Jonathan Brokaw
Bosch Rexroth Corp.
Mobile Hydraulics Div.
Wooster, Ohio
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An essential requirement of modern hydraulics is that various implements on a machine operate simultaneously, yet with precise control. Valves must handle changing loads and flows even though hydraulic demands vary, depending on the materials being handled and the nature of movement. |
It's no secret that OEMs want hydraulic controls that simplify mobile-equipment design and are versatile enough to handle a range of applications. Modular load-sensing valves fit this bill and are an increasingly popular alternative to traditional monoblocks.
The latest generation of loadsensing directional-control valves incorporates both flow and pressure control into sandwich-style units.
Although it is possible to control equipment operations by either flow or pressure, doing so requires additional external plumbing that increases complexity and cost. Placing flow and pressure control in the same valve simplifies hydraulics, gives engineers added design flexibility, and reduces the number of components and connections — thus increasing reliability.
One example of the latest generation of valves is the Rexroth M4, a pressure-compensated directional-control valve for medium and high-pressure duty on forestry, agricultural, and construction equipment. Its mechanical stroke limiters adjust to set maximum flow. And load-sensing pressure limiters on each port can maintain constant flow under varying loads. This ensures the valves deliver just the right flow to each function, independent of overall loads on the hydraulic system.
In a crane, for example, hydraulics power the outriggers for leveling and stabilizing, extends, retracts, and rotates the boom, and so on. For each operation, the valves precisely control hydraulic-motor speed and cylinder position over a wide range of loads and conditions.
The M4 also has load ports, which can include a companion measuring port for sensing realtime flow and pressure. Optional relief valves on work ports protect systems from shock, pressure spikes, and cavitation.
Three valve sizes, M4-12, M4-15, and M4-22, accommodate flows of 35, 53, and 105 gpm, respectively. Because pressure and flow demands usually differ not only from one type of machine to another, but for various functions on the same machine, engineers can stack different-size M4 valves together. Up to 10 Size 12 and 15 valves or eight Size 15 and 22 valves can be ganged together in a single, sealed unit.
All three versions offer opencenter control for fixeddisplacement pumps and closed-center control for variable-displacement pumps. And each operates in manual, hydraulic, or electric mode as on/off or proportional valves. Hand-lever mechanical overrides on each valve section work with electrical or hydraulic actuation in two ways:
Engaged: The lever follows every movement of the control spool. Operators use this visual feedback for safety.
Disengaged: The lever remains stationary as the valve cycles. This generates less friction and lower hysteresis for faster reaction times and higher accuracy without requiring position feedback on the spool.
ELECTRONIC CONTROL
"Intelligent" electrohydraulics clearly have a bright future in mobile systems. They bring finer control, greater safety, sophisticated diagnostics, and are easier to operate. And software tools are widely available to speed design and commissioning. Development and diagnostic software usually has Windows-based graphic interfaces and uses highlevel block programming based on IEC 61131-3. Software modules are available for most standard functions and applications, including timers and Flash downloads; standard routines, including ramps, curves, and PID control; and applications such as drive programs with load-limiting control.
Designers mate electronics to load-sensing valves in two principle ways: through external controllers or on-board electronics. External devices, such as Rexroth's RC family, use 16-bit controllers for open and closedloop applications and handle analog and digital signals at both input and output ports. Pulsewidthmodulated solenoid outputs produce highly accurate proportional control with low hysteresis. The design withstands vibration, temperature extremes, water and dirt, and electromagnetic interference — key features of any mobile controller.
A potential drawback to external control is complex wiring, if equipment uses point-to-point wire-harness connections. However, some electronically controlled hydraulic applications are relying on CANbus, an open architecture that uses only four wires to connect devices in a serial daisy chain. It significantly simplifies communication networks.
Electronic control can also be built into the valve. Bringing control closer to the action offers several advantages, including:
- Faster response and lower hysteresis, for greater precision.
- Simpler wiring than with external control.
- Off-load processing tasks from the central controller.
On-board valve electronics do not eliminate the central controller, but they can permit a smaller and less-costly one. Onboard control supplements the main unit by executing CANbus commands and providing operating feedback. Ganged valve modules also act as a single unit in the CANbus daisy chain, further simplifying wiring.
Multitasking hydraulics
A typical application for M4 valves is on a truckmounted loading crane. A single, variabledisplacement pump supplies the six-section valve that controls various crane functions:
A key requirement is that these functions operate simultaneously, yet with precise control. The valves must handle changing loads and flows even though hydraulic demands vary, depending on materials being handled and the nature of movement. In such applications, variable-displacement pumps hold an advantage over their fixed-displacement counterparts. Fixeddisplacement pumps always deliver the same flow, regardless of demand. So if the crane sits idle, all the flow dumps to the tank in an inefficient waste of energy. Variabledisplacement, loadsensing pumps, however, only provide the flow required by operations at the valve. The pumps destroke to low-pressure standby until one or more functions engage. This improves system efficiency. For safety reasons, static controls that stabilize the truck typically rely on a different set of valves. |
MAKE CONTACT
Bosch Rexroth, (330) 263-3434, boschrexroth-us.com