OSHA Hazard Information Bulletins The Limitations of Radiofrequency Presence Sensing
Devices.
OSHA
Safety and Health Information Bulletins - Table of Contents
- Information Date: 19870921
- Record Type: Hazard Information Bulletin
- Subject: The Limitations of Radiofrequency Presence Sensing Devices.
September 21, 1987
MEMORANDUM FOR REGIONAL ADMINISTRATORS |
THROUGH: |
LEO CAREY
Director
Office of Field Programs |
FROM: |
EDWARD BAIER
Director
Directorate of Technical Support |
SUBJECT: |
Safety Hazard Information Bulletin on the Limitations of
Radiofrequency Presence Sensing Devices |
Introduction:
The purpose of this bulletin is to review and supplement information on the
limitations of detection devices of the radiofrequency type. It is apparent
that some of the limitations are either unrecognized or are greater than
generally known.
Various types of presence sensing devices can be used to safeguard many
different kinds of machines as per OSHA 29 CFR 1910.212 and 1910.217. These
devices are widely used on robots, power presses, textile manufacturing
equipment and many other industrial machines. Presence sensing devices
provide maximum visibility of a machine's danger zone. This is desirable in
most point of operation machine guarding. However, accident records and
safety analyses of radiofrequency presence sensing devices show that improper
application, use, selection, design or mounting of these devices can result
in serious accidents. It is recommended that radiofrequency presence sensing
devices not be used as the primary safeguards or only safeguards for workers
exposed to serious hazards.
General Considerations
The basic function of a radiofrequency presence sensing device is to
interrupt a machine's operating cycle when an object is detected within the
device's sensing field. For the device to be effective, it must:
1. be installed and tuned so that effective sensing occurs at
least at a minimum safe distance;
2. be able to detect penetration of a minimum hand size;
3. stop or prevent the operation of a machine as long as any part
of an employee's body is within the machine's danger zone;
4. be interlocked with other guarding devices or have fixed guards
to prevent human access to a machine's danger zone except through the sensing
field;
5. protect exposed employees during all phases of machine
operation including start-up, shutdown, and maintenance; and
6. generate no false "safe-to-operate" signals and ignore received
false "safe-to-operate" signals due to component malfunction or environmental
conditions such as unexpected electrical or magnetic fields, humidity, and
moisture.
A radiofrequency presence sensing device may not be able to provide
effective protection if:
1. it is not installed and tuned so that its sensing field is at a
safe distance;
2. penetrating object is too small to be detected in time by the
sensing field;
3. a penetrating object is between the field and the danger
zone;
4. its source of power fails while the machine remains
operative;
5. it is installed on equipment not designed to stop upon field
interruption at any point of the machine's operating cycle, such as full
revolution presses.
Capacitance Proximity Sensing (CPS)
CPS devices (radiofrequency presence sensing devices) consist of a control
unit, a coupler, a sensor antenna and a cable connecting the control unit to
the sensor. The antenna should have a geometrical shape that makes the
generated sensing field form an envelope around the space where human
detection is desired. This sensor antenna is made of electrical conducting
material and is isolated from the ground. An oscillating electrical signal
in the radiofrequency range of 150 - 400 KHz is generated in the control unit
and sent through the cable and coupler to the sensor. At the sensor antenna
the signal creates an electric potential and generates an electromagnetic
field. The developed field can be much larger than the volume defined by the
antenna. The shape of the field is affected by the location of objects
within its boundaries and by shielding at the sensor, and is not normally
well-defined. Physical changes in the space surrounding the antenna alter
this electromagnetic field. An electronic bridge circuit incorporating an
adjustable reference capacitance is used to detect changes in the field
capacitance. At a tuned level the bridge becomes unbalanced, thus triggering
machine stoppage.
Studies have shown that well-grounded conductors (people) are more easily
detected by CPS system because a grounded conductor causes a greater change
in capacitance in a CPS detection bridge than a weakly-grounded conductor.
In fact, ungrounded conductors may not be detected and poorly grounded
objects must penetrate the field more deeply for CPS devices to detect the
presence of such objects. Attention should be paid to potential problem
situations such as those where employees can move from ground to an insulated
platform while remaining in an antenna's detection space. Detectable changes
of the sensing field in the presence of an operator may diminish when the
operator is isolated from ground. Intermittent energizing of outside
electric fields (such as those from electric motors or power supplies) and
the movement of objects into the sensing field (such as maintenance gear)
also should be checked to insure that they do not change the capacitance of
the CPS system. Changes in machine tool or die layout may also adversely
affect the detection field.
CPS devices employ a "sensitivity" control as an operating adjustment to
compensate for changes in the field due to added or subtracted shielding such
as tooling and support hardware. This adjustment has the effect of moving an
effective sensing point toward or away from a machine's danger zone.
Unfortunately, this can permit effective sensing at less than a safe
distance. Also, holes in the sensing field can be created if the effective
sensing point is collapsed to the antenna. In this case the system might not
detect the presence of an operator's arm in the sensing field. The safety
distance between the sensing plane and the danger point cannot be set upon
initial installation, but must be reset for each application.
Recommendation:
Specific standards have not been adopted for the installation, adjustment,
and maintenance of radiofrequency presence sensing devices. Employers using
such devices should have had safety hazard analyses performed to determine
potential failures or limitations of the devices in their applications.
Employers should be aware that the performance of a CPS device may be
affected as environmental and physical conditions are changed.
In 1979, National Institute for Occupational Safety and Health (NIOSH)
issued a warning on the use of radiofrequency presence sensing devices. The
warning emphasizes that the following procedures be followed:
1. Care must be taken to set a device's sensitivity for the
operator's grounded condition and allow for variations in the operator's
working position during a workday.
2. Supervisors and workers should avoid drastic grounding changes
once sensitivity is set and a device is in use. This includes changes in what
the operator is standing or seated on.
3. Manufacturer's instructions should be followed carefully in the
construction and placement of radiofrequency antennas.
4. The manufacturer of a device should be consulted about specific
problems, such as extreme sensitivity variations in particular machine
applications.
Because of the uncertainties involved with the ability of radiofrequency
detectors to measure intrusion accurately due to ground changes, field
interference and other physical and environmental conditions, CPS devices
should not be used as the primary or only safeguard to protect workers who
are frequently exposed to a serious hazard. When used for applications
involving infrequent human intervention such as perimeter guarding, CPS
devices may provide adequate protection. Changes in sensing distance may not
be critical if there is no need for an operator to normally reach into the
danger zone. A combination of a CPS device and an automatic feeding device
or other safe guarding method provides better protection.
Compliance and consultation personnel must be aware of the hazard addressed
in this bulletin. Please disseminate this information to Area Offices, State
Plan States and Consultation Projects. A companion information bulletin is
being prepared on photoelectric type presence sensing devices and will be
issued in the very near future.
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