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Machine Safety Terminology


Access Guarding
Typically used to guard an access point to automated machines, such as a robot or a palletiser where accessibility or visibility are desired. Because the safety distance is usually relatively high, low resolution object detection capability may be considered. See Figure 1. See also Perimeter Guarding.

Auto Power-up
A feature which, when switched ON, enables the safety device to be powered up (and recover from a power interruption) without the necessity of a Key Reset. When Auto Power-up is ON, the safety device automatically begins internal diagnostics upon power-up and automatically resets the system if it passes the diagnostic check. With Auto Powerup OFF, a manual reset is required.

Auxiliary Monitor or Alarm Contact
A low-load-capacity, non-safety-related relay contact whose primary purpose is to communicate system status to a PLC.

See Fixed Blanking and Floating Blanking.

Contains the circuitry that provides the proper voltage to the system, controls the sensing units, receives and processes information from the sensing units and the safety monitoring means and provides outputs to interface to the machine. Note that sometimes this controller circuit is built-in (e.g. EZ-SCREEN Systems).

Control Reliability
A method of ensuring the performance integrity of a control system. Control circuits are designed and constructed so that a single failure or fault within the system does not prevent the normal stopping action from being applied to the machine when required or does not create unintended machine action but does prevent initiation of successive machine action until the failure is corrected.

Detection Zone
The curtain of light generated by the safety light screen. When the detection zone is interrupted by an opaque object of a specified cross section or larger, a trip condition (or latch condition) results.

Diverse Redundancy
A design feature in which two components of different design, running from two different instruction sets (if programmed components), constantly check all system components, including each other.

E-stop Button
The function offered by some controllers which produces a lockout condition when an (optional) E-stop Button is engaged. The E-stop Button must meet certain mechanical and electrical requirements.

The light-emitting component of a safety light curtain, consisting of a row of synchronised modulated infrared LEDs. The emitter, together with the receiver (placed opposite), creates a light curtain called the Detection Zone.

Fixed Blanking
A feature that allows the safety light curtains to be programmed to ignore objects (such as brackets or fixtures) that are always present within the detection zone, so that the presence of these objects does not cause the switching outputs to trip. If any of the fixed objects are moved within or removed from the detection zone, a lockout condition results.

Final Switching Device (FSD)
The two output relays (FSD1 and FSD2) of the safety device which respond to an intrusion of the detection zone by interrupting the circuit connecting them to the MPCEs of the guarded machine.

Fixed Guarding/Hard Guarding
Screens, bars or other mechanical barriers that prevent a person from reaching over, under or around the point of operation of the guarded machine. See Figure 2.

Floating Blanking
A feature that allows safety light curtains to be programmed to produce an intentionally disabled light beam within its light curtain, which appears to move up and down (float) in order to allow the feeding of an object through the light curtain (Detection Zone) at any point along the length of the light curtain without causing a trip or latch condition. Some systems offer one- or two-beam floating blanking to allow multiple objects (usually workpiece material) to move through the detection zone without tripping the Final Switching Devices depending on configuration. See Figure 3 above.

Failure Mode and Effects Analysis (FMEA)
A testing procedure by which potential failure modes in a system are analysed to determine their results or effects on the system. Component failure modes that produce either no effect or a lockout condition are permitted; failures which cause an unsafe condition (a failure to danger) are not. All Banner safety devices are extensively FMEA tested.

Full-Revolution Devices
A type of machine drive arranged such that, once started, the machine can only be stopped when the full cycle is complete. Examples include positive key clutches and similar mechanisms. Typically, safety light screens may not be used with full revolution devices.

Guarded Machine
The machine whose point of operation is guarded by a safety device and whose MPCEs and MSCE are connected to the FSDs of the safety devices.

Internal Lockout
A lockout condition that occurs due to an internal problem, typically indicated by the red status LED flashing.

Key Reset
A key-operated switch used to restore the FSDs and SSD to the ON state from a lockout condition. Also refers to the act of using the switch to reset the safety device.

Latch Condition
In latching output models, the FSD output operates when an object equal to or greater than the diameter of the specified test piece enters the detection zone. In a latch condition, the outputs simultaneously de-energise and open. The latch must be reset after the detection zone is cleared by momentarily closing the normally open contact of the Latch Reset switch.

Lockout Condition
A condition that is automatically attained both (1) when its power supply is interrupted and restored and (2) in response to certain failure signals. When a lockout condition occurs, a Key Reset is required to return the system to the RUN condition.

Machine Primary Control Element (MPCE)
An electrically powered element which directly controls the machineís normal operating motion in such a way that it is last (in time) to operate when motion is either initiated or arrested.

