Intelligent drives for increased safety

Dec 11, 2007 6:48 AM

Safety standards require that workers are protected from harm by machine movements. Now, drives integrate safety for better reaction times.

While many solutions provide a "safe stop," it is equally important to stop in a controlled manner. Rather than simply removing power, driveintegrated safety functions actually control motion, providing quick and orderly stops. Depending on the application, drives can decelerate axes at the best possible or fastest speed. Drives can also be de-energized to remove all torque, or can be held in position under energy for jogging at safely reduced speeds.

Dual-channel monitoring

From the signal-level perspective, drive safety looks like this: Two independent channels transmit and process all safety-related information. Both the primary and secondary channels are located in the basic drive unit, however, the secondary channel is an optional safety module. Each constantly compares data to ensure correct safety parameters are in place. Deviations result in errors, whereby the drive enters safety mode and axes are brought to a standstill. This safety channel cross-checking easily identifies problems such as:

Safety function activated on only one system
Wrong safety function activated
Different monitoring parameters
Faulty safety function
Accidental hardware and software errors

Machines are set to safe operation via the dual channels (on the drive) by changing the mode switch from normal to special. After all selected drives switch to safe status, the master holds the protective door interlock open so an operator can enter safely. Drive-based safety is advantageous because it eliminates implementing power protection in the network and motor feed areas. And, operators can switch to the special operating mode from any location while drives remain in position-monitoring mode.

Once at a safe standstill, operators can jog axes at safely reduced speeds for setup, maintenance, troubleshooting, or repair. If axes carrying heavy weights are located in the access area, dropping is an additional danger. After a request for access, the drive tests the corresponding holding brake function before enabling the protective door to be held open. While someone is underneath the axes, both the safe operation-stop and tested brake operate in parallel so that redundant stopping systems prevent uncontrolled falling of the axes. Access request signals connect redundantly to the drive, which internally monitors limits using redundant software and hardware modules.

Productive and flexible

Productivity demands call for minimal setup and machine downtime. As such, drive-integrated safety lets a machine return to normal production in little time. Because power isn't removed from the drive, it returns to work without waiting for capacitors to recharge or for the operator to recover from an emergency stop. Also, without a loss of position, machines don't have to "home" before restarting production.

All machines have different safety requirements. To accommodate various machines, drive-integrated systems offer flexible, password-protected safety parameters. For example, printing equipment may limit speed to 5 m/min, while general automation equipment may limit safe speed to 2m/min. General guidelines exist, but machine builders usually conduct their own hazard analysis to determine specific safety values. Machine safety can also be pre-certified so OEMs don't have to worry about the certification process.

Designed to meet European safety standards

Before machine builders or OEMs put a machine into circulation in Europe, manufacturers must analyze risks that may occur during use, in accordance with the European Machine Guideline. For uncontrolled machine movements, risk analysis must include duration and frequency within the hazardous area, escapes for the operator, and severity of injuries. The resulting analysis can be used to define the safety category for safety-related components in accordance with EN 954-1.

EN 954-1 is a European standard for safe operation at open guarding. The standard contains several classifications of machine components' ability to withstand faults. In Europe, demand is growing for equipment that meets EN 954-1 safety guidelines. For international OEMs, EN 954-1 compatible equipment is a competitive advantage.

This standard defines several categories, or levels, of safety. Category B is the basic requirement on which all other categories are built. Basic requirements involve selecting components so the safety system withstands fundamental conditions, such as operating stresses, vibration, or process materials (fluids and other agents). This category does not require safety systems to operate after a fault. On the other hand, Category 3 safety, or single-fault safety, means that a single fault, detected by cyclic tests, doesn't lead to a loss of the safety function.

For more information, contact Bosch Rexroth at (800) 739-7684, visit www.boschrexrothus.com, or email the editor at ctelling@penton.com