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Guarding a hydraulic press brake has always been a challenge. Traditional light curtains provide acceptable protection, but often result in lost productivity due to extended set-up time between operations and slower cycle times. Mechanical safeguarding methods, such as restraints, protect without the issues of set-up time. But they only protect the operator, are costly to maintain, are often disliked or misused by operators, and can result in ergonomic issues.

New laser guarding systems resolve these problems and can actually increase productivity (see sidebar). These complete safety solutions install either during the manufacture or as a retrofit of existing machines. They don't inhibit productivity, but do reduce temptations to bypass or "work around" the system.

THEORY OF OPERATION

The operating principle is simple. The transmitter and receiver mount onto the ram of the press brake, transmitting a laser beam below the entire length of the upper die (Figure 1). This allows the operator to stay close to the workpiece as the ram operates at high speed. A continuous band of laser light that senses the zone below the die protects hands and fingers. The system detects material or items accidentally left on the die that could cause injury or damage and stops the ram, preventing contact with the obstruction.

Figure 1. Mounting Example of LazerSafe on Hydraulic Press Brake

The critical speeds and stopping distance of the moving member of the machine are continuously monitored. If the crawl (bending) speed or the stopping distance is exceeded, the controller stops the machine. This obsoletes the need for a separate stopping performance monitor. With a response time of 13-ms, the system stops instantly due to a direct interface with key components of the press brake (valves, modules, and PLCs) and high laser accuracy.

A COMPLETELY INTEGRATED SOLUTION

The dual channel, closed-loop design here doesn't rely on signals from other components of another system whose integrity could be compromised for various reasons. Optical encoder feedback measures the ram position on every machine stroke and the controller monitors the ram stopping distance on every stop. The system monitors the fast closing speed and slow bending speed, plus the point where the speed changes (mute point). Continual monitoring of each function quickly detects any performance degradation, signals an error code and immediately stops the machine. This fault mode provides opportunities to maintain the press brake for peak performance.

Since many of these parameters tie to the hydraulic system, the laser system provides advanced warning of hydraulic system failures that lead to dangerous situations and extended downtime. Insuring that the press brake operates within pre-determined tolerances minimizes downtime and prevents performance-related scrap. The laser system completely integrates guarding functionality with full machine surveillance, muting, and control of speed points.

The closed-loop design records the laser position when there is an obstruction in order to automatically set the mute point. The distance checked between the mute point and the material ensures the material is in the lowest and safest possible position. This can minimize tool travel time at the crawl speed to shorten cycle times even more. Once the mute point is set, the laser system also detects warped or incorrect size of material.

The laser system has four main elements:

• The controller features dual circuit, dual processor design and manages the guarding, safety monitoring of the press brake, muting, and position monitoring. It integrates into the existing press break control.


• The laser transmitter and receiver units incorporate dual laser beams to support even higher-speed press brake operation without compromising safety. These come with a full mounting bracket system.


• The optical encoder mounts to the ram and provides critical information to the controller on movement, speed, and positioning of the ram on every stroke.


• The remote operator control panel houses the system controls and indicators. Magnetic backing allows it to locate anywhere on the press brake. This simple interface allows selection of the appropriate operating mode or set-up. It can be locked with a key to prevent operators from overriding critical safety functions.

LASER SENSING AREA

A unique innovation is a flat, wide laser beam that guards the area directly under the tip of the die plus those in front of and behind the die center as well (Figure 2). The receiver configuration detects the front, center, and rear segments of the beam. This accommodates back gauges, bending box or tray configurations. Each detection area has an individual indicator that signals obstructions on the operator control panel.

Figure 2. Laser Beam Presence Detection Areas Used by LazerSafe

System Benefits

This fundamentally different system approach delivers:

• Comprehensive operator protection at extremely close proximity to workpiece.


• Increased productivity as tools close at high speeds without compromising safety.


• Complex shape forming with the "Tray/Box" and "Field Muted" operating modes.


• Closed-loop encoder feedback monitors ram speed and stopping distance.


• A flat band of continuous laser light detecting obstructions as small as 4-mm while remaining tolerant to vibration.


• The mute point is automatically determined, easily set, and continuously monitored.


• The rear section of the laser band easily mutes to ignore a back gauge in "up-close" situations.


• Failure detection by real-time monitoring of the process.

PRODUCTION FRIENDLY

Several operating modes suit various press brake applications. The Box Mode makes boxes or flanged parts without additional set-ups. In box mode, the control recognizes the obstruction of the laser beam with the box flange and still allows the ram to close to the mute point at high speed without retraction. This eliminates repeated set-up changes, accommodates the changing shape of the work piece, and prevents any need to override the control to continue the bend.

Beyond safety, modern guarding systems must maintain accuracy and reliability in a harsh physical environment without imposing extra burdens on the operator or employer. For example, the sensing field in properly applied light curtains is fixed at least 100 mm from the center line of the die. The operator must move out of this sensing field for the tools to close. This increases cycle time.

But the laser system detects obstructions so accurately the operator doesn't have to move between cycles. The press brake isn't stopped unnecessarily due to "false tripping." Cycle times are shorter without compromising safety.

Operator adjustments are easy for different production processes (e.g. changing of the die). Aligned correctly, the system maintains parallel projection below the top tool, with a simple process for properly adjusting the height and depth of the upper die. The receiver lens design allows the operator to usually reposition only the laser transmitter when adjusting for different tool heights. The laser system tolerates vibration in the horizontal and vertical axes without introducing processing or time delays.

Without question, laser guarding systems offer the best combination of operator protection and productivity available for hydraulic press brakes.

Chris Soranno is a machine & process safety engineer for STI Machine Services, 1-866-260-4784, www.stimachineservices.com.

Hydraulic press brake productivity directly relates to the descent and return speeds of the ram and return height. Maximizing the travel speed to the mute point and minimizing the closing speed distance, the LazerSafe system can actually enhance productivity. Assuming a press brake operating at 4-in/sec has a stroke length of 3-in, change of speed height of 0.5-in above the material, a bend depth of 0.2-in and a material thickness of 0.04-in, the average cycle time when guarded with LazerSafe is 3.955-sec. Cycle times for the same machine fitted with a safety light curtain range from 4.57-sec to 8.98-sec. Systems where slower response times require that the change of speed take place at a higher location, typical cycle times are 4.23-sec to 4.86-sec. This is a 6 to 27 percent productivity improvement.