epc110
Pulse Modulated Operation (e.g. OUTN
int
to OUTN)
The epc110 chip operates the LED and the receiver path on a pulse modulated concept. Thus, the LED is operated with short pulses whereas
the receiver channel does synchronous demodulation of the received light pulses by reading the current pulses of the photo diode. This
concept allows a very high sensitivity, high speed operation, and a high suppression of input ambient or foreign light (DC currents) generated
by sunlight or other DC light sources like light bulbs. The pulse length is set by the parameter TPULSE.
In the standalone mode the whole system is clocked by the scan period defined by the programmed parameter TPER. In the controlled mode
the timing is propagated from the external sig nal EN (positive edge, synchronized on the internal clock).
In order to eliminate interference caused by modulated light, e.g. a flashing light or by other light barriers, the input signal from the photodiode
is amplified, filtered, and processed by an integrated signal processor. If the photo diode signal meets the required frequency, pattern and
amplitude, the output(s) are triggered. The following timing diagram shows the basic concept.
t
Pulse
Emitter LED
Beam interrupted
on
off
yes
no
t
Cycle
Photo diode current I
PD
1
OUTN
INT
or OUTH
INT
OUTN or OUTH
[V]
t
F
Release time
missing pulses
t
R
Response time
valid (non-missing) pulses
2
n
M
1
2
n
V
Figure 6: Pulse modulation concept
It is in fact a digital filter which counts missing and non-missing pulses to change the ou tput state of OUTN or OUTH.
Working principle of the digital filter e.g. for the signal OUTN
int
to OUTN Filter:
The aim of this programmable filter is to suppress single pulses, so they cannot trigger the output and generate a false signals.
This filter is based on a counter, which is counting up (increment) the valid pulses and counting down (decrement) the missing pulses in a
weighted manner. Separate weighting factors can be selected for valid pulses (parameter INC) and missing pulses (parameter DEC). If the
counter reaches the upper limit (maximum count, response time), the signal OUTN is set to LOW. Similar in the opposite direction, if the
counter reaches zero, the lower limit (minimum count, release time), the signal OUTN is put to HIGH. With the parameters INC and DEC the
filter has the advantage of individual selectable gradients of the slopes. Counter will never exceed maximum nor minimum limit. In between it
acts as an integrator of both parameters.
IF Pulse then
–
IF Pulse = valid then
Counter = Counter + (INC * 1024 for INC > 0 and 32 * 1024 for INC = 0)
IF counter > 2
15
(maximum limit) then Counter = maximum limit
IF counter = maximum limit then OUTN = 0
IF Pulse = missing then
Counter = Counter - (2
DEC
)
IF counter < 0 (minimum limit) then Counter = minimum limit
IF counter = minimum limit then OUTN = 1
–
ELSE wait for Pulse
Lets assume that the photo diode does not receive light pulses for a long time: This means the light beam is interrupted. Then OUTN is at high
level. If the light beam is not anymore interrupted, the photodiode receives light pulses which are strong enough to trigger the OUTN
INT
threshold and the internal pulse evaluation unit (designated in Figure 5 with 'Pulse evaluation') starts to count the receiving pulses. If the
number of received pulses reach the set level n
V
, the output OUTN turns to low level. Thus, single pulses cannot trigger the output and
generate a false signal.
The same procedure is used when a beam changes from not interrupted to interrupted. The internal pulse evaluation unit counts the missing
pulses. If the number of missing pulses reaches the set level n
M
, OUTN is turned to high level.
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
8
Datasheet epc110 - V2.1
www.espros.ch
EPC [ ESPROS PHOTONICS CORP ]
