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6 CALIBRATION
6.1 Concept
Calibration is the procedure for determining the parameters needed to regulate the dispensing process, thus
granting machine accuracy.
The aim of calibration is to obtain a correlation chart between the volumes that we want to dispense and a
parameter that can be regulated by the machine. In this case the parameter that the machine can count is
the number of steps generated to move the stepper motor.
Below is a simplified explanation of a few of the theoretical concepts regarding the calibration of an
automatic dispenser with bellow pumps and stepper motor.
This model is based on the following hypothesis:
•
The pump has a constant volumetric efficiency (the volume dispensed for each step is constant) that
does not depend on the fluid characteristics;
•
The fluid is incompressible and there is no valve switching time.
Considering the above mentioned hypothesis, we can define the Resolution of a circuit as follows:
In carrying out this function in regard to the variable Number of steps, the result is:
The MPX is defined as the inverse of the resolution:
therefore:
At a certain speed, the number of steps (N) required to dispense a volume V of colorant is calculated by
multiplying the volume V by the MPX for that speed.
In other words, for an ideal circuit, given a volume of 1 cc to be dispensed, the number of steps to carry out
is equal to the MPX, that can be easily calculated considering the technical characteristics of the SAPS circuit
given in the section "Dispensing circuit".
MPX
(steps/cc)
In a real circuit, you must take into account a lot of deviations from this ideal model due to:
•
differences in the mechanical assembly of the SAPS;
•
rheological properties of the fluid;
•
compressibility of the fluid and his elasticity;
•
pressure needed to open the delivery valve and to close the suction valve.
All these deviations from the ideal model create the necessity of adding a new parameter to our model and it
makes also impossible to state that MPX is equal to 800 (steps/cc).
[
MPX
where Offset is a value, expressed in number of steps, that takes into account the delay that is present from
when the stepper motor starts to move and the fluid starts to flow out of the nozzle.
Version 1.0 - rev0 - May 2006
Dispensed
volume(cc)
Number
of
steps
Dispensed
volume(cc)
Resolution
(cc/steps)
1
Resolution
(cc/steps)
MPX
(steps/cc)
∗
Dispensed
8000
(steps
=
10
(theoretic
al
volume
(steps/cc)
∗
Dispensed
=
Resolution
(cc/steps)
=
Number
of
steps
=
MPX
(steps/cc)
volume
(cc)
=
Number
per
one
full
stroke)
displaced
by
one
full
]
volume(cc)
+
OFFSET
of
steps
=
800
(
steps
stroke)
=
Number
of
steps
6-3
/
cc
)
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