MLX90711 Datasheet, PDF (2/12 Pages) List of Unclassifed Manufacturers – Position/movement Sensing Auto-Shutoff

MLX90711
Position/movement Sensing Auto-Shutoff
Introduction
The Automatic Shut-Off is a safety system which
turns off the electrical power of a load based either
on a movement detection or a position detection.
This detection will be taken into account after a well
defined time delay. A typical application is the safety
feature in irons which will switch off the heating
element:
if the iron is connected to the mains and left
immobile in the ironing position for a time larger than
TH;
OR
if the iron is connected to the mains and left
vertically up on heel rest for a time larger than TV.
OR
if the iron is connected to the mains and left
vertically down on heel rest for a time larger than TD.
The heating of the iron is turned on again
immediately after detecting a different position (or
detecting a movement).
Sensing Principal
Melexis developed a new concept of Auto Shut-Off in
which the position or movement detection is realized
with a patented capacitive detection. This sensor
uses the movement of a ball, but the detection is
capacitive, and therefore, the system does not need
a conductive contact. Consequently, the system is
insensitive to erosion and dust.
The sensitivity of the asic is very high, and a ball with
a diameter as small as 3 mm can be used.
This detection system described in the following
section is protected by a patent (Patent European
application Nr. EP0589092; US application Nr.
US5627316).
General Description
The detection is done based on the ratio of two
variable capacitors. These capacitors can be the
parasitic capacitances of a small conductive ball
placed on a cavity in the pcb. As the pcb is rotated in
its lenghty direction, the ball is running forwards and
backwards on the pcb. Copper tracks placed on the
PCB at the edges of the hole will be used as the
terminals of capacitors. We can define two
capacitors made up of 2 copper tracks with a
(conductive) ball in between. If the ball rolls just
between 2 copper tracks, the distance between the
copper tracks and the ball will decrease, thus
increasing their capacitance. The circuitry on chip
performs a measurement of the difference between
these 2 caps. It controls the sense terminals (CA0,
CA1, SUM), processes their data and drives 2
outputs: RD (to drive a relay) and LEDP (to drive an
led). The information from the detection system
results in a momentary down, up or horizontal
position. This information can be used to generate a
position detection or can be interpreted as a
movement detection.
When used as a position sensor, the output of the
sensor consists of 3 digital signals, indicating the
position of the sensor: the ball is in the middle
(horizontal), is at the front side (down) or is at the
end (up). The position of the ball is sensed every
15.6 msec. For this the analog output voltage of the
sensor circuit is sent to 3 comparators with
hysteresis. If the result is equal for 3 consecutive
samples, the corresponding digital signal (DOWN,
UP or HOR) becomes valid. This signal is used to
evaluate the Auto Shut-Off function. The Auto Shut-
Off is active only when UP remains high during a
time period larger than TV or when HOR remains
high during a time period larger than TH or when
DOWN remains high during a time larger than TD.
The signals UP, HOR and DOWN are debounced.
When the Auto Shut-Off is active the relay driver is
activated. Using a NC (normally closed) type relay,
this can switch off the power of the heating element.
When the module is moved, the ball can roll forwards
and backwards in such a way that the states UP,
HOR and DOWN never remain stable for a time
more than TV, TH and TD respectively and the relay
output is not activated.
As long as the vertical inclination is enough to keep
the ball in the "vertical" area, the state UP remains
active.
When used as a movement sensor the output of the
sensor has only one signal which indicates that the
position of the ball has changed. If this signal is
inactive for the predefined time period TASO, the
chip enters the Auto Shut-Off state. The position of
the ball is sensed every 15.6 msec. When the Auto
Shut-Off is active the relay driver is activated. Using
a NC type relay, this can switch off the power of the
heating element.
When the module is moved, the ball can roll forwards
and backwards and due to this the chip continuously
detects a change of state of the sensor.
Power On Reset
In the chip there is a block "Power On Reset" (POR),
which tracks the level of the power supply voltage
Vdd1 and defines the state of the chip.
If Vdd1 is less than the high POR level (PORHI) after
initial plugging into the mains, the chip is in the "reset
state". When Vdd1 becomes higher than the PORHI
level, the operation of the chip starts: start up
sequence, heating up and then the chip enters in
"normal operation mode".
If Vdd1 becomes less than the low POR level
(PORLO), (PORLO=PORHI-HYS), for more than
7.8msec the chip enters the "reset state".
MLX90711 Position/movement Sensing Auto-Shutoff Page 2
Rev 1.0 15/Sep/00