Presence-sensing

Recent developments in machine controls and changes in U.S. standards now permit the initiation of some machine cycles without an overt action on the part of the machine operator. These systems, known as presence-sensing device initiation (PSDI), automatically cycle the machine upon the sensing-device determination that the operator is clear of hazard areas. Similar systems have been utilized outside the United States, but normally the design and function of the machines have been approved by government or professional third parties. Many safety professionals recommend that PSDI be utilized only if the machine control has been designed and periodically tested by the original machine manufacturer. Under no conditions should any part of the body other than hands be placed inside dies or other points-of-operation while machine power is active. 

Muting— The deactivation of a presence-sensing safeguarding device during a portion of the robot cycle. 

There are many kinds of point-of-operation devices. They include gates or moveable barrier devices, presence sensing devices, pull-out devices, sweep devices, hold-out or restraint devices, two-hand controls, and hand-feed tools. 

Presence-Sensing Devices Presence sensing devices detect whether the operator s fingers or hands are or could be in the point-of-operation. If they are, the machine will not operate or will stop quickly enough to prevent injury. Presence-sensing devices are not a suitable alternative for some machines. Each type of device has particular limitations. Guards should protect all areas not protected by presence-sensing devices. 

Figure 13-12. Example of a photoelectric presence sensing device on a machine. (From OSHA 3067.)...

One must check presence sensing devices from time to time to be sure they are working properly. Sometimes an indicator light combined with the control circuit notifies the user of a failure. Assuming a device is working properly when it is not could result in an injury. 

Mats One kind of presence-sensing device is a mat that detects whether someone is standing on it. Position these mats in locations near a machine where someone could reach into the point-of-operation. When someone is on a mat, it can shut off a machine. 

A machine will have a presence-sensing device and an interlocking barrier gate. How far should you locate a two-hand trip control from the point of operation ... 

Mechanical-power presses are required to have switches and brakes that are operator proof The size of some presses makes it imperative that they are switch-guarded as well as mechanically guarded. This section describes what actions must be taken to produce the most failsafe device possible. Minimum braking action and the proper brakes that must be installed on presses of differing specifications are outlined. Criteria for the use of presence-sensing devices and their proper utilization is described. 

Presence sensing devices Two-hand controls Awareness Means... 

Presence-sensing devices may not be used to initiate the slide motion except when used in total conformance with paragraph (h) of 29 CFR 1910.217, which requires certification of the control system. Any machine using full-revolution clutches must incorporate a single-stroke mechanism. A main disconnect capable of being locked in the off position must be provided with every power press control system. 

The radicfrequency (capacitance) presence-sensing device uses a radio beam as part of the machine control circuit. When the capacitance field is broken, the machine stops or will not activate. Like the photoelectric device, this device is only to be used on machines that can be stopped before the worker can reach the danger area. This requires the machine to have a friction clutch or other reliable means for stopping. This device allows freer movement for the operator but does not protect against mechanical failure. In addition, antennae sensitivity must be properly adjusted this adjustment must be maintained properly. 

The electromechanical presence-sensing device has a probe or contact bar that descends to a predetermined distance when the operator initiates the machine cycle. If an obstruction prevents it from descending its full predetermined distance, the control circuit does not actuate the machine cycle. This device allows for access at point of operation, but the contact bar or probe must be properly adjusted for each application. This adjustment must be maintained properly. 

In special circumstances, where plysical guarding is not firlfy possible, the application of quick-stop devices can be incorporated. In particular, rrse is made of photo-electric beams or other presence-sensing systems. As is the case with automatic guards, these types of devices present technical difficulties and reliability concerns if not properly installed and integrated into the operations of the machine. 

After power one and before the initial presence sensing device initiation (PSDI) stroke. 

Inspect presses equipped with a presence sensing device for use in the PSDI mode (or for supplemental safeguarding on presses used in the PSDI mode) to ensure ... 

When will a presence-sensing guard stop a machine ... 

Other machine features that offer protection include moveable barriers, automatic feed systems, presence-sensing devices, emergency stop control, hand control devices and mechanisms, such as pull-out devices, restraint devices, and two-hand control. Reduced speed control is used during setup, cleaning, and maintenance. Other safety mechanisms include antikickback devices, run controls, and foot controls. 

Presence-sensing safeguarding device a device designed, constructed, and installed to create a sensing field or area to detect an intrusion into the field or area by personnel, robots, or other objects... 

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. 

The basic function of an RF 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... 

An RF presence-sensing device may not be able to provide effective protection if... 

Specific standards have not been adopted for the installation, adjustment, and maintenance of RF presence-sensing devices. Employers nsing snch devices should have had safety hazard analyses performed to determine potential failnres 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. 

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