KERN & SOHN VFS 2T1 Addendum - Manual (Page 15)

EPM205 - Technical reference - Rev. A Page 15
Advanced Printing Systems
5. HOW TO OPTIMIZE SPEED, CONSUMPTION AND MAX. PEAK CURRENT
The printing speed is always a compromise between 3 parameters :
· Paper feed speed (function of voltage).
· Head activation time Ton (function of voltage and temperature).
· Maximum peak current available (function of voltage and maximum number of dots simultaneously
activated and directly related to power supply capabilities).
For a given voltage it is easy to determine the maximum paper feed speed (MaxPFS), as indicated on the
table of section 4.1. So, if the two others parameters (current and Ton) are not limiting, maximum paper feed
speed (MaxPFS) becomes equal to maximum printing speed (MaxPS).
MaxPFS gives a time (by inverting) known as S.L.T. (scanning line time). In this time, the head must be
activated. If this time is not long enough, MaxPS will be subsequently affected.
Then, the method of driving the head is a critical point in a thermal printing application. There are two basic
methods to limit the current in the head:
1. Divide the head into fixed blocks (by 64 dots for example) and use the strobe lines to control the blocks
to be activated. This method is easy implement, however the printing speed will be very slow because
the MaxPS will be the invert of the activation time multiplied by the number of blocks the head is
divided into.
Example: at 6 V with the EPM205, the activation time is 2.53ms*(5 2 /6 2 ) = 1.76ms. If the maximum
current available for the head is 2.4A, the maximum number of dots to be simultaneously activated will
be 2.4A/(6V/160Ω)=64 dots. So the number of activation per S.L.T. will be 384/64=6, giving a S.L.T.
of 6*1.76ms=10.6ms. Therefore, the MaxPS will be 1/(8*10.6*10
-3 ) = 11.8 mm/s. and MaxPFS is
62mm/s. So there is a big margin and the printing speed is relatively slow.
2. Divide the head dynamically performed by counting the number of dots actually activated. The software
counts the actual number of “black” dots while loading the print-head. When the number of black dots
reaches the maximum value (in this example the value will be 64) the software fills the remaining dots
with “0” and activates the strobes line. As a result the activation is the maximum number of black dots
allowed, by the given value, thereby optimizing the number of times the head needs to be activated.
Printing standard text, the average number of black dots is generally less than 64 and sometimes reach
128.
Example: referring to the above example, the MaxPS is multiplied by 6, or sometimes by 3. Let take
that 30% of the lines contains from 64 to 128 black dots, the average MaxPS will be 62*0.7 +
(11.8*3)*0.3 = 54 mm/s, getting very close to the MaxPFS, and optimizing all the parameters.