Current-Sensing End Stop
It is the method that uses the rise in the motor's current draw as a stop signal when the motion reaches the mechanical end stop. As the rotor slows, the back EMF falls and the current climbs; when the driver sees the defined threshold it cuts the output in that direction. AXI's AntiSwitch function is based on this principle and makes external limit switch wiring unnecessary.
The method works subject to two conditions. First, there must be a distinguishable difference between the normal operating current and the current at the moment of stalling against the stop. Second, the mechanics must withstand the stall force during the short time until the threshold is caught.
- Wiring benefit: no separate switch, cable and terminal are needed for each end point; site installation and the fault surface are markedly reduced.
- Direction independence: the load in the two directions is usually different (lifting up is heavier than lowering down). That is why on the PT500 the A1 and A2 pots offer independent threshold setting for the two directions.
- Its limit: if there is an obstacle mid-travel, the driver takes that for an end point too. Unexpected resistance appears as the motion being cut short.
In practice you calibrate the threshold so that it stays above the heaviest normal operating current but below the stall current. The aim is not for the end point to be a repeatable position but for the motion to end safely.
Context where this term is used: DC Motor Driver Selection Guide