Machine safeguarding is essential to operator safety and productivity. Traditional mechanical devices such as mechanical barriers, sliding gates and pull-back restraints fall short in providing fail-safe protection. They can be damaged, left open or overridden, and often slow down production. What is worse, they can fail and you may not even know it!
Light-based Operator Safety Light Curtain
Safety light guards are an advanced method of safeguarding personnel around most hazardous machinery. Safety light guards provide freedom, flexibility and reduced operator fatigue when compared with traditional guarding, methods applicable, for solid guards, safety light guards simplify routine tasks like machine set-up, maintenance and repair.
Safety light guards use infrared light to provide a safety curtain. A photoelectric transmitter projects an array of synchronised, parallel infrared light beams to a receiver unit. When an opaque object interrupts one or more beams, the light curtain controller sends a stop signal to the guarded machine.
The transmitter contains light emitting diodes (LEDs), which emit pulses of invisible infrared light when energised by the light curtains timing and logic circuitry. The light pulses are both sequenced - one LED is energised after another - and modulated - pulsed at a specific frequency.
Corresponding phototransistors and supporting circuitry in the receiving unit are designed to detect only the specific pulse and frequency designated for it. These techniques offer enhanced safety and rejection of external light sources. The controller unit contains user controls, supporting logic, status and diagnostic indicators, power supply and output relays. It provides one convenient location to connect all wiring.
Safety Standards for Safety Light Curtain
Today, there are a number of Safety Regulations laid down by various nations for protection of the operator from hazardous working environments. In the United States, the Occupational Safety and Health Administration (OSHA) regulate installation and use of machine guarding. In addition in OSHA, American National Standards m construction, care and use of machine tools along with its safeguarding. These standards also cover power presses, hydraulic presses and press brakes. Safety Photo Guards standards fall into two categories: Application standards and Construction standards.
Construction standards provide design, construction and testing information of these photo guards. For eg. one significant difference between safety photo guards and standard photoelectric sensors is a design concept known as Control Reliability. It is a mandatory requirement under both OSHA and ANSI.
Some of the important factors required for proper installation are listed below:
1) Beam Spacing and Minimum yet Resolution
Light Guards differ in the spacing he parallel, infrared light beams (or channels), which make up the sensing field. Beam spacing is important due to close relationship with a light curtains minimum object resolution. This defines the minimum object anywhere in the sensing field and is considered a measurement of light curtain sensitivity.
Minimum object resolution is determined by adding together the beam lens diameter and the beam spacing. This distance covers two of the channels and anticipates an object entering the sensing field between two beams.
2) Safe Mounting Distance
To function as a safety guard, a light must be mounted far away enough from the machines point of operation (pinch point) to provide sufficient time for:
a) The light curtain to detect the obstruction and send a stop signal to the machine controller.
b) The machine controller to signal the machine braking system.
c) The machine to stop hazardous motion.
All of this must happen before the operators hand reaches the machine pinch point. Calculating the distance from light curtain sensing field to the installation. This distance is called the minimum safe distance. The minimum safe distance is determined by a number of factors, including hand speed, light curtain response time, minimum object resolution and the time it takes for the machine to cease hazardous motion.
The information necessary for this calculation comes from a variety of sources: the hand speed factor is provided as a constant in the formula; the response time and resolution of the light curtain manufacturer. The stopping time of the machine is often measured by a device called a stopping performance monitor (or brake monitor), especially in machinery that is subject to mechanical brake wear, resulting in deterioration of braking degradation, the machine manufacturer may provide the stop time.
3) Additional Safeguards
A light curtain is only one part of a machine guarding solution. Additional guarding, such as mechanical fences, barriers or shields is required if the light curtain does not protect all access to the machines pinch point. For example, if the light curtain is used at the operator station, a mechanical barrier may be appropriate guarding for other sides to the machine. Additional guarding is also needed to prevent an employee from reaching around the light curtain in anyway to gain access to a hazardous machine area or standing between the machine and the light curtain.
4) Machine and Controller Reliability
Safety light curtains are general-purpose devices. They are not designed for any specific type, model or brand of a machine. It is important that your machine controller has the same level of safeguarding including self-checking and redundancy that is contained in the light curtain. The safety related functions of machine controller including pneumatic, electric logic or hydraulic controls must be control reliable.
The reason for this requirement is that without this control reliable functions, a component failure within the machine controller may cause an unsafe condition. This could result in a failure of the machine controller to stop hazardous machine motion even though the light curtains sends a stop signal to the guarded machine. Always contact the machine manufacturer for advice and assistance regarding the control reliability of the machine controller and the connection of any safety device.
5) Special Perimeter Guard
For perimeter guarding installations, the guarded machine or robot controller must be wired in such a way that any stop signal generated by the light curtain causes an immediate stop of the hazardous motion. The machine or robot must only be re-started by the actuation of a manual reset switch.
Light curtains do not offer protection from machinery that may eject parts, materials, debris, liquids or other flying objects. On such machines, metal or plastic guards, complete with a suitable safety interlock switch, wired to the machine stop circuit, and should be installed to prevent operations with the guard open or removed.
Safety light curtains that conform to ANSI & OSHA specifications are generally provided with the following features:
1) Individual Channel Bypass
Channel select disables selected, fixed areas in the light curtains sensing field. This is a helpful when stationary object such as tooling; tables and work fixture permanently obstruct a specific portion of the sensing field.
2) Safety Light Curtain Bypass
During tool change or machine set up, the safety light curtain may need to be bypassed to allow the operator access to the machine. Since this is a controlled operation, a bypass feature is provided with a key switch to enable the supervisor to bypass the guard.
3) Status and Diagnostic Indicator LEDs
There are two conditions here:
a) Light Curtain Aligned - Dual LEDs on the transmitter unit come on when the safety light curtain is aligned and operational. When obstructed, the beams are cut and the dual LEDs go off. Dual LEDs are used to process and monitor signals for detecting any obstruction.
b) Self-Test LED - The self-test LED on the control panel is normally ON during operation. When the self-checking circuitry that is monitoring the unit for proper operation has detected a fault, this LED will go off and report an error. The control units will then go to Standby state and prevent further operation.
To ensure proper operation of safety Light Guards, the following precautions need to be observed: ·Light guards should only be used on machinery that can consistently and immediately stop anywhere in its cycle or stroke.
· Never use light guards on a full revolution clutched power press or machine.
· If the light guard does protect all access to the point of operation, the unprotected access must be guarded by the other appropriate devices or methods.
|Posted : 8/13/2005|