HMI Control Standards

spartacus06

Member
Join Date
Jul 2017
Location
MI
Posts
43
Hello,

I am in the process of updating our control panels and was wondering if anyone could give me a little more information on the guidelines with regards to HMI vs Push-Button control. Should starting/stopping equipment always be hardwired (hydraulics, 480 power, etc.)? HMI controls should only be used for controls/feedback after equipment is already energized (valves, feedback, vfd control, etc.)

Please share your thoughts
 
We have traditionally been against touch screens, for the concern that the touch function becoming damaged and the machine inoperable.
But nowadays we have overcome this concern, and we are using touch screens routinely. It is a lot more flexible than hardwired switches and buttons. That being said, we have a backup solution if the panel fails - a PC HMI, or an additional panel, or a CPU webserver or the machine can be rudimentarily controlled by interface signals.

One exception though, is if the operator has to watch the process while holding a button pressed for safety (not personnal safety, just for the proces to work in the intended way), we still use a physical button or switch. With the button or switch you have feedback by touch even when not looking at the operating element.
 
As a starting point, if you are:
  • controlling the machine then use physical buttons.
  • changing settings, then use an HMI button.

You really should watch how the operators use the HMI on a running machine to see when a physical button would be better.
But cost, space, time and redesign could make you use an HMI button instead.

Personally I would consider making start/stop buttons physical buttons for 2 reasons:
1. If the motor is stopped/started frequently, the screen will get worn out where the buttons are.
2. If the HMI fails or freezes while the motor is running, the only way to stop it is E-Stop, which may be undesirable.
I also agree.
The buttons I would always recommend for a discrete machine are Start, Stop, and E-Stop.

As a point of reference, most touch screens are rated for 1 million presses (with a finger, at a normal and gentle pressure).
One exception though, is if the operator has to watch the process while holding a button pressed for safety (not personnal safety, just for the proces to work in the intended way), we still use a physical button or switch.
I also agree.

Pressing HMI buttons while looking somewhere else is not a good idea:
(1) When holding a button the operator can drift their finger off an HMI button; stopping the action.

(2) When tapping a button their hand can drift and they may end up tapping the wrong button; as they are not looking at the HMI.

If neither of those is a worry, then use the HMI button.
 
Look at nfpa 79 electrical standard for industrial machinery.

depending on the system, a touch screen to start / stop motors should NEVER be used. There are exceptions and we use Scada to start / stop motors, but we have done risk assessments, machine guarding, all motors are in unmanned locations, all motors has lockable field disconnects, radio communications between maintenance and operators. our process has backup procedures/controls and no one can be harmed if we loose comms to the plc (pc has died before and backup procedures worked along with the manual controls).

why?
what if you loose power to the hmi or it dies?
what happens if the cell membrane no longer works (it does happen)
what if the comms to the plc controller dies?
what if the plc inputs fail and no longer works ( I had this happen several times)

regards,
james
 
One exception though, is if the operator has to watch the process while holding a button pressed for safety (not personnal safety, just for the proces to work in the intended way), we still use a physical button or switch. With the button or switch you have feedback by touch even when not looking at the operating element.

Great idea. I'm all for modernizing just about anything that can be, but I think this is one thing that is hard to replicate on a touchscreen.

what if the comms to the plc controller dies?

Using a safety PLC with safety drives, this is never an issue. You lose comms, or any other unsafe condition is detected, the system shuts down in the way that you have predefined as safe.

Granted its probably partly a function of my location and primary industry (automotive), but I've hardly even seen a drive system without safety back to the PLC in the last 5 years.
 

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