9
From the instant that a persons foot
touches the mat to the instant that
dangerous motion actually ceases is
called the overall system response time.
The overall system response time, T, is
given by the following calculation:
T = t1 + t2
where t1 = the maximum response time
of the safety device between
the actuation of the sensor and
the generation of
the
stop
signal = 30ms
and t2 = the response time of the
machine between receiving a
stop signal from the safety
device and the dangerous
parts coming to rest.
The dangerous parts will obviously
continue to move during this time. The
sensor must therefore be dimensioned
such that the nearest point at which a
person could first touch the mat is at a
certain minimum distance from the
dangerous parts, to prevent the person
from reaching the dangerous parts
before they have stopped.
This must take into account the worst
case conditions illustrated in Figure 5
where a person could be a full stride
onto the sensor before the sensor is
actuated. This means that a certain
distance from the front edge of the
sensor and the nearest dangerous parts
must be maintained.
MINIMUM SEPARATION
DISTANCE 'S'
DANGEROUS
PARTS
WORST CASE
POINT OF
DETECTION
Figure 5
—
Minimum separation distance
BEST CASE
POINT OF
DETECTION
N
E
A
R
E
S
T
D
A
N
G
E
R
P
O
IN
T
F
R
O
N
T
E
D
G
E
O
F
A
C
T
IV
E
Z
O
N
E
