Safety Application Note

Transcription

1 Safety Application Note Safety Application Note Rockwell Automation A Modular Safety System Design Concept Gobal E-Stop with Local Machine Guarding This application note shows a modular safety design concept with separate but related e-stop and guardlocking interlock functions. This concept is intended modular manufacturing systems, where the number of machines needed to complete the system varies. Each machine has its own interlocks and e-stops. This design facilitates the expansion of the safety system, where the e- stops are intended to stop the complete system, and the interlocks are only intended to stop their local machine. The block diagram below characterizes this modular approach. Module 1 Module 2 Module n E-Stops Machine 1 E-Stops Machine 2 E-Stops Machine n Guarding Machine 1 Guarding Machine 2 Guarding Machine n The Guardlocking Interlock Circuit There are several ways to implement the local machine interlock control. Figure 1 shows an example using guardlocking interlocks and multiple safety rated AC drives. As an option, guarding without locking can be substituted guardlocking. Guardlocking Safety Function The guardlocking interlock has the following safety function: 1. The machine can only run when all the interlocks are closed and locked. Modular Safety Relay Design Concept Rev B Page 1 of 8 April 9, 2007

2 Safety Application Note Rockwell Automation 2. Unlocking any interlock will initiate a machine shutdown sequence (if not already shutdown). 3. While any interlock is open, the machine cannot operate. 4. The closing of any interlock shall not start the machine. Reset Function In this example, access to the machine is limited to partial body access. With this limitation, the interlock safety system is set to automatic reset. The machine control logic must be designed to prevent automatic restart of the machine when all the gates are closed. In this example, pressing the start button can start the machine after the guards are closed and locked. The guardlocking interlocks are chosen three reasons: 1) the stopping time of the machine, 2) vibration on the machine and 3) machine cycle interruption. The type of locking function chosen is mechanical lock / power to unlock. Stopping Time of Machine If non-locking interlocks were used, the operator may be able to open the guard and reach the hazard, as some of the moving parts have not come to a complete stop. The Guardlocking interlocks keep the guards locked until the machine has come to a stop. Vibration Although vibration is typically minimal and within the specification of the safety components, the guardlocking approach prevents nuisance trips due to vibration on the machine, as well as misalignment of the guard that may occur over time. Machine Cycle Interruption The best permance of the machine is maintained when the production cycle is completed. Non-locking interlocks would allow operators to inadvertently open a guard and stop the machine in the middle of a cycle. Guardlocking interlocks can keep the guards closed until the machine has completed a full cycle. Each machine may have multiple axes of movement. A risk assessment must be permed to determine which axes might generate a hazardous condition. Those axes that may generate a hazardous situation must be controlled by the safety system. In Figure 1, two safety rated drives are part of the safety system they are connected in parallel to the MSR127.. Machine Start Sequence To start the machine, all the guards must be closed. The operator must release the Lock Release button. If all the guards are closed and locked, the signals to the PLC through the 33/34 contacts open. The signal from the safety relay contacts 41/42 also open. The drives are enabled. The PLC can also get feedback from the drives over the DeviceNet communication - the drives are ready to start. The operator presses the start button. The PLC gets a negative feedback from each interlock (33/34) and the safety relay (41/42) as indicators that the safety system is ready to go. This approach cannot distinguish between the guard closed and a broken wire. Optionally, the 33/34 circuit on the MSR127 can be monitored by the PLC as positive feedback that the safety system is ready. Modular Safety Relay Design Concept Rev B Page 2 of 8 April 9, 2007

3 Safety Application Note Rockwell Automation Machine Stop Sequence The typical machine sequence is to bring the machine to a normal machine cycle stop (Stop Category 2). To do this, the operator presses the Stop button. Through DeviceNet, the PLC issues a stop command to the drives. To gain access to the machines, the operator presses the Lock Release button (a maintained button). The PLC issues a lock release command which applies power to all the solenoids. The PLC receives signals through the 33/34 contacts from all the interlocks that they are unlocked. Operators can now open the guards. In addition, the safety relay sends a signal to the PLC through the 41/42 contact to indicate that the machine is in a safety stop. Safety Permance Rating The rating of the interlocking design is Category 3 per EN A single fault will not lead to the loss of the safety function. An accumulation of faults may lead to the loss of the safety function. The safety rated drive is rated to Category 3. The series connection of multiple gates is considered category 3, as a fault across one contact can be masked by opening and closing another guard. The rating utilizes fault exclusion of any single point failure of the interlocking switches, due to the proper design, installation and operating procedures. The machine design limits the withdrawal and insertion speeds of the actuator to within the manufacturer s specification. Mechanical stops prevent the guard from banging into and damaging the interlock. Operating procedures are used to periodically prove the permance of the switch. Figure 2 is similar to Figure 1, except the drives are replaced by dual contactors, and the contactors are powered by the PLC. The outputs of the PLC are fed through the outputs of the MSR127 safety relay. When the safety relay is satisfied, a signal is fed from the 41/42 contacts of the MSR127 to the PLC. The PLC can then control the contactors. Since there is no electrical speed reduction (i.e. braking), this approach perms a Category 0 (coast to) stop. Mechanical braking can be added to achieve a Category 1 stop. The E-Stop Circuit Category 0 Stop Figure 3 shows an example of a typical e-stop circuit a machine. The machine may have more than one E-Stop device (e.g., pushbutton, cable pull). If so, the dual channel e-stop devices are connected in series and then connected to an MSR144RTP safety relay. The MSR144RTP safety relay was chosen because it has easily expandable outputs. By itself, the MSR144RTP has two safety outputs and two auxiliary outputs. Using ribbon cables, MSR230P modules can be added to the MSR144RTP when additional machines are needed. Similarly, MSR238P modules can be added when an off-time delay output is needed to achieve a Category 1 stop. The safety outputs (13/14 and 23/24) of the MSR144RTP are connected to the inputs of the interlock safety relay (the MSR127TP). When the e-stop button is pressed, the machine executes an uncontrolled (Category 0) coast to stop. This is because it opens the input to the safety relay which immediately disables the drives. Modular Safety Relay Design Concept Rev B Page 3 of 8 April 9, 2007

