A lift can physically reach the first floor and still fail to recognize that it has arrived.
This may sound like a complicated PLC, hydraulic, or control-system problem, but sometimes the actual cause is much simpler: the first-floor limit switch is installed in the wrong position.
We recently helped a customer troubleshoot exactly this situation. The lift had already reached the first floor, but the control system continued to show a downward movement. The first-floor landing door also failed to open because the PLC had not received the correct first-floor position signal.
After checking the system, we found that the lift was reaching its mechanical lower limit before activating the first-floor limit switch. By moving the limit switch upward so that it was triggered before the mechanical end of travel, the PLC could correctly recognize the first-floor position and the problem was completely resolved.
This case highlights an important principle in lift and elevator installation: a mechanical stop and a position limit switch are not the same thing, and their positions must be correctly coordinated.
A Limit Switch Installation Problem That Can Cause Incorrect Floor Detection
In hydraulic lifts, home elevators, and other vertical lifting equipment, limit switches play an important role in determining the position of the platform or cabin.
A limit switch may look like a simple mechanical component, but its installation position can directly affect how the PLC understands the position of the lift. To understand how limit switches work in lift systems and why mechanical limit switches remain widely used, see our detailed guide: Limit Switches in Lift Systems: How They Work and Why We Still Use Mechanical Limit Switches.
We recently helped a customer troubleshoot an unusual problem:
The lift had physically reached the first floor, but the control system still indicated that the lift was moving downward. At the same time, the first-floor landing door did not open as expected.
The lift had already reached its mechanical lower limit, so why did the control system still think it had not reached the first floor?
The problem was caused by the installation position of the first-floor limit switch.
The Symptoms
The customer reported several related symptoms.
When the lift was descending from the second floor toward the first floor, everything appeared normal at first.
However, after the lift reached the first floor:
-
The lift had physically reached the first-floor position.
-
The control system still indicated a downward movement or "Down" status.
-
The first-floor position signal was not detected correctly.
-
The first-floor landing door did not open.
-
The PLC continued to consider the lift as being in the process of descending from the second floor to the first floor.
This can be confusing because mechanically, the lift was already at the bottom position.
The key question was:
Why did the PLC not recognize that the lift had reached the first floor?
The Difference Between a Mechanical Stop and a Position Limit Switch
The most important point in this troubleshooting case is that a mechanical lower limit and a floor-position limit switch have different functions.
A mechanical stop is designed to physically prevent the lifting platform or elevator cabin from traveling farther downward.
In other words, when the lift reaches its mechanical lower limit, it simply cannot continue descending.
However, the PLC does not necessarily know that the lift has physically reached this mechanical stop.
The PLC relies on electrical signals from sensors and limit switches.
For example, the control logic may work approximately like this:
Second Floor → Down → First-Floor Limit Detected → Stop → First-Floor Position Confirmed → Open Door
The first-floor limit switch therefore needs to send its signal before the lift reaches the mechanical end of travel.
If the lift reaches the mechanical stop first but the first-floor limit switch has not yet been activated, the PLC may continue to believe that the lift is still traveling downward.
What Was Wrong With the Limit Switch Installation?
After analyzing the customer's installation, we found that the first-floor limit switch had been installed too low.
The position of the limit switch was very close to, or below, the actual mechanical stopping position of the lift.
This meant that the sequence was effectively:
Lift Descends → Mechanical Lower Limit Reached → Lift Stops Mechanically → First-Floor Limit Switch Still Not Activated
The lift could no longer move downward, but the PLC had not received the first-floor position signal.
From the PLC's perspective, the lift had not yet reached the first floor.
This explains why the system could continue displaying a downward status and why the landing door did not receive the correct condition to open.
Why the Landing Door Did Not Open
The landing door control is normally related to the confirmed floor position.
For safety reasons, the system should not simply open the landing door because the lift has stopped somewhere near the bottom.
The control system needs to confirm that the lift has actually reached the correct floor position.
A simplified control sequence could be:
Down command → Lift descending → First-floor limit activated → Down movement stopped → First-floor position confirmed → Door opening permitted
In the customer's case, the third step was missing.
