FANUC direction-dependent high-speed position switches: why one coordinate can give different outputs
Using the 30i/31i/32i-B direction-dependent high-speed position switch, compare range checks with crossing events. Following output state along an outward-and-return path shows why current position alone need not determine the current switch value.
Contents
If an axis stops twice at the same position and a position-switch output is 0 once and 1 the other time, coordinate detection is not necessarily unstable. For a direction-triggered high-speed position switch, output also depends on which trigger point was crossed previously.
Checking a position range and recording a crossing are different mechanisms
The FANUC 30i/31i/32i-B descriptions manual B-64482EN/03, §1.48–1.50, page 68 (public mirror of the original manual) distinguishes ordinary position switches from direction-dependent high-speed position switches. The latter monitor coordinate and direction: crossing one coordinate in its specified direction sets output to 1; crossing another in its specified direction sets it to 0. The corresponding high-speed position-switch and direction-dependent functions must be configured. The word high-speed alone does not establish the decision logic.
An original event model: both trigger points use positive travel
Assume a positive crossing of X=40 mm sets output to 1, and a positive crossing of X=80 mm sets it to 0; initial output is 0. Only these two events are tracked below, with no reset, mode change or other state intervention along the way:
| Movement | Event matching the specified direction | Output after movement |
|---|---|---|
| 0→30 mm | None | 0 |
| 30→50 mm | Positive crossing of 40 | 1 |
| 50→30 mm | Reverse crossing of 40; not a configured event | Still 1 |
| 30→90 mm | Positive crossing of 40, then of 80 | Finally 0 |
Output is 0 on the first arrival at 30 but 1 on the second arrival at 30. Both coordinates are correct. The difference is whether a set-to-1 event occurred previously and whether a set-to-0 event has occurred since.
Why this cannot be reduced to output 1 between 40 and 80
A direction-independent interval check would naturally put X=30 outside the range and yield 0 both times. In the event model, crossing 40 in reverse does not encounter a qualifying reset event, so output remains 1. Confusing these mechanisms readily leads to the mistaken question of why the signal remains active after the axis has left the position.
A single coordinate screenshot therefore cannot validate this signal's state. At minimum, include the preceding set and reset events and intervening movement directions in one path history. Also distinguish the CNC output value from the value after subsequent PMC logic.
High speed does not mean zero delay or a safety switch
The high-speed function changes monitoring and output capability, but cannot make external sampling, PMC processing and actuator response instantaneous. This state table describes logical order only; it assumes no particular sampling period or delay. Analysing the difference between trigger position and external action requires separate interface and timing evidence.
Nor does the name alone make this position switch a safety-certified personnel-protection signal. This article supplies no signal-forcing or parameter-rewriting steps. More monitoring-function resources are in the OPTION section.