It’s not unusual for cranes to reach 100 ft or more into the air at major construction sites. Traditionally, cranes are transported to a work area and assembled on-site. More recently, as truck-mounted cranes become bigger and more powerful, they have found favor because they are quicker to set up than traditional cranes. Truck-mounted cranes have a telescoping hydraulic boom mounted on commercial truck chassis. Their portability and lower setup costs have led to their widespread use at construction and utility sites around the world. But as loads get heavier and lifting distances become higher, designers of truck-mounted cranes must provide the stability to ensure that safety remains the top priority.
Truck-mounted cranes use outrigger systems to ensure stable operation. The outriggers extend from the main body of the truck and contact the ground several feet away from the truck. This distributes the crane’s load over a much larger area, thereby increasing stability. Manitowoc Company Inc., Manitowoc, Wis., takes this a step further by using smart cylinders in the A-frame outrigger systems of its National Crane line of truck-mounted cranes. The crane’s hydraulic system is driven from a power takeoff on the truck’s transmission. The crane operator then runs all crane functions through a series of lever-operated valves at a control station.
An important function for lifting, moving, and lowering heavy loads is to ensure that outrigger beams are properly positioned. The outriggers are attached to the truck frame and are extended downward by hydraulic cylinders at an angle to create an A-frame structure that is wider at its base than at the top. This provides a stable framework to level and support the loaded and extended crane.
Adding smarts to outriggers
For the past several years, National Crane has added outrigger-monitoring systems (OMSs) to its cranes. With the OMS, operators monitor the horizontal extension of the crane’s outriggers at a control station. The OMS used with A-frame model cranes includes an ELA position-sensing linear-displacement transducer (LDT) from Rota Engineering, Dallas, an electronic control module (ECM), and bicolor indication LEDs at each station.
The ELA is an externally mounted LDT that uses Hall-effect technology to sense the location of a magnet embedded in the cylinder’s piston through the cylinder’s carbon steel barrel. A microprocessor then assigns an analog voltage to the magnet’s corresponding absolute position. For example, when the cylinder is fully retracted; the voltage may be 0.55 V. As the cylinder extends, the voltage gradually increases until 4.5 V is reached at full extension. Accuracy of the transducer is typically ±0.5 mm (0.02 in.) — more than adequate for most mobile equipment. That position is then sent to the ECM and compared to the known maximum horizontal extension. After this, an indication is given to the operator about the outrigger state. The position update happens within milliseconds.
Mark Hoffman, of Rota Engineering, pointed out that because mobile equipment has a human operator, position feedback from cylinders generally only needs to be within hundredths of an inch. Put simply, he says that magnetostrictive LDTs are overkill for most mobile-equipment applications. He suggests that an LDT with slightly less precision, but substantially lower cost, would enable designers to provide cylinder position feedback more often — not just for the most critical applications that justify high cost.
Simple electronic display
The electronic control module on the A-frame units serves only to monitor the position of the outriggers and provide feedback to the operator. As the analog voltage from the ELA transducer is read into the ECM, it sends a signal to a set of bicolor LEDs — one set per operator’s station. The indications available are:
- Red for system error (sensor out of range, electrical short, etc.)
- Blinking red to indicate the operator is not at a valid working position as directed by the operation manual
- Green to inform the operator that full horizontal extension has been accomplished. The ECM can be configured through the use of a service tool to also help diagnose any issues related to the OMS.
Made for mobile
Designed for use with mobile equipment, the ELA transducer matches this application well because of several physical and intrinsic attributes. The most important of these is the ability to mount the sensor along the exterior of the hydraulic outrigger-cylinder barrel. Although the cylinder gains added functionality, in many cases it retains the same form and fit as the original cylinder; the smart cylinder is essentially a drop-in replacement. The envelope in which the cylinder is mounted does not change. Only additional harnessing and the ECM are added — plus there are minor physical changes to the rear stabilizers.
The cylinder bores used in A-frame outriggers range from 3 to 4.5 in. Strokes may be as long as of 66.9 in., depending on lifting capacity. According to Hoffman added, “Eliminating the expense of gun-drilling the piston rod and machining the end cap reduces the cost of creating this smart cylinder. The cylinder’s structural integrity remains the same, and it is easier to assemble, install, and service than cylinders with magnetostrictive sensors.”
Other positive attributes: the Hall-effect sensor is noncontact for long service life, its temperature rating is high, it performs well in high shock and vibration applications, and its aluminum housing resists damage from impact and corrosion. The external transducer can be replaced in the field without difficulty.
Cylinders can be supplied with magnets already fitted, so that if the stroke-sensing function is required later, the transducer can easily be added. The magnet assembly for the EL transducer is designed to match the bore of the cylinder. A slot is milled into the piston to accommodate the magnet assembly. Service life is not a factor because the magnet assembly is made of the same quality as piston-wear rings.
A different kind of linear sensor
Model ELA linear-displacement transducers (LDTs) use Hall-effect technology and mount externally to mobile hydraulic cylinders. Unlike other types of in-cylinder LDTs, they can be used in double-ended cylinders. They can also be used effectively in steering and long-stroke cylinders, where gun drilling can become cost prohibitive and are easily field replaceable.Hall-effect LDTs can be manufactured for strokes exceeding 50 ft and for use 20,000 ft below the surface of the ocean and other demanding environments.
LDTs from Rota Engineering use a microprocessor that transmits and receives signals from Hall-effect chips mounted to a printed-circuit board. The circuit board is contained within a stainless-steel or aluminum housing, depending on application requirements. A piston-mounted magnet causes a voltage drop when it passes over the Hall-effect chip. The microprocessor calculates the position of the Hall-effect chip and correlates the voltage drop to a proportional voltage, current, PWM, or CANbus output.
Hoffman explains, “Hall-effect sensors do not have as high a resolution as magnetostrictive sensors, which can achieve resolution measured in ten-thousandths of an inch. Hall-effect LDTs, however, generally have resolution of 0.012 to 0.020 in. The tighter resolution of magnetostrictive LDTs is needed for many process applications, such as a rolling mill. Most of the time, though, 0.020-in. resolution is more than sufficient for mobile hydraulic applications.”
An additional benefit of the Hall-effect technology is small size. In most instances, the pin-to-pin dimension of a cylinder need not be increased to accommodate a Hall-effect LDT. Also, the surface-mount technology tolerates high levels of vibration, and potting can provide additional vibration resistance.