An access control device arranged to conditionally authenticate a user for access to a physical space, the access control device comprising: a processor; a fixed sensor that is mounted on a fixed physical structure that is moveable in a restricted way, such that the fixed sensor does not have free movement in relation to the physical space: and a memory storing instructions that cause the access control device to detect a user in the proximity of the access control device and, when a user is detected in the proximity of the access control device, cause the access control device to perform the consecutive steps of: obtain first sensor data being based on a mobile device of the user sensing a tap of the user on the fixed physical structure in the vicinity of the fixed sensor; obtain second sensor data being based on the fixed sensor sensing a vibration corresponding to the tap of the user; determine whether the first sensor data matches the second sensor data by determining whether the first sensor data reflects the tap of the user and corresponds to the tap of the user also reflected in the second sensor data, wherein the first sensor data matches the second data only when a time difference between corresponding taps is less than a threshold time difference; and send an unlock signal to a physical lock device, connected to the access control device, when the first sensor data matches the second sensor data. a processor; a fixed sensor that is mounted on a fixed physical structure that is moveable in a restricted way, such that the fixed sensor does not have free movement in relation to the physical space: and a memory storing instructions that cause the access control device to detect a user in the proximity of the access control device and, when a user is detected in the proximity of the access control device, cause the access control device to perform the consecutive steps of: obtain first sensor data being based on a mobile device of the user sensing a tap of the user on the fixed physical structure in the vicinity of the fixed sensor; obtain second sensor data being based on the fixed sensor sensing a vibration corresponding to the tap of the user; determine whether the first sensor data matches the second sensor data by determining whether the first sensor data reflects the tap of the user and corresponds to the tap of the user also reflected in the second sensor data, wherein the first sensor data matches the second data only when a time difference between corresponding taps is less than a threshold time difference; and send an unlock signal to a physical lock device, connected to the access control device, when the first sensor data matches the second sensor data. obtain first sensor data being based on a mobile device of the user sensing a tap of the user on the fixed physical structure in the vicinity of the fixed sensor; obtain second sensor data being based on the fixed sensor sensing a vibration corresponding to the tap of the user; determine whether the first sensor data matches the second sensor data by determining whether the first sensor data reflects the tap of the user and corresponds to the tap of the user also reflected in the second sensor data, wherein the first sensor data matches the second data only when a time difference between corresponding taps is less than a threshold time difference; and send an unlock signal to a physical lock device, connected to the access control device, when the first sensor data matches the second sensor data.