Manufacturers and operators of mobile equipment like joystick controls because they give the operator a feel for his command — 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.
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
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.
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.
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.
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
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) 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
This file type includes high resolution graphics and schematics when applicable.