Pneumatics troubleshooting gets easier

Ed Bickel
Bosch Rexroth Corp.
Pneumatics Business Unit
Lexington, Ky.

A newly developed pneumatic-control system provides basic valve operation, as well as detailed monitoring of electrical solenoids, sensors, wiring, and connections.

Called Drive Diagnostic Link (DDL), the system checks that every valve's supply voltage is within acceptable tolerances, solenoid coils operate properly, and instantly pinpoints short circuits. And should it detect a fault, the control system displays appropriate text messages to alert operators and service technicians, often before a failure occurs. Checks are carried out as soon as a machine is switched on and continuously as it runs, ensuring the pneumatic subsystem operates properly at all times.

By integrating "intelligence" with the control system, DDL makes it easier to use diagnostic capabilities with Bosch Rexroth pneumatic systems. And better diagnostics means better response to impending problems, permitting proactive maintenance with less downtime and, ultimately, higher productivity and reliability.

SMART AND SIMPLE
Production-plant operators increasingly dictate that advance fault notification, typically called "preventive maintenance," be included in new control packages. Pneumatic systems traditionally generate diagnostic information through cryptic LEDs or indirectly through externally mounted sensors. Neither approach satisfactorily provides meaningful information because they only signal faults after the fact.

Take, for example, the case where a part breaks and prevents an air cylinder from completing its stroke. Without sophisticated diagnostics, personnel only know something went wrong, not whether the problem is mechanical, pneumatic, or electrical. Technicians generally need considerable experience to diagnose the cause of faults and determine a correct course of remedial action. DDL, on the other hand, identifies impending valve failure and permits repair or replacement in advance, before it affects an entire production line.

While DDL isn't for every pneumatics installation, any system that requires instantaneous feedback down to the input or output level is an ideal candidate. When a pneumatic fault can disrupt a wide range of operations, real-time diagnostics becomes a critical factor in improving reliability and maintenance. It scales easily and can be used in applications ranging from relatively simple machines to entire production plants.

DDL also simplifies the design of pneumatic systems because it is compatible with common fieldbus protocols including Profibus DP, DeviceNet, Interbus S, CANopen, ControlNet, and EtherNet/IP.

DDL's daisy-chained bus structure eliminates up to 90% of the hard wiring normally needed. This makes it easy to create more-sophisticated capabilities without adding complexity. The labor savings to build a system can be quite significant and less wiring means fewer mistakes and quicker installation. Testing and commissioning are typically faster, with fewer glitches. And problems, if they occur, are easier to trace and remedy.

DEEP DIAGNOSTICS
DDL monitors the electrical-control system, continuously checking each valve for:

• Supply voltage tolerances on electronics power.
• Supply voltage tolerances on valve or other output power.
• Solenoid coil opens or shorts.
• Short circuits.

For instance, resistors are set up in line with an operating output, and monitor output drivers such as valve coils or external outputs that are wired through a DDL module. Resistance values are converted to serial information and fed back to the PLC or PC controlling the network.

Resistance levels slightly outside the normal operating range are usually far from a failure point. But they provide ample warning to schedule corrective maintenance prior to an unexpected shutdown.

For example, the supply voltage for valve electronics is nominally 24 V, with a tolerance range of 21.6 to 26.4 V. DDL continuously monitors this level and reports if it falls outside the range.

Low voltage can indicate a power-supply issue that is, or soon will be, a major problem in a system's 24-V dc power loop. Detecting a rise in current on an output warns of an impending breakdown in a valve coil or an external device hardwired to the system's output module.

ONE NODE, MANY I/O
Structurally, DDL has a serial master/slave configuration. Two types of modules are the bus module, which controls communication with the fieldbus, and participant modules that contain valves and I/O. These include:

• Valve drivers with up to 24 valves and coils.
• Valve terminals with integrated DDL and up to 16 valves.
• Pressure control valves with electricalto-pneumatic transducers.
• Digital and analog I/O modules.

In addition, DDL also supports external, nonintegrated analog I/O such as linear potentiometers, thermocouples, servodrives, and other 4 to 20-mA or 0 to 10-V devices.

Participants communicate with the bus module at 125 or 250 kbaud. The diagnostic information is at a bit level, with individual bits indicating the status for various parts of the system. The advantage of bit-level information is that it communicates a considerable amount of data in only a few bytes. The PLC converts this low-level information to meaningful text messages.

Each participant module is assigned an address, either automatically or set through a rotary hex switch, for communication with the bus module. The bus module, in turn, handles all communication over the fieldbus.

This arrangement means the entire DDL chain has only one fieldbus address, even though the system supports multiple valves and I/O modules over a 30-m bus length. The serial DDL link is transparent to the fieldbus.

Because the PLC must scan only one device to obtain diagnostic information, DDL decreases loading on the fieldbus. The DDL network is designed for fast, secure data transfer. The bus module has a 0.8-msec cycle time. System cycle time depends on baud rate, number of participants, and data length (up to 128 bits). Representative cycle times are shown in the table.

The system handles up to 1,024 I/O points on a single fieldbus address, depending on the fieldbus used. Modules use five-pin, M12, keyed connectors for simple cabling of the daisy-chain links. The modules also contain LEDs for local troubleshooting.

On start-up, the bus module polls all participants to detect missing modules or problems, and continues to do so during operation. DDL aids predictive maintenance by identifying problems so they are corrected before a failure. The ability to detect small changes in supply voltages, solenoid opens, and shorts yields a surprising amount of information about the health of a pneumatics system.

Some DDL participants can also accommodate fieldbus failures by adjusting software parameters. Because the DDL can control all attached I/O independent of the fieldbus, a failure does not prevent the DDL link from responding with appropriate measures. And the system quickly and clearly displays operating status by linking diagnostic data to report messages from sensors, regulator valves, and PLC programs.

What makes DDL especially useful in promoting productivity and lowering costly downtime is the valve-level diagnostics that give greater insight into the health of a pneumatic systems.

REPRESENTATIVE CYCLE TIMES

MAKE CONTACT

Bosch Rexroth
Pneumatics Business Unit
(859) 254-8031
boschrexroth-us.com