moment’s notice and the wide availability of push buttons now in the market makes it easy to fit one into nearly any design. For an emergency stop, the push button should be large enough and/or illuminated distinctly, strong enough to be struck hard and located in an easily reachable position on the machine.
Figure 2: A simple keypad includes an e-stop button, but custom units can get much more involved.The push button is no longer so humble (Figure 2). Today's push button, even the e-stop button, is not necessarily a hardwired device, but a networked intelligent device that complements full graphical HMI displays, either built into the HMI panel's bezel, connected to the HMI panel as an add-on module or mounted as remote push-button stations to allow quick response to commonly performed tasks such as stop, start, forward, reverse, jog.
The new push buttons have the appearance of a keypad, are plugged into the same network cabling as the machine I/O and are treated as I/O modules in the automation software development environment. They may be standard catalog products or highly customized. They may incorporate e-stop switches and other devices, such as RFID access control and USB ports.
Nor are they conventional electromechanical push buttons, but membrane switches with a lifespan of 1 million cycles or more, illuminated by multiple LED colors that can correspond to HMI screen colors or stack lights, flashing/solid light modes, are easily reprogrammed and easily customized by inserting legend strips. They may be built to high IP ratings, hygienic design and even Class I, Div. 2.
HMI touchscreens have a lifespan of about 2-15 years, depending on the usage and installed environment. In case of potential deterioration of touchscreens in harsh and abusive environments, the operators should have the ability to start or stop the process or completely stop the machine in case of emergencies with physical buttons. The control panel on which the HMI is installed should always have hardwired start, stop and emergency-stop push buttons installed on it.
Digital HMIs can be very useful and efficient, especially when several data points need to be monitored. However, often only a couple or few critical points need to be monitored on a piece of machinery or equipment. In those instances stand-alone alarm trips can often provide far greater flexibility at a much reduced cost.
An HMI typically requires a PLC or some remote I/O product that interfaces to the analog signals and provides a digital communication link to the HMI panel, another cost and source of maintenance. A stand-alone alarm trip on the other hand, provides the built-in analog circuitry with onboard PV display and LEDs for local alarm indication. In addition, they have the added benefits of universal input power (24 dc/115 ac/230 ac); two to four SPDT mechanical relays that can directly control pumps, lights and klaxon horn; faceplate programming so alarm levels can be reconfigured without custom or complex HMI software; analog output retransmission of PV; local manual reset capability for latching alarms; onboard power for two-wire instruments; IEC 61508 SIL1/2/3-compliant versions available for safety-instrumented-system (SIS) applications; and password protection for unauthorized access or programming.
Depending on the environment, the button or switch type, style, size and location will vary. If the operator is normally using gloves, the button or switch would need to be bigger and at a distance from the other buttons to prevent another missile alert like what happened in Hawaii.
There are several opportunities where taking advantage of HMIs can not only improve safety, but also minimize downtime on equipment. One of the simplest and most powerful production enhancements is taking advantage of diagnostic information of your safety system. Since safety functions, by their very definition, are designed to shut down a machine’s operations if a problem is detected and keep that machine shut down until that problem is fixed, it becomes critical to understand what your safety system is doing. Today’s integrated safety systems have the ability to identify and display a tremendous amount of fault codes, leading to shorter troubleshooting time. The question is, are those fault codes being taken advantage of?
The two biggest areas where physical buttons/switches are still recommended for safety-related interfacing with equipment are emergency stops and bypassing functions. Emergency stops, according to NFPA 79, are still required to be independent physical electromechanical devices, such as push buttons, pull cords and push-bars, so it wouldn’t make sense to include them in an HMI. Bypassing functions, also referred to as manual suspension of safety functions, are not required to be separate devices, according to ANSI B11.19; however, they are highly recommended. Separate physical electromechanical devices are typically much easier to ensure proper circuit performance and meet the monitoring requirements of safety systems. HMIs, however, can still be very useful at managing authorized access—passwords for specific personnel—and for displaying useful information for these functions, such as guard status or control zone information.
In the design of a control system with an HMI, safety is the biggest consideration in deciding where physical buttons should be used. When appropriate security is used, starting a process, changing set points and acknowledging faults are commonly completed through a HMI application. In determining which buttons to leave out of a HMI, opting for a physical button instead, the safety of personnel should be your first thought. Emergency-stop buttons should be physical buttons, present on a control panel or in the field, because of the high reliability of the switching mechanism used to interrupt the signal to other devices. Similarly, some fault reset buttons are isolated to the location of the fault. When intervention to the system is required, like resets to motor overloads, broken parts and physical jams in conveyors, resetting these faults from a remote location can exacerbate a situation and cause injury to personnel. Imagine someone in an office environment repeatedly presses an HMI reset button for a jam to an overhead conveyor in an attempt to get a line running. This could cause product to spill over a guardrail and strike someone below. Without a physical presence at the location of the fault they are not capable of safely resetting the system. Other considerations for physical push buttons may be control power on, key lock switches for maintenance/auto mode, machine jog buttons for maintenance and two-hand controls for safety applications.
Emergency stops (e-stops) would be the best example. There are domestic and international codes that define this button; however, simply put an e-stop is a manually operated device, activated by a single human action, which is designed to open a circuit to one or more pieces of equipment without creating any additional hazards. This device must remain in its actuated open position until normal operation can be restored. Only then can the circuit be reset to resume normal operation.
An e-stop provides a safe and quick disconnection of power in emergency situations. When an emergency occurs, an operator will press the e-stop actuator to begin the e-stop sequence. The actuator will mechanically actuate a normally closed contact, which will open the circuit. As a result, power supplied to the equipment will be disconnected, ensuring the equipment stays off until intentionally reactivated.
Push-pull, turn-to-release and key-release e-stops are the most common types of e-stops.
Emergency-stop buttons hardwired to remove power to the machine are probably the mos+
t important, to ensure safety. Next might come start and stop buttons to control the running of the machine. Others might be setup switches such as run/jog. Lastly, there are pilot lights for indication such as running, machine stopped and alarms.