Method and apparatus for accurate positioning of printing units

1. A method for accurate positioning of a unit movable by a drive system, the method comprising:

adjusting tension of a resilient element of the drive system such that hysteresis motion errors of the resilient element remain below a predetermined threshold, wherein adjusting comprises:

measuring, for two or more tension values, the hysteresis motion errors of the resilient element; and

selecting a tension value for which a hysteresis motion error is below the threshold.

2. The method of claim 1 , wherein measurements of hysteresis errors at two different tension values are used to predict a desired tension value which corresponds to a reduced hysteresis error.

3. The method of claim 1 , wherein measuring comprises:

instructing the drive system to move the unit in a first direction to a first end position;

instructing the drive system to move the unit in a second direction, opposite the first direction, to a desired middle position;

measuring a first middle position to which the unit has arrived;

instructing the drive system to move the unit a first distance in the second direction to a second end position;

instructing the drive system to move the unit the first distance in the first direction towards the desired middle position;

measuring a second middle position to which the unit has arrived;

calculating the distance between the first middle position and the second middle position; and

changing the tension of the resilient element if the distance between the first middle position and the second middle position that represents the hysteresis error is above a predetermined threshold.

4. The method of claim 3 , wherein measurements of hysteresis errors at two different tension values are used to predict a desired tension value which corresponds to a reduced hysteresis error.

5. The method of claim 3 , wherein the resilient element is a belt movable by a drive wheel.

6. The method of claim 5 , wherein the unit is attached to the belt.

7. The method of claim 1 , wherein the unit is a printing block.

8. A method for reducing hysteresis errors in a drive system having a resilient element, the method comprising:

receiving a request to move a unit connected to the drive system a desired distance, smaller than a first threshold and larger than a second threshold, from a first position to a second position in a desired direction opposite to a direction of a previous movement command; and

generating a first and second consecutive commands, the first command is to move the unit in a first direction and the second command is to move the unit in a second direction opposite to the first direction and the first and second commands are to move the unit a distance larger than the first threshold, such that the sum of the first and second commands is equal to a single command to move the unit the desired distance in the desired direction.

9. The method of claim 8 , wherein the first threshold is between 40 millimeters and 60 millimeters.

10. The method of claim 8 , wherein the second threshold is between 1 millimeters and 3 millimeters.

11. The method of claim 8 , wherein the first direction is the same as the desired direction.

12. The method of claim 8 , further comprising:

receiving a request to move the unit a desired distance larger than the first threshold from a third position to a fourth position in a direction opposite to the direction of a previous movement command; and

adjusting a command to move the unit the desired distance using an offset parameter.

13. The method of claim 12 , wherein the offset parameter corresponds to a backlash correction parameter.