Digital Indeed...

For the CGI double width page pack, why do we say it is digital?

The CGI page packs use three principal technologies which when combined; result in an extremely precise delivery of ink.

First, the page pack incorporates a simple piston and barrel combination that when rotated by the prime driver displaces (pumps) ink in a predictable manner. The piston/barrel arrangement combines to produce a fixed amount of ink for each revolution of the prime driver. The prime driver is a stepping motor. The construction of the page packs determine the amount of ink which will be delivered for each revolution of the piston. This accuracy of construction and the resulting tolerances create accuracies of +/- .05 of standard densities.

Second, the page pack utilizes a special class of motor called a stepping motor. The stepping motor consists of a wound stator (field windings) and a magnetized rotor. This motor is unique in that the rotor has 400 discreet positions as it moves through one revolution. The motor contains no commutaters nor does the motor require periodic maintenance.

The rotor may occupy only one of these 400 locations. In this way, a wave drive signal is capable of causing the motor to move a discreet number of steps. Knowing the number of steps the motor has moved allows one to know the exact location of the motor rotor. In this way the motor moves in a digital way, one step at a time. This motor is not referred to as an analog motor as one cannot impress a simple voltage to cause rotary motion similar to an AC or DC motor. The stepping motor is not a speed device but rather a precise position device.

Third, the stepping motor movement is governed by a CPU. The CPU simply generates a series of step commands to the wave generating circuits. The wave generating circuits cause the motor to advance one position at a time out of the 400 possible steps or positions.

The CPU keeps track of the number of steps the motor has been commanded to make and therefore knows the amount of ink delivered. The system works completely in the digital domain. At no time are the line shaft pulses converted to an analog value. The CPU, stepping motor and piston all move in a step like or digital manner. This can best be seen by the jittery motion of the pistons when at work.

To further this statement, if you were to place all of the cranks at the same phase angle and then apply a continuous line shaft signal to the system, you will observe that the cranks never fall out of phase with each other as they rotate. Only a digitally controlled system could produce this type of angular accuracy.