Rockwell Automation Publication 440L-IN012A-EN-P - November 2017
9
GuardShield Type 4 and GuardShield Remote Teach
The ANSI Safety Distance Formula
The ANSI safety distance formula, which is the Rockwell Automation®
recommended formula, is as follows:
D
S
= K x (T
S
+ T
C
+ T
r
+ T
spm
) + D
pf
D
S
Minimum safety distance between the safe guarding device
and the nearest point of operation hazard, in inches.
K
Hand-speed constant in inches per second. The ANSI
standard value is 1600.2 mm (63 in.) per second when the
operator begins reaching toward the point of operation
hazard from rest.
NOTE
: ANSI B11.19 2000 E8.3.2.3 and
Annex D states “One of the accepted values for K is the hand
speed constant (it is considered as the horizontal motion of
the hand and arm while seated). The common value of K is
1.6 m/s (63 in./s). Although other values (typically greater)
are also used. The hand speed constant does not include
other body movements, which can affect the actual approach
speed. Consideration of the above factors should be included
when determining the speed constant for a given application.
The employer should consider all factors, including the
physical ability of the operator, when determining the value
of K to be used.”
T
S
Stop time of the machine tool that is measured at the final
control element. Measurement starts at maximum machine
velocity.
T
C
Response time of the control system
T
r
Response time of the presence sensing device (safety light
curtain) and its interface, if any. The device manufacturer
states the value or you measure it.
T
spm
Additional time is allowed for the stop performance monitor
to compensate for variations in normal stopping time Ts.
Check ANSI B11.19 Annex D for details.
D
pf
Depth penetration factor. It is an added distance to allow for
how far into the protective field an object, such as a finger or
hand, can travel before being detected. D
pf
is related to the
object sensitivity of the safety light curtain. Object sensitivity
(resolution) is the smallest diameter object, which is detected
anywhere in the sensing field.
Example (for US):
In a perpendicular safety-light curtain application with object sensitivity
(effective resolution) less than 64 mm (2.5 in.), the D
pf
can be
approximated based on the following formula:
D
pf
(inches) = 3.4 × (Object Sensitivity – 0.276),
But not less than 0.
For GuardShield Type 4 light curtain:
Object sensitivity = 30 mm (1.18 in.)
D
pf
= 3.4 x (1.18 - 0.276 in.) = 3.08 in.
T
S
= 241 ms = 0.241 s
T
C
= 10 ms = 0.01 s
T
r
= 20 ms = 0.02 s
T
spm
= 20% of Ts (calculated by the machine manufacturer) = 49 ms =
0.049 s
K = 63in./s (determined by the machine manufacturer)
D
S
= 63 x (0.241 + 0.01 + 0.02 + 0.049) + 3.08 in.
= 63 x 0.32 + 3.08 in.
= 20.16 + 3.08 in.
= 23.24 in. (600 mm)
European Safety Distance Formula
A safety distance must be maintained between the light curtain and the
point of danger. This safety distance confirms that the point of danger
can only be reached after the dangerous state of the machine has been
removed.
The safety distance as defined in EN ISO 13855 and EN ISO 13857
depends on:
• Stopping/run-down time of the machine. (The stopping/run-down
time is shown in the machine documentation or must be determined
by taking a measurement.)
• Response time of the protective device, for example, GuardShield
Type 4 light curtain (see
).
• Response time of the safety circuit:
• Reach or approach speed.
• Resolution of the light curtain.
Figure 11 - Safety Distance from the Point of Danger
WARNING:
Not including the response time of a device or devices
in the stop time calculation results in insufficient safety distance
for the application, which results in operator injury.
IMPORTANT
A stop time measuring device measures T
s
+ T
c
+ T
r
+ T
spm
.
IMPORTANT
An operator is not able to stand between the protective field
and the point of operation. EN ISO 13855 requires a maximum
distance “X” (see
) of ≤ 70 mm (2.76 in.)
at an installation height of 300 mm (11.9 in.). Check EN ISO
13855 for other installation heights.
Safety Distance S (D
s
)
Protective Field Height
X
Point of
Danger
