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ACR1 autochange rack

The ACR1 autochange rack is the functional part of the system which is located within the CMM working volume.  It is powered and controlled by the ACC2-3 controller via a rack cable which can be up to 30 metres (98 feet) long.

The ACR1 stores up to eight 25 mm diameter autojointed probe and extension bar combinations which can be exchanged automatically by the CMM.  The ACR1 can be mounted in either the horizontal or vertical orientation depending on the application.

The two cut-aways on the ACR1 are to allow larger diameter probes to be used, such as SP600M.  In this case special short ports are required making the ports either side of the probe redundant (or with limited use).

A full set of short ports (i.e. eight) are supplied with the ACR1 allowing the rack to be fully customised.

Datum probe

The rack includes a fixed touch-trigger datum probe at its left end which incorporates a datum stylus and an alignment stylus.  This datum probe is used to align the probe head to the ACR1.

Datuming the probe head autojoint on the datum stylus relates the position of the head to the rack, and the location of the probe configurations held within the rack.  The alignment stylus is included to assist with the head alignment.

Sensing system

The rack performs probe change routines using light beams to detect the probe head and includes motor-driven screwdriver blades to lock and unlock the probes.  The light beams are located in front of and behind the probe docking port lids, and a light beam sensing system determines which function to perform by monitoring which light beam is broken first.

If the front beam is broken first, this indicates to the system that the head is entering the port with either a probe or extension bar attached - the screwdriver blades will be set to the locked position to enable the head to dock correctly.

If the rear beam (within the main unit) is broken first, this indicates that an empty probe head or extension bar is entering the rack - the screwdriver blades will be set to the unlocked position.  After both beams have been broken, the system waits for the appropriate command or time to lock or unlock the probe.


The accessories supplied with an ACR1 are a datum stylus, an alignment stylus, mounting screws, a stylus tool, eight short replacement ports, a S10 autojoint key and the necessary hexagonal wrenches.

 ACR1 autochange rack

Rack LED sequences

There are four LEDs on the rack which indicate the status of the probe change cycle (see table below).


LED off

LED on

Probe active

Change cycle

Cycle errorLock errorModes

Normal mode - touch probe disabled

Normal mode - touch probe enabled

Change cycle in progress

Error in change cycle *

Error in lock / unlock operation*

Datum mode 1 - touch probe enabled

Datum mode 2 - rack probe enabled

There is a limited ovetravel and crash protection facility built into the rack.  If it is deflected by more than 1 mm (0.039 in) it sends a STOP signal to the CMM control to stop the machine.

* See 'Fault finding' section for error mode analysis.