Machine Response Time
The time between the interruption by the FSDs of the electrical supply to the MPCEs and the instant when the dangerous parts of the machine reach a safe state by being brought to rest.

Machine Secondary Control Element (MSCE)
A machine control element independent of the MPCEs, capable of removing the source of power from the prime mover of the relevant dangerous machine parts.

Minimum Safety Distance
See Safety Distance.

Minimum Object Detection Size
See Object Detection Capability.

Non-latching Controllers
See Trip Condition.

Object Detection Capability (ODC)
The minimum-diameter object that a light curtain system can reliably detect. Objects of this diameter or greater are detected anywhere in the sensing field. A smaller object can pass undetected through the light curtain if it passes exactly midway between two adjacent light beams. See also Specified Test Piece. See Figure 4. Click here to see also Resolution.

Output Relays
The devices that are used to initiate a stop signal. The output relays (FSD1, FSD2 and SSD) use positively guided contacts.

Perimeter Guarding
See Access Guarding.

Point of Operation
The area of the guarded machine where a workpiece is positioned and a machine function (i.e.,shearing, forming, punching, assembling, welding, etc.) is performed upon it.

Positively Guided Contacts
Relay contacts that are mechanically linked together, so that when the relay coil is energised or de-energised, all of the linked contacts move together. If one set of contacts in the relay becomes immobilised, no other contact of the same relay is able to move. The function of positively guided contacts is to enable the safety circuit to check the status of the relay. Positively guided contacts are also known as captive contacts, locked contacts, forced-guided contacts or safety relays. See Figure 5 above.

Presence-Sensing-Device Initiation (PSDI)
An application in which a presence-sensing device is used to actually start the cycle of a machine. In a typical situation, an operator manually positions a part in the machine for the operation. When the operator moves out of the danger area, the presence-sensing device starts the machine (without using a start switch). The machine cycle runs to completion and the operator can then insert a new part and start another cycle. The presence-sensing device continually guards the machine. Single break mode is used when the part is automatically ejected after the machine operation. Double break mode is used when the part is both inserted (to begin the operation) and removed (after the operation) by the operator.

The light-receiving component, consisting of a row of synchronised photo transistors. The receiver, together with the emitter (placed opposite), creates a light curtain called the detection zone.

Safety Distance
For a normal approach, reference ISO/DIS 13855 (EN 999) gives the following formula:

S = K x T + C where

S = The minimum safety distance in mm, from the danger zone to the centre line of the detection zone. Minimum allowable safety distance is 100 mm (175 mm for non-industrial applications) regardless of the calculated value.

K = The standard maximum hand speed in mm/s according to reference ISO/DIN 13855 (EN 999). K = 2000 mm/s for values of S between 100 mm and 500 mm. If S > 500 mm, then K may be reduced to 1600 mm/s but S must remain > 500 mm.

T = The overall response time of the machine, the time between the physical initiation of the safety device and the machine coming to a stop or the risk being removed. This can be broken down into two parts: Ts and Tr where

T = Ts + Tr

Ts = The response time of the machine measured between the application of the stop signal and the machine coming to a stop or the risk being removed. Ts is usually measured by a stoptime measuring device.

Tr = The response time of the safety device.

C = The additional distance in mm, based on the intrusion of the hand or object towards the danger zone prior to actuation of the safety device. If the Object Detection Capability is not greater than 40 mm: C = 8 (d - 14). Always consult the manual for complete details on safety distance calculation! See Figure 6.

Secondary Switching Device (SSD)
The output device which, in a lockout condition, interrupts the circuit connecting it to the MSCE.

Self-checking (circuitry)
A circuit with the capability to electronically verify that all of its own critical circuit components, along with their redundant backups, are operating properly.

Single-stroke Press
See Full-revolution Device.

Specified Test Piece
An opaque object of the minimum cross section required to place the safety light screens into a trip or latch condition when inserted into any part of the detection zone. See also Object Detection Capability.

Supplementary Guarding
Additional electro-sensitive safety device(s), possibly employed along with fixed guarding measures, used for the purpose of preventing a person from reaching over, under or around the detection zone into the point of operation of the guarded machine.

Trip Condition
In trip output models, the FSD output operates when an object equal to or greater than the diameter of the specified test piece enters the detection zone. In a trip condition, FSD1 and FSD2 simultaneously de-energise and open. A trip condition clears automatically when the object is removed from the detection zone. See also Non-latching Controllers.

The (temporary) suspension of the safety function to allow material to flow in or out of the guarded part of the machine.

A type of machine protection where an operator needs to place both hands on two actuating devices (such as buttons) in order to initiate a machine cycle. Note that this only keeps the operatorís hands out of the dangerous area. See Figure 7 below.



Fig 7:  Two Hand Control