Bot position sensing

1. A storage and retrieval system comprising:

a storage structure having storage shelves, each storage shelf having slats for supporting stored items where the slats are spaced apart from each other by a predetermined distance;

an autonomous transport vehicle including at least one sensor configured to sense each of the slats and output a signal indicating when a slat is sensed; and

a controller for verifying a location of the autonomous transport vehicle within the storage structure based on at least the output signal and configured to compare a location of the autonomous transport vehicle at a time the slat is sensed with a predetermined location of the slat and updating a verified location of the autonomous transport vehicle if the locations substantially coincide.

2. The storage and retrieval system of claim 1 , wherein the controller is further configured to ignore the output signal of the at least one sensor where the locations do not substantially coincide.

3. The storage and retrieval system of claim 1 , wherein the controller is configured to continuously update an estimated location of the autonomous transport vehicle based on a last known verified location of the autonomous transport vehicle.

4. The storage and retrieval system of claim 3 , wherein the autonomous transport vehicle includes at least one wheel encoder and the controller is further configured to obtain wheel encoder information for updating the estimated location of the autonomous transport vehicle.

5. The storage and retrieval system of claim 1 , wherein the autonomous transport vehicle is configured to align transfer arm fingers of the autonomous transport vehicle with spaces located between the slats of a respective storage shelf based on the determined location of the autonomous transport vehicle for extending the transfer arm fingers into the spaces without contacting the slats.

6. The storage and retrieval system of claim 1 , wherein the autonomous transport vehicle includes a case unit detection sensor configured to detect case units located on the storage shelf and the controller is further configured to determine a position of the autonomous transport vehicle based on the sensed case units.

7. A storage and retrieval system comprising:

at least one multilevel vertical conveyor having at least one shelf having support fingers;

at least one wall adjacent the multilevel vertical conveyor, the wall including protrusions substantially aligned with the support fingers;

an autonomous transport vehicle including at least one sensor configured to sense each of the protrusions and output a signal indicating when a protrusion is sensed and wherein the autonomous transport vehicle includes a transfer arm having transfer arm fingers, the autonomous transport vehicle is configured to align the transfer arm fingers with spaces located between the support fingers of the at least one shelf based on the determined location of the autonomous transport vehicle for extending the transfer arm fingers into a path of the shelf without substantial contact with the supporting fingers; and

a controller configured to determine a location of the autonomous transport vehicle relative to the support fingers based on the output signal from the at least one sensor.

8. The storage and retrieval system of claim 7 , wherein the at least one shelf includes at least two item holding locations and the autonomous transport vehicle includes a transfer arm, the autonomous transport vehicle being configured to align the transfer arm with one of the at least two item holding locations based on the output signal from the at least one sensor.