UNITED STATES
DEPARTMENT OF LABOR

• Safety and Health Information Bulletins /
• The Limitations of Radiofrequency Presence Sensing Devices

September 21, 1987

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.