4 Safety Application Note Rockwell Automation The MSR230P is used to communicate the e-stop safety function to other machines. If no additional machines are used, then this module can be removed from the system. When additional machines are used, two safety contacts (e.g., 13/14 and 23/24) from the MSR230P are fed to the input of the e-stop safety relay of the previous machine. The other two safety contacts (e.g. 33/34 and 43/44) are fed to the input of the e-stop safety relay on the next machine. This design approach creates a domino affect when an e-stop is pressed. For multiple machines, the maximum response time of the e-stop function will occur when an e-stop is pressed on the first machine. The e-stop signal cascades through all the MSR144RTP relays. Take example a system with 5 machines, and the e-stop is pressed on the first machine. The response time both the MSR144RTP and the MSR127 is 15ms. Theree the last machine will begin is shutdown after a 90ms (6 x 15ms) delay. The E-Stop Circuit Category 1 Stop If a Category 1 e-stop is preferred over a Category 0 e-stop, then the MSR238P can be added to the e-stop relay. This is shown in Figure 4. When an e-stop is pressed, dual diverse signals are sent from the MSR144RTP (13/14 and 41/52) to the PLC to initiate a machine shutdown sequence. After the timer in the MSR238P expires, the safety contacts (17/18, and 27/28) open and the MSR127TP disables the drives. Author: Steve Dukich Global Component Technical Specialist Machine Safeguarding Modular Safety Relay Design Concept Rev B Page 4 of 8 April 9, 2007

5 Safety Application Note Rockwell Automation +24V DC From Immediate Action (Delayed Action) Guardlocking Interlock (e.g. Spartan or 440G-MT) Guardlocking Interlock (e.g. Spartan or 440G-MT) 13 MSR144-n 14 (17 MSR238-n 18) 23 MSR144-n 24 (27 MSR238-n 28) S21 S11 S MSR127TP 440R-N23132 S22 S12 S PowerFlex AC Drive with DriveGuard DeviceNet Dig. Comm Safe-Off Option Enable Gate Control Power Supply Gate Control Circuit Remove Jumper L1 L2 L3 R S T U V W PowerFlex AC Drive with DriveGuard DeviceNet Dig. Comm Safe-Off Option Enable Gate Control Power Supply Gate Control Circuit Remove Jumper L1 L2 L3 R S T U V W Start Lock Release Stop Input 1756-IB IQ IB IB IB6 PLC Processor Output DeviceNet 1756-OW16I 1756-DNB 1769-OW8I 1769-SDN 1746-OW SDN 1734-OW PDN 1793-OW ADN Motor Motor Optional Signals from MSR238P Cat. 1 Stop 24VC DC Com Figure 1. Machine Guardlocking System with Safety Rated Drives Modular Safety Relay Design Concept Rev B Page 5 of 8 April 9, 2007

6 Safety Application Note Rockwell Automation +24V DC From 13 MSR MSR L1 L2 L3 L1 L2 L3 Guardlocking Interlock (e.g. Spartan or 440G-MT) Guardlocking Interlock (e.g. Spartan or 440G-MT) S21 S11 S MSR127TP 440R-N23132 S22 S12 S K1 K3 K2 K4 Start Lock Release Stop Input 1756-IB IQ IB IB IB6 PLC Processor Output 1756-OW16I 1769-OW8I 1746-OW OW OW4 M M 24VC DC Com K1 K3 K2 K4 100S Contactors or 700S Relays Figure 2. Machine Guardlocking System with Contactors Modular Safety Relay Design Concept Rev B Page 6 of 8 April 9, 2007

7 Safety Application Note Rockwell Automation +24V DC To Machine Interlock Relay To Previsous Machine From E-stop Relay of Previous Machine 33 MSR230-(n-1) 34 To Next Machine 43 MSR230-(n-1) 44 S11 S52 S21 X1 X2 S33 S MSR144-n MSR230-n From E-stop Relay of Next Machine S12 S22 X3 X4 Y2 Y MSR230-(n+1) MSR230-(n+1) 24 To Next Machine To Previsous Machine To Machine Interlock Relay 24VC DC Com Figure 3. E-Stop Safety Circuit Category 0 Stop Modular Safety Relay Design Concept Rev B Page 7 of 8 April 9, 2007

8 Safety Application Note Rockwell Automation +24V DC To PLC Initiate a Stop To Machine Interlock Relay To Previsous Machine From E-stop Relay of Previous Machine 33 MSR230-(n-1) 34 To Next Machine 43 MSR230-(n-1) 44 S11 S52 S21 X1 X2 S33 S MSR144-n MSR238-n MSR230-n From E-stop Relay of Next Machine S12 S22 X3 X4 Y2 Y MSR230-(n+1) MSR230-(n+1) 24 To Next Machine To Previsous Machine To PLC Initiate a Stop To Machine Interlock Relay 24VC DC Com Figure 4. E-Stop Safety Circuit Category 1 Stop Modular Safety Relay Design Concept Rev B Page 8 of 8 April 9, 2007