Limit switches

The x- and y-frames have IR or Hall effect magnetic sensors for determining the x- and y-positions of the tool. In addition, a pair of microswitches mounted on the main frame are used as limit switches. These detect when the x- and y-frames are in their base positions. They reliably put the tool at the front right corner of the frame. From there it uses the IR or Hall effect sensors to find its way to any other place in the working area. In other words, the limit switches provide a starting point to return to if the software loses count of the markers.

The limit switch for the y-direction is microswitch MS1. This is bolted at the top of the front right leg so that the y-frame closes it when it comes to the front limit of its travel.

Both MS1 and MS2 are closed when the x-frame is at the front right corner of the main frame.

MS2 is bolted underneath the right side-rail of the main frame. A finger projects from one end of the x-frame (see opposite). The finger is shaped so that it projects under the end of the y-frame to contact the microswitch on the main frame. Its vertical end presses against the lever of MS2 as the x-frame reaches the right end of its travel.

The finger attached to the x-frame reaches down beneath the right side-rail of the main frame. Its upturned end presses against the actuator lever of MS2 when the y-frame is at the front and the x-frame is at the right. Determine the dimensions of the finger by measurement when the x-frame is in position.

The finger attached to the x-frame reaches down beneath the right side-rail of the main frame. Its upturned end presses against the actuator lever of MS2 when the y-frame is at the front and the x-frame is at the right. Determine the dimensions of the finger by measurement when the x-frame is in position.

This arrangement means that the x-frame can close MS2 only when the y-frame is in its base position. Software can take care of this requirement — first move the y-frame until M1 closes, then move the x-frame until M2 closes.

Note that M1 and M2 are both on the main frame. Their leads run directly to the PIC1 board, from where the winch motors are controlled. If the gantry is not to become a birds' nest of connecting wires tangling with the x- and y-frames we need to keep the number and length of connecting wires to a minimum. This is a major reason for having the up/down tool winch on the y-frame, rather than on the x-frame where its action takes effect. For the same reason, MS1 and MS2 are on the main frame, only a few centimetres away from the PIC1 processing board. Power supply for PIC2 is a problem. Do we have a battery on the x-frame (taking up space, adding weight and increasing friction) or do we run three wires from the main frame (tangling risk and impeding motion)? We eventually settled for the second alternative.

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