Manufacturers and operators of mobile equipment like joystick controls because they give the operator a feel for his command &emdash; the greater the displacement of the joystick, the greater the response from the actuator. Some approaches to interfacing hydraulic valves with joysticks include:
* mechanical linkages,
* hydraulic pilot circuits, and
* electrical commands to electrohydraulic valves.
With mechanical linkages for commanding hydraulic valves via dual-axis joysticks, the apparent disadvantage is the complexity of the linkage arrangement, making it difficult to install and adjust. The simplest linkages are levers and rods, but these become cumbersome if a straight path from the joystick to the hydraulic component does not exist. When the mechanical linkage must be routed over, under, around, or through obstacles, mechanical push-pull cables have found favor.
All mechanical linkages may require operators to generate objectionably high actuating forces if the mechanical advantage of the assembly is not designed in. Otherwise, operator fatigue may result. And as with all mechanical arrangements, regular lubrication and adjustment for wear may be necessary.
A neater arrangement uses hydraulically piloted valves and joysticks. With this setup, low-pressure fluid is routed to the joystick, which, in turn, routes fluid to the appropriate pilot-operated hydraulic devices based on joystick position The advantages of this system over a mechanical linkage arrangement are simplified installation, lower actuating force required by the operator, and less maintenance. Among the disadvantages of piloted joysticks are the potential for leakage, noise, and heat from hydraulic fluid in close proximity to the operator. In winter weather, cold oil adversely affects response and increases operator effort. Hydraulic pilot installation still involves the routing of multiple hoses or soft-metal tubing.
An electric joystick uses a power supply and sends electric signals to command an electrohydraulic valve. Because thin wires are so much easier to route through a machine than mechanical cables, hoses, or tubing, electric joysticks greatly simplify installation and and provide the freedom of remote mounting. Serial communications make this process even easier. Electronic control also provides the advantage of being able to create unique response curves for lever position versus flow and/or pressure and the incorporation of integral safety interlocks. The valves can be located extremely far from the joystick. The principal disadvantage of the electric joystick until recently has been the higher cost of electrohydraulic valves over their manually driven or pilot-operated counterparts. The cost trade-off between hydraulic and electric joysticks is about even.
Electric joysticks
There are two major categories of electric joysticks — displacement and non-displacement &emdash; as well as a hybrid non-displacement sensor with a displacement lever. The displacement type uses the motion of a lever, transducing that motion eventually into an electrical output. The non-displacement type has a lever similar to the displacement type. However, the motion of the joystick is not transduced directly to an electrical output. Instead, the force applied to the lever is transduced via a strain gage or similar medium to electrical output. An advantage of the non-displacement joystick is the absence of moving parts. A disadvantage is the loss of tactile feedback to the operator normally associated with the motion of a joystick. The hybrid non-displacement sensor with a displacement lever uses a heavy spring to connect the sensor to a lever and create the desired joystick feel.
The three popular designs of displacement-type joysticks are potentiometric movement, inductive coupling, and Hall effect. The potentiometric joystick, Figure 1, uses a rotary or linear potentiometer to convert mechanical displacement to electrical output. The conversion from curvilinear motion of the joystick lever to potentiometer movement usually involves shafts, gimbals, gears, and torsion springs. These mechanisms contain many parts, which can make these joysticks vulnerable to damage and shortened lifespan &emdash; especially if they are exposed to machine vibration.
Potentiometric joysticks are available in high-cycle-life construction with published life in excess of 106 full cycles. Most joysticks spend the greater part of their working lives in a neutral position, with the wipers of their pots being dithered continuously due to the vibration of the machinery onto which they are mounted and the mass of the control levers. This low amplitude cycling (dithering) of the pot wiper may prove more destructive to the pot than full-cycle stroking would. The constant dithering and high accumulation of cycles over a narrow area may cause the conductive element to wear-in locally, causing a "dead" or "noisy" spot.
One big advantage to potentiometric joysticks is high noise immunity. When choosing either an inductively coupled or Hall-effect sensor based joystick, care must be taken to insure that the potential EMI/RFI interference is not sufficient to self-activate the joystick output. Various designs by various manufacturers have different response levels and frequencies. The sensor in one manufacturer's unit is a strain gauge based hybrid non-displacement sensor with a displacement lever that has been placed so as to be insensitive to EMI/RFI in excess of 100 volts per meter.
Inductively coupled movement
An inductively coupled joystick uses a variable-transformer-type relationship. A primary coil sets up a field that is induced into a set of secondary coils. Through movement of either the drive coil or a ferrous shaft, which commutates the field, the induced field will vary proportionally. The closer the drive coil is to a secondary coil, the stronger the pickup field. The secondary coil that is farther away from the primary coil will have a proportionally smaller pickup. Figure 2 shows the relationship of primary and secondary coils.
The principal advantage of this mechanism over that of the potentiometric movement is that no contacting or wiping electrical parts exist. Further, the mechanical complexity is much less. The model shown in Figure 2 has only three moving parts (lever, centering cup, and helical compression spring), so life of the control is significantly extended.
Protection from stray electrical fields affecting the joystick's inductive field is provided by a synchronous detection system. The pickup from the four secondary coils must equal the induced signal provided by the primary, so the effects of adjacent electrical fields essentially are ignored.
