6
2
AND
4
D
IGIT
Q
UADRATURE
C
OUNTER
O
PERATIONAL
M
ODES
The modes of operation describe the behaviour of the display when the count is below zero or
above the maximum number the 2 or 4 digit display is able to convey i.e. 99 and 9999
respectively. Counts below zero are only applicable to a system using Quadrature inputs which
has the ability to turn in both directions, however the behaviour of the counter after the full scale
of the display has been exceeded will still be important.
Q
UADRATURE OR
S
INGLE
I
NPUT
It is possible to use both sensor inputs if a quadrature input is required. Alternatively either one
of the inputs may be used if a quadrature input is not required.
W
HAT IS
Q
UADRATURE
?
A quadrature sensor input arrangement allows the system to monitor 2 inputs and determine the
direction of travel whenever a sensor changes state. This can relate to rotation or linear
movement depending on the mechanical system and sensor arrangement.
The sensors and sensor triggers are arranged such that after a sensor changes state, the other
sensor must change state next if movement continues in the same direction. If the direction of
movement changes and movement continues, the last sensor to change state will revert to its
state prior to the previous change of state next time it is triggered.
For a graphical representation of this see the section on Quadrature Input Decoding.
This can be achieved in a number of ways such as:
•
A wheel or rail with a number of magnetic targets at regular intervals
•
A wheel or rail with a number of teeth at regular intervals
•
A helical / threaded drive shaft with a wheel as described above
It is very important to get the arrangement of sensors and targets correct. If a single sensor
changes state twice before the other sensor changes state, or is intermittent, then the count will
be unreliable.
Correctly implemented Quadrature schemes are inherently immune to noise / contact chatter.
U
NIPOLAR
I
n Unipolar mode the display is always positive as indicated by the sign indicator on the display.
As the count is decremented, one less count than zero be comes ‘+9999’ and ‘99’ on the 4 and 2
digit displays respectively. Similarly as the count is incremented one more count than the +9999
or 99 b0 or 0.
B
IPOLAR
In Bipolar mode, the display with one count less than zero is –1, and with one more count than
+9999 is ‘o000’ which represents an overflowed zero equivalent to 10,000 which cannot be
displayed on a 4 digit display. More counts in the positive direction will yield –9999 on the
display followed by –9998, -9997 etc through –1, 0, +1, +2 and up to +9999 again. Decrementing
the count through zero from say +5 will yield +4, +3, +2, +1, 0, -1, -2, -3, -4 etc.
