Reverse Acceptance Threshold [RAT] Modification

This modification applies to 4QD series controllers, top boards issues 4 through 8.

As the voltage to a motor is reduced, it will slow down and when the voltage gets low enough, the motor will stop. However – this ‘stop’ voltage is not zero but some small voltage which depends on the motor and the rolling friction. The zero-speed voltage also depends on the gradient of the slope on which the vehicle is moving.

When the reverse switch is operated while the vehicle is moving, the controller automatically slows down, reverses and starts up again (dual ramp reversing). Clearly, to reverse, it is not necessary to slow to zero motor voltage and, by altering the threshold at which reverse occurs, the vehicle’s performance can be altered.

This is the function of the R.A.T. control. It also controls the level at which the electromechanical parking brake power is released. There is also a small time delay on the parking brake.

It is adjustable between two internal thresholds

To our knowledge, no customer has ever bothered to adjust it from the original preset minimum level – so it has turned out to be a good idea which has not proven very popular. However on certain vehicles it has been found that the lower threshold is too high and the parking brake is coming on while the vehicle is still moving – logically this will occur on 48v vehicles (where the threshold corresponds to a higher motor voltage) or on low rolling friction vehicles where the motor stops at very low voltages. On some of these it has indeed been found that the brake comes on when the vehicle is still moving.

The top board of the controller can be modified so that the RAT adjustment minimum is zero armature voltage: details follow. These apply specifically to issue 8 boards (30-15-08) but details are nearly identical back to issue 4. Issue 9 boards are already changed to the new standard.

You will need a 10K preset: we use Piher type PT10H for the modification but any preset that will fit the same holes as the existing one will suffice. You will also need a 2K2 resistor: a miniature (CR16) size is neater, but a standard CR25 size will suffice.

rat modification
Disconnect the battery. Remove the controller top board (4 corner screws and a 14 way ribbon). The diagram below shows the corner of the board with components. The RAT control is a small blue preset (2K0), arrowed. Remove it.

Fit the new 10K preset in its place.

drawing of track modifications
The second diagram shows the solder side in the same corner of the board.

  • Cut the track arrowed A.
  • Solder a 2K2 resistor between points arrowed B. This is effectively across the points where the preset track was connected, to replace the 2K of the preset.
  • Link the ‘free’ end of the preset (arrowed C) to the 0v line which runs around the periphery of the board (E).
  • Check your work is correct. Check also for poor joints and shorts to adjacent bits of track.
  • Optionally, cut the track to the top end of the preset (lower ‘B’ arrow) and fit a 22K resistor across this cut (i.e. in series with the top of the preset).

If you have a voltmeter, the voltage on the wiper of the preset (to the right of the copper ‘T’) will now adjust betwen zero and about 700mV (or zero and 240mV with the extra 22K resistor in circuit). On the original circuit the lowest setting was about 240mV – which was sometimes too high, so you can now set the threshold to less than 240mV: take a guess depending on how fast your machine was running when the brake came on. Too low a setting and your machine will run-on or roll back on a hill.