Deadband Removal

This page specifically refers to modifying pre issue 15 boards.

With issue 15, there is an on-board jumper to alter the deadband.

With issue 17 boards, the half-speed reverse feature was changed to make the response more linear. This removes a good part of the need for the reduced deadband feature.

If in doubt, leave the deadband header in the factory default position.


There is a ‘dead band’ in the controller at zero speed: you have to advance the throttle from zero by a finite amount before anything happens at the output.

Be aware that there may be other ‘dead bands’ in the system, for instance all potentiometers have a dead band at their bottom, where there is no electrical effect. See Pot Dead band for more on this.

In some applications a controller deadband is beneficial, but in a growing number of applications, it is less desirable. For instance, in Robots a dead band is not desirable.

Fortunately the controller’s dead band may be quite easily removed by a small alteration to the top board of the controller.

The modification is in two parts: the first part is the modification proper and the second part should only be done where the controller is used in Single-ended mode.

The effect

This description applies to controllers with top board issue 16 or earlier (but see also Scope of the modification, below).

The graph shows the output, plotted against input. These graphs are with half speed reverse engaged and with the gain set so that 10v input gives 30v output.

Trans
The pecked lines show the standard response: the dead band is about 17% of pot travel if the gain is set so that full pot travel corresponds to full battery voltage output. If the gain is adjusted otherwise, deadband will be affected.

The solid lines show the response after the modification described herein, Note that the effect of the modification is particularly noticeable on the performance when half speed reverse is in effect (or when the gain is reduced, so the controller’s full output is not used).


The modification

The modification is in two parts and is very simple. The first part is done in the area of the top (control) board near the Gain control (marked G). The second part is only to be done when the board is used in single ended mode: do not do this part if you are using the board in joystick mode (set by the J link on the board).

Preliminary

  • Disconnect the battery.
  • Remove the controller’s cover
  • Unscrew the four corner screws that hold the top board in place.
  • Unplug the 6 way input connector
  • Unplug the 14 way ribbon connecting together the two boards.

First part: all uses

Cut through the track indicated in red and link with a thin piece of insulated wire as shown by the blue line.

Gainmod

What it does

Originally the circuit that controlled the ramps was ‘passive’ – inserted as a follower between the input circuit and the speed control proper. The dead band is introduced in this ramp circuit and the gain is adjusted before it, in the input circuit. So the dead band actually remains constant, but the amount of input needed varies with the gain control. In reverse (where gain is halved) you needed twice the input before the controller responded.

This modification alters the negative feedback loop (in which the gain control operates) so that the ramp circuit is now inside this loop and the negative feedback automatically controls the ramp output rather than the input to the ramp circuit. Therefore the dead band in the ramp circuit is effectively removed by the negative feedback.

Second part: single-ended mode only

Reversing works by detecting either a signal on the reverse input or a reverse signal in the joystick circuit. Unfortunately analogue ‘or’ circuits are not as simple as digital ones and if you do not do this mod, then, when revere is selected, there is some feed from the reverse input to the speed. That did not matter previously as it was well within the old dead band – which you have just removed!

The diagram shows the area of the board just behind the 6 way input connector.

IC1
Locate the 2K2 (red – red – red – gold) resistor beside IC1, arrowed in the diagram. Cut (or unsolder) the lead at the end arrowed in red. Do not disturb the other end of the resistor as it is a feed-through connection which must remain intact.


Scope of the modification

This modification may be applied retroactively as required to top board issues 3 through 14 on all 4QD series controllers. Issue 14 controllers supplied from the factory in 2003 will have this already modified if appropriate and it is permanently fitted on the top board from issue 30-15-15 onwards as two 3 pin ‘jumper links’ – effectively one for each drawing above.

With issue 17, although the jumpers are still present, the way of doing half speed reverse was changed so the speed is consistently halved over the whole range so modifying the dead band is unlikely to be useful.