Safety
Presence Sensing Devices
Light Curtains
ANSI Safety Distance Formulas (Recommended for U.S.)
The ANSI safety distance formula, which is the Rockwell Automation recommended formula, is as follows...
| Where | |
|---|---|
| Ds | 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 63 inches per second when the operator begins reaching toward the point of operation hazard from rest. |
| Note | ANSI B11.19 1990 E4.2.3.3.5 states “The value of the hand speed constant, K, has been determined by various studies and although these studies indicate speeds of 63 inches/second to over 100 inches/second, they are not conclusive determinations. The employer should consider all factors, including the physical ability of the operator, when determining the value of K to be used.” |
| Ts | Stop time of the machine tool measured at the final control element. |
| Tc | Response time of the control system. |
| Note | Ts and Tc are usually measured by a stop time measuring device. |
| Tr | Response time of the presence sensing device (safety light curtain) and its interface, if any. This value is generally stated by the device manufacturer or it can be measured by the user. |
| Tbm | Additional time allowed for the brake monitor to compensate for variations in normal stopping time. |
| Dpf | 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 safety light curtain’s object sensitivity. Object sensitivity is the smallest diameter object which will always be detected anywhere in the sensing field. |
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".
Additional notes for Point of Operation Controls
Object Sensitivity
The POC 14 has an object sensitivity of 14mm (0.55 inches) and the POC 30 has an object sensitivity of 30mm (1.18 inches) The Dpf for the POC 14 is 0.93 inches and the Dpf for the POC 30 is 3.07 inches.
Note: Activating Fixed or Floating Blanking increases the minimum object sensitivity. An increase in object sensitivity requires that the safety distance be recalculated and the safety light curtains are to be remounted at the new safety distance.
Safety Distance to Point of Operation
The safety device requires a certain minimum distance between the sensor and the point of operation. This ensures that the point of operation can be reached only after the hazardous motion has been stopped. The safety distance (according to EN 775, 999 and 294) is dependent on the following:
- Machine stop time
- Response time of the safety device
- Resolution of the safety device
- Hand speed
The following formula should be used when calculating the safety distance Ds:
Ds = K x (Ts + Tc + Tr + Tbm) + Dpf
or
Ds = K x (∑Tall) + Dpf
| Where | |
|---|---|
| Ds | Minimum safety distance between device and the nearest point of operation hazard (in mm (inches)). |
| K | Hand speed constant: 1600mm/sec or 63 in/sec. |
| Tall | The sum of all stopping times. |
| Ts | Stop time of the machine tool measured at the final control element (in seconds). |
| Tc | Response time of the control system. |
| Note | Ts and Tc are usually measured by a stop-time measurement device and confirmed periodically. |
| Tr | Response time of the presence-sensing safety device as stated on the product label. |
| Tbm | Additional time allowed for the brake monitor to compensate for variations in normal stopping time (i.e. brake wear). |
| Dpf | Added distance due to the depth of penetration factor. Can be obtained using the chart below or 3.4 x (object sensitivity - 7mm). |
The minimum object sensitivity is stated on the product label. If beam blank-outs are used, this opening should be added to the object sensitivity.