The lift physically stopped, but the first-floor limit signal was not activated.
Therefore, the PLC did not receive the necessary confirmation that the lift had arrived at the first floor.
As a result, the landing door remained closed.
The Solution: Move the First-Floor Limit Switch Upward
We recommended moving the first-floor limit switch upward.
The objective was very simple:
The lift should activate the first-floor limit switch before it reaches the mechanical lower limit.
After adjustment, the sequence became:
Lift Descends → First-Floor Limit Activated → PLC Recognizes First Floor → Down Movement Stops → Mechanical Stop Remains as Final Protection
This is the correct relationship between the position limit and the mechanical stop.
The limit switch should provide the normal stopping and position-detection signal, while the mechanical stop should serve as a final physical protection against excessive downward travel.
After the customer adjusted the position of the first-floor limit switch, the problem was completely resolved.
The PLC correctly recognized the first-floor position, the downward status ended normally, and the first-floor landing door could operate as expected.
A Simple Way to Understand the Problem
Think of the system as having two different "stopping points."
1. Control stopping point
This is determined by the limit switch.
The PLC receives the signal and understands:
"The lift has reached the first floor."
The controller can then stop the movement and continue with the next operation, such as enabling the landing door.
2. Mechanical stopping point
This is the physical end of travel.
It prevents the platform or cabin from moving any farther.
It should not normally be the point at which the PLC first discovers that the lift has reached the floor.
The ideal relationship is:
First-Floor Limit Switch → Mechanical Lower Stop
Not:
Mechanical Lower Stop → First-Floor Limit Switch
The first-floor position signal should occur first.
Troubleshooting Checklist
If a lift reaches the first floor but the PLC still shows "Down," the following items should be checked.
1. Check the physical position of the lift
Confirm whether the platform or cabin has actually reached the intended first-floor level.
2. Check the first-floor limit switch
Manually inspect whether the mechanical actuator is being contacted by the lift at the correct position.
3. Check the PLC input
Look at the PLC input status and confirm whether the first-floor limit switch changes from OFF to ON when the lift reaches the first floor.
This is especially useful during commissioning and troubleshooting.
4. Check the installation height
Make sure the first-floor limit switch is positioned high enough that it is activated before the lift reaches the mechanical lower limit.
5. Check the wiring
If the mechanical position is correct but the PLC input does not change, check:
-
Limit switch wiring
-
Terminal connections
-
Normally-open / normally-closed configuration
-
PLC input voltage
-
Loose or damaged cables
6. Check the PLC logic
If the limit switch is physically activated and the PLC input changes correctly, but the system still shows "Down," the problem may be in the PLC program.
The first-floor limit signal should be correctly incorporated into the stopping and floor-position logic.
Why This Small Adjustment Matters
A limit switch is a relatively small component, but its installation position can have a significant effect on the entire control system.
The lift itself may appear mechanically normal. The hydraulic system may work correctly. The motor and pump may also work normally.
Yet a small positioning error in one limit switch can cause:
-
Incorrect floor detection
-
Incorrect movement status
-
Failure to stop through the intended control signal
-
Landing door operation problems
-
Confusing PLC input/output behavior
-
Difficult commissioning and troubleshooting
This is why limit switch positioning should be considered part of the overall control-system design rather than simply a mechanical installation detail.
Final Takeaway
When troubleshooting a lift that physically reaches the first floor but still indicates "Down," do not immediately assume that there is a hydraulic or PLC programming problem.
First, check the relationship between the first-floor limit switch and the mechanical lower stop.
The first-floor limit switch should normally be activated before the lift reaches its mechanical end of travel.
Once the limit switch is correctly positioned, the PLC can recognize the first-floor position at the correct time, stop the movement through the intended control logic, and allow subsequent operations such as landing-door opening.
In this customer case, simply moving the first-floor limit switch upward solved the problem completely.
A small limit-switch adjustment can sometimes solve what initially looks like a complicated PLC or elevator control problem.
For a related design decision on how our hydraulic home lifts handle final limit protection, read: Why We Eliminated the Upper and Lower Final Limit Switches in Our Hydraulic Home Lifts.
0 comments