|Calculating Safety Distance|
When installing safety light curtains, a minimum safe distance that the light curtain can be mounted from the point of hazard must be calculated. The distance between the hazard and the light curtain must be such that the time it takes the operator to reach the hazard (with respect to approach speeds of parts of the human body) is greater than the stopping time of the machine.
In the United States there are two formulas that are used to properly calculate the safety distance. The first, the OSHA formula, is the minimum requirement for the calculation of the safety distance. The second formula, the one recommended by United Machine Corporation, is the ANSI formula, which incorporates additional factors to be considered when calculating the safety distance.
OSHA Safety Distance Calculation Formula
The OSHA safety distance formula as specified in CFR Subpart O 1910.217 is as follows:
Ds = 63 X TS
DS Safety Distance
63 Is the OSHA recommended hand speed constant in inches per second
TS Is the total stop time of all devices in the safety circuit, measured in seconds. This value must include all components involved in stopping the hazardous motion of the machinery. For a mechanical power press it is the stopping time measured at approximately the 90Âº position of the crankshaft rotation.
Note: The TS number must include the response times of all devices, including the response time of the safety light curtain, the safety light curtain controller (if used), the machine’s control circuit and any other devices that react to stop the hazardous motion of the machinery. Not including the response time of a device or devices in the stop time calculation will result in insufficient safety distance for the application. This may result in operator injury.
ANSI Formula for Safety Distance
The ANSI calculation of the safety distance for light curtains, as used in B11.1-1988 and B11.19-1990, is as follows:
Ds = K x (Ts + Tc + Tr + Tbm) + Dpf
Ds The minimum safe distance from the danger zone to the light curtain (in.)
K 63 in./sec (suggested). The hand speed constant as defined in ANSI B11.19-1990: "…the hand speed constant, K, has been determined by various studies and although these studies indicate speeds of 63 in./sec to over 100 in./sec, they are not considered conclusive determinations. The user should consider all factors, including the physical ability of the operator, when determining the value of K to be used."
Ts The stopping time of the machine, in seconds, measured at approximately the 90° position of crankshaft rotation as determined with a measuring device.
Tc Response time, in seconds, of control circuit to activate machine stoppage as determined with a measuring device.
Tr Light curtain response time as specified in seconds.
Tbm Additional stopping time attributed to the response time of a brake monitor. If no brake monitor is available, a percentage factor must be added to account for braking system deterioration due to wear. For new brakes, 20% is recommended; for older brakes, 10% is the recommended factor.
Dpf Additional distance based on the depth penetration factor (refer to Table 10 in 1910.217). By knowing the minimum object size that can be seen by the light curtain, the distance an object can penetrate the sensing field before the light curtain initiates a stop signal can be determined.
Example: Using the ANSI formula, a light curtain response time (Tr) of 15ms, a machine stopping time (Ts+Tc) of 180ms, a brake monitor response time (Tbm) of 40ms and a 3.2 inch depth of penetration, the calculation would be as follows (Remember that the hand speed constant, K, is set by OSHA at 63 inches per second):
|Ds = K x (Ts + Tc + Tr + Tbm) + Dpf |
|Ds = 63 x (0.180 + 0.015 + 0.040) + 3.2" |
|Ds = 63 x (0.235) + 3.2" |
|Ds = 14.805 + 3.2" |
|Ds = 18.00" |
So, the minimum safe distance the safety light curtain must be mounted from the hazard is 18 inches.
EN 999 Formula for Safety Distance
EN 999 is the European standard that deals with the positioning of protective equipment with respect to approach speeds of parts of the human body. It is recommended for use on machines that are intended for sale or use in Europe. Quite similar to the ANSI formula, the formula from EN 999 is as follows:
S = (K x T) + C
S The minimum distance in mm from the danger zone to the light curtain sensing field.
K 1600 (suggested). This parameter is based on research data showing that it is reasonable to assume an approach speed by the operator of 1600mm/sec. The circumstances of the actual application must be taken into account. As a general guideline, the approach speed will vary from 1600 to 2500mm/sec.
T The overall stopping time of the system, i.e., the total time, in seconds, from the initiation of the stop signal to the cessation of the hazard.
C An additional distance, in millimetres, based on possible depth of penetration toward the hazard area. This will depend on whether it is possible to reach over, around or through the light curtain before the switch contacts are opened. Standards EN 294 and
EN 811 provide more information on calculation of reach distances.
Example: Using the same measurements (converted to mm) from the ANSI calculation above, the EN 999 equation yields:
|S = (K x T) + C |
|S = (1600 x 0.235) + 81.28 |
|S = (376) + 81.28 |
|S = 457.28mm |
Therefore, the minimum safe distance for the same application would be 457.28mm based on EN 999.
Interference from Reflective Surfaces and Other Opto-Electronic Devices
When reflective surfaces (shiny/polished metals, foils, glossy painted surfaces, etc.) are in close proximity to opto-electronic devices such as light curtains, the system is susceptible to faults due to deflection of the optical beams. A situation such as this can result in the light curtain not detecting an object or personnel in the sensing field.
|Figure 82: POC alignment/distance |
When installing light curtains, the potential for interference can be calculated to determine the minimum distance that the light curtain must be mounted from the reflective surface in question. As a general rule, no reflective surfaces should be contained within the beam angle of the light curtain emitter or receiver with misalignment taken into account. Using the following formula:
D = R/2 (tan 2a)
D Distance to the reflective surface (worst case)
R Distance between the light curtain emitter and receiver
a The angle of acceptable misalignment (angle of divergence) as determined from the light curtain specifications
It is also necessary to take into account other opto-electronic devices near the safety light curtain. It is quite possible for light curtains to interfere with each other or "crosstalk.”
|Figure 83: Multiple POC |