Hall-effect technology
Development of newer joystick technology has focused on enhancing linearity and electromagnetic immunity while providing additional output capabilities within a smaller working envelope. One significant technology — the Hall effect — has emerged as being completely capable of providing all these desired attributes and enhancements. If a magnetic field is present when an electrical current flows through a conductive material, the electrons are uniformly distributed throughout the conductor, Figure 3A. Introducing a magnetic field to the electric current disrupts the current and causes its course to be changed, Figure 3B. When the input current is held constant, as in a joystick application, the Hall voltage is directly proportional to the perpendicular component of the magnetic field. Therefore, if the magnets change position, the voltage changes and can be quantified as joystick movement.
 |
|
Figure 3A.
|
Additional axes
The mechanical simplicity of the inductively coupled and Hall effect joystick movement lends itself to incorporation of multiple movement axes. The traditional two-axis joystick can have a third or even a fourth axis added to it, retaining the basic mechanical simplicity, yet having no wiping contacts. For example, a twist movement of the handgrip may produce a third axis, and a thumb-operated wheel can provide a proportional fourth axis. A grip-mounted switch could also be incorporated to add simple functionality.
 |
|
Figure 3B.
|
Joysticks with coupled non-contact sensing elements have been replacing the more traditional models on continuous duty cycle applications &emdash; such as large excavators used in strip mines, personnel platforms, construction equipment, and other heavy-duty applications. These applications and other have found electric joysticks, particularly the inductively coupled type, to enhance their reliability.
Joysticks with inductively coupled movement have been replacing traditional joysticks on continuous duty cycle applications &emdash; such as large excavators used in strip mines and aerial basket controls for utility trucks using fiber-optic couplings. These and other applications have found electric joysticks, and more particularly the non-contact sensing element type, to enhance their reliability.
Joystick controller helps move mountains of cargo
Aircraft Maintenance Support Services, Bridgend, Wales, a manufacturer of ground support equipment for military and civilian customers worldwide, has specified the Penny & Giles JC150 joystick controller for its latest Atlas 2000 air-portable cargo transporter/loader.
The four-wheel-drive vehicle, which is sold to air forces and international peacekeeping and relief agencies, weighs less than 12 tonnes and will load and transport up to 18 tonnes of cargo. It is used to load military and civilian aircraft, from a C130 Hercules to the B747 Main Deck, which requires a 4.5-m vertical lift.
The Penny & Giles JC150 provides full control of the Atlas 2000's platform through its processor control system. The functions include lowering and raising from 1 to 5.5 m and pitching and rolling fore and aft, as well as left and right.
Commenting on the choice of joystick, Phil Summers of AMSS says the decision was an easy one as the Atlas 2000's predecessor, the Atlas Mk2, is fitted with a Penny & Giles JC600 joystick and both are still giving excellent service around the world.
New designs improve joysticks
The rapid growth of electronic controls on all types of mobile equipment is changing customer expectations and lever innovations. Adaptive solutions to problems with "old technologies" are rapidly appearing. One is the unique solution to the strain gauge force lever's usual problem of, "It doesn't move . . . how do I know where the force is applied and how much?" Quadrastat Corp., Pomona, Calif., has solved the problem inherent to the decade-old design by using a stiff spring as the connector from the lever handle to the stationary post containing the strain gauge. The result: a moving handle strain gauge lever.
Further redevelopment of the output amplifier design resulted in a unit that operates in a 100 V/m EMI/RFI environment. The resulting package has a lever that will survive almost any mobile equipment environment, both electrically and mechanically. The joystick features a 2.1- by 2.6-in. mounting footprint, along with an extremely high maximum side load capability of over 200 lb.
Another innovative solution is the lever sensor by Elobau Sensor Technology, Germany. This company has taken the analog Hall Effect sensor that has been around for several years and modified it to be tolerant of EMI/RFI of 100 V/m. The result: a lever capable of operating in the mobile equipment electrical environment. The Elobau sensor unitlever is finger-tip size (compared to full arm motion) and is approximately 3.6-in. above the mounting surface with a footprint of 2.4- by 2.4-in.
The one area that continues to elude lever manufacturers is the ability to design new grips that are being requested. Mobile equipment designers continue to request that manufacturers place a large number of switches (and indicator lights) in extremely small grips to conserve cab space.
One particularly amusing request was for eight pushbutton switches and six indicator lights in a rectangular head that measured 2 in. by 2 in. — plus a deadman lever in the grip. Even if someone was able to place all of the switches and lights into the 2-in. by 2-in. area, what equipment operator could activate a selected switch with his or her thumb?
For more info: selected joystick manufacturers
ADDCO, Inc.
240 Arlington Ave. E.
St. Paul, MN 55117
(651) 558-3561
www.addcoinc.com
Bosch Rexroth
1700 Old Mansfield Road
Wooster ,Ohio 44691
330-263-3300
www.boschrexroth-us.com
Camozzi Pneumatics, Inc.
2160 Redbud Blvd., Ste. 101
McKinney, TX 75069
(972) 548-8885
www.camozzi.com
Hoke Inc.
405 Centura Ct.
Spartanburg SC 29303
(864) 574-7966
www.hoke.com
Hydro Electronic Devices, Inc. (HED)
1715A Innovation Way
Hartford, WI 53027
(262) 673-9450
www.hedonline.com
IC Fluid Power Inc.
63 Dixie Hwy. / P.O. Box 97
Rossford, OH, 43460
(419) 661-8811
www.icfluid.com
J.R. Merritt Controls, Inc.
55 Sperry Ave.
Stratford, CT 06615
(800) 333-JRMC
www.jrmerritt.com
Kawasaki Motors Corp., U.S.A.
5080 36th St. SE
Grand Rapids, MI 49512
(616) 949-6500
www.kawasakipmd.com
P-Q Controls, Inc.
95 Dolphin Rd.
Bristol, CT 6010
(860) 583-6994
www.p-qcontrols.com
Penny + Giles Controls, Inc.
1100 E. Woodfield Rd.
Schaumburg, IL 60173
(847) 995-0840
www.penny-giles.co.uk