HMI Pushbuttons Types and Use

Hello,

When it comes to process control (e.g starting/pausing/stopping a batch or sanitation function) with HMI pushbuttons, what is the standard or preferred or best types of buttons to use for START, STOP and PAUSE ? Should they be of type MOMENTARY OR MAINTAINED pushbuttons ?

Thanks

I can't speak for everyone, but it is more reliable to use a momentary HMI bit to trigger an action and let the PLC do the latching to maintain. If your HMI is providing a maintained contact and you lose the HMI, you lose the maintained contact.

What about for a STOP button ? would you use momentary as well ?

Always momentary.

The rung in attached picture, how would you use the STOP pushbutton as a momentary ? Because as a momentary, it will never actually stop the RUNNING state.

Thanks

mbartoli said:

I can't speak for everyone, but it is more reliable to use a momentary HMI bit to trigger an action and let the PLC do the latching to maintain. If your HMI is providing a maintained contact and you lose the HMI, you lose the maintained contact.

Click to expand...

Ditto!

I let the Hmi set a pb flag, then reset it in the plc as soon as it sees it. And create a oneshot of it at the same time. Toggle pushbuttons are the devil's own.

wanher3 said:

The rung in attached picture, how would you use the STOP pushbutton as a momentary ? Because as a momentary, it will never actually stop the RUNNING state.

Thanks

Click to expand...

Yes it would.

wanher3 said:

The rung in attached picture, how would you use the STOP pushbutton as a momentary ? Because as a momentary, it will never actually stop the RUNNING state.

Thanks

Click to expand...

I do it this way too. Emergency Stop is for emergencies, because it's hard-wired. You don't depend on the HMI for emergency stop conditions, so there's no need to get crazy with programming.
Use hard-wired buttons for emergency operations.

wanher3 said:

Hello,

When it comes to process control (e.g starting/pausing/stopping a batch or sanitation function) with HMI pushbuttons, what is the standard or preferred or best types of buttons to use for START, STOP and PAUSE ? Should they be of type MOMENTARY OR MAINTAINED pushbuttons ?

Thanks

Click to expand...

In the early days of HMIs, back when we were still using DOS, we didn't even have "momentary" pushbuttons. We typically only had bit sets and bit clears. A momentary pushbutton on a modern HMI is really just a bit set on button down/bit clear on button up function.

In those early days, we'd typically make all of the pushbuttons "bit sets" and unconditionally clear the entire array of pushbutton bits at the end of the program scan. That way, they functioned in the program as if they were one-shots. That method wouldn't work reliably with PLCs that update their data tables asynchronously with the program sweep, unless you move everything into a memory buffer array and reference bits from those buffer registers in the program.

Hardwired pushbuttons are electrically failsafe when wired Normally Closed, but there is no safety advantage to adhering to that model with an HMI interface.

paraffin power said:

I let the Hmi set a pb flag, then reset it in the plc as soon as it sees it. And create a oneshot of it at the same time. Toggle pushbuttons are the devil's own.

Click to expand...

Toggle pushbuttons can be useful for things like binary Mode Selections. If you want to override the operator's selection within the PLC program under certain conditions, you can use Latch and Unlatch functions to easily do so.

I wouldn't want to use a toggle to run a motor.

Pushing the "Start"-pushbutton sets a bit and pushing the "Stop"-pushbutton clears the bit.

Ladder would be:

   bit             output 

---| |--------------( )---

Things like Emergency stop, circuit breaker etc would also clears the bit.

Use the status of the bit to change color of the pushbutton or to show the status of the motor in some way so the user would know if the motor was started or not.

If you also have physical start and stop buttons as well as HMI you just make them affect the same bit.

For instance:

start_button         bit 

---| |-------------(LATCH ON)--



stop_button          bit

---| |-------------(LATCH OFF)-



or if the stop button is NC



stop_button          bit

---|/|-------------(LATCH OFF)-