The drive unit
29
If the speed of the chair exceeds 0.2 - 0.3 m/s, the idler will no longer be able to follow the shape of the teeth and it
will come away from the top of the tooth. The idler will then not have sprung back in time to let the next cam pass.
The idler will then be seized by a cam in the cam ring and forced against a locking pin (
5
) by the edge of the idler.
The sideways movement operates a switch mechanism (
7
) that is detected by an opto coupler (
6
). As a result of
this, the control board will initiate an emergency stop sequence. The drive motor will stop and the brake will be
activated. All of this happens in a fraction of a second.
At that moment the idler forms a mechanical stop between the cam wheel and the locking pin, so that the over
speed gearwheel cannot turn any longer. Because the over speed gearwheel also engages in the rail rack, the seat
will come to a halt.
Emergency end stop
The idler can also be pushed back by the emergency limit switch (
8
). This is operated by the emergency end stop
located on the rail. If, in case of technical problems, the drive unit goes past the stopping position at the end of the
rail, then the emergency limit switch comes into contact with the emergency end stop. The emergency limit switch is
moved so that the idler is pushed outwards.
A similar situation now occurs as for the exceeding of the maximum speed. A mechanical lock is created and the
power circuit to the drive unit is interrupted to prevent the seat riding off the rail. The emergency limit switch must be
manually moved back to the centre position to allow the drive unit to move again. This can also be established by
using mode -3-.
4.1.3
The horizontal system
Construction
The horizontal system keeps the seat horizontal by correcting for the rail’s angle of inclination. Via a double worm
wheel transmission, a motor drives a gear that is directly connected to the horizontal shaft of the chair assembly.
The transmission is constructed in such a way that the seat can never tip on its own when the horizontal motor is not
running.
The motor must deliver the required force to keep the chair horizontal in a comfortable way during travel. A
brushless DC motor is used for this job.
The motor is part of a control loop in which two sensors (gyro and chair sensor) measure the position (angle) of the
chair’s seat as compared to the set horizontal position. A third sensor (safety sensor) is used for checking the chair
sensor and the absolute angle.
