Continuing evolution keeps joysticks the most intuitive means to interface man with machine.
By Gerry Larocque
J. R. Merritt Controls Inc.
Hydraulic joysticks offer the simplicity of controlling pilotoperated valves directly, and without electronic controls that can add cost to systems. This makes them most attractive for smaller machines where the close proximity of pressurized hydraulic fluid is not objectionable.
Joysticks using high-life conductive plastic potentiometers are the predominant type of electric joystic in use today.
Electric joysticks have evolved into a variety designs with functions built into hand grips, allowing operators to perform multiple tasks and significantly increase productivity.
The advent of hydraulics allowed controlling much higher levels of power via valves. A hydraulic joystick connects directly to low-pressure ( pilot) hydraulic valves, allowing an operator to control the high-pressure "working side" of the hydraulic system. The benefits of hydraulic joysticks over mechanical linkage or cable systems include simpler installation for complex control applications, the ability to substantially increase power, and minimize maintenance. As with mechanical controls, another significant advantage is the ability of the operator to feel any force feedback through the joystick.
The disadvantages of pilot valve joysticks are the close proximity of pressurized hydraulic fluid to the operator, the temperature effect on hydraulic fluid performance, and the considerable cost of routing multiple hydraulic lines throughout the system.
Hydraulic pilot valve joysticks gained widespread use on construction equipment, aerial work platforms, and forklifts. When this was the state-of-the-art technology, hydraulic joysticks were used in most applications of this type, because the advantages over mechanical linkages were many.
As technology of other control alternatives has advanced, the hydraulic joystick still maintains a competitive position in smaller equipment where the compactness of the hydraulic system minimizes the cost of fluid power transmission. The first applications to venture into the newer technology of electric joysticks have been larger equipment with extensive hydraulic systems. With long hose and tube routings and operators at a considerable distance from the working end of the equipment, these applications have the most to gain from electric joystick technology.
Most electric joysticks operate on low-voltage DC circuits and can be located a long distance from the electrohydraulic valve they control. Wiring from the joystick to the valve is much easier and costs only a fraction of what would be required to route hoses and tubes from the joystick to the hydraulic valve. This allows the freedom to design a more ergonomic operator station without compromising the cost of the equipment. The primary disadvantage as compared to a manual or pilot-operated valve is the higher cost of the electrohydraulic valves.
Electric joystick types
Electric joysticks are manufactured in a wide variety of types, ranging from large hand-operated displacement and force controls to very small finger-operated devices. Each has its own niche application. Joysticks can be further classified into discrete control, which use switch contacts, or proportional control, which use potentiometers (pots), digital encoders, rotary variable differential transformers
displacement and force controls to very small finger-operated devices. Each has its own niche application. Joysticks can be further classified into discrete control, which use switch contacts, or proportional control, which use potentiometers (pots), digital encoders, rotary variable differential transformers (RVDTs), induction coils, or Halleffect devices. Force (nondisplacement) controls primarily use induction coils or strain gauges as output devices. Small finger-operated joysticks are also made in displacement and force types. Displacement joysticks in this category generally use potentiometers and Hall-effect devices for proportional output and microswitches for discrete output.
Switching-type joysticks use high current and voltage mainly on equipment that requires stepped, discrete control of a motor drive. At the other end of the spectrum are finger-operated joysticks that control microswitches. These are often used as simple discrete (bang-bang) controls — one switch closure in each direction — for single-speed directional control.
The most-common use of a joystick is for proportional control of machinery through an electric motor control, electrohydraulic valve, hydraulic pump or computer.
Encoders and RVDTs are highly reliable, non-contacting output devices. Because of their large size and high cost, they are normally used in large controllers that are integrated into equipment that requires highly reliable components, where failure and downtime come at an extraordinarily high cost. Cranes used in metal-producing mills and container-handling terminals are examples of applications using these devices. Strain gauge-type joysticks are generally relegated to use as small force-type joysticks.
This leaves joysticks using pots, induction coil or Hall effect as the most-commonly preferred controllers. Of these three types, the joystick using a potentiometer as a primary output device is predominant.
Pot joysticks using conductive plastic pots have published life spans in excess of 10 million cycles. In applications that don't have severe vibration, this duty cycle results in a very respectable lifetime for the joystick, if it is operated and maintained properly. Because pots are small devices, they can be included as components on both large and small joysticks. This allows flexibility in the design of machine control because the joysticks can be placed almost anywhere.
Electronically, the pot output can be used directly, producing a variable DC voltage, or that signal can be processed further through circuitry to produce a pulse width modulated (PWM) signal, a digital serial stream (RS232, etc.) or other variants of analog and digital signals. Potentiometers also are relatively immune to electrical emissions from outside sources. This makes their operation predictable around high voltage power transmission lines or in any application where the joystick may come in close proximity to a strong electric field.
As the most-widely used type of joystick, pots are found controlling the movements of excavators, agricultural equipment, aerial work platforms, scissor lifts, skid steer loaders, pavers, compactors and a wide variety of other construction equipment.
The disadvantage of using a potentiometer in a joystick is realized only if the application involves very high duty cycle or vibration. A potentiometer is a variable resistive device using a delicate wiper contacting a resistive element. Although the joystick may be in neutral (center) position for much of the time a machine is active, vibration from the machine can continuously dither the pot wiper on the element and produce heavy wear in the center region. Devices using contacts are always subject to wear and potential failure, but the use of non-contacting technology is a way to avoid this.
The two most popular types of non-contacting joystick technologies are induction and Hall effect. Both offer highly reliable output at reasonable costs. In addition, the gear train used to drive potentiometers is eliminated, enhancing the reliability of the joystick even more. Inductive joysticks work on the principle of varying an induced field in a set of secondary coils by changing the relative position of a primary coil or ferrous shaft to the secondary coils. Protection from outside electrical fields is provided by an internal detection system. The size and number of coils limit how small these joysticks can be practically manufactured.
Hall-effect joysticks operate on the principle of varying voltage output by disturbing a current flow through interjection of a magnetic field. An advantage of Hall-effect technology over inductive technology is miniaturization of the output device and redundancy at a very modest cost. When properly designed and packaged, Hall-effect devices offer very high resistance to electromagnetic and radio frequency interference. Because the increase in reliability comes at little-to-no increase in cost, these joysticks can be used in all of the applications where the more-traditional potentiometertype joysticks fit.
As equipment or vehicle operator cabs has become more sophisticated and ergonomic, electric joysticks have evolved even further with the adaptation of a variety of hand grips to them. Providing an operator the ability to perform multiple tasks by manipulating a variety of switches on a handgrip coupled to the input shaft of the joystick is a means of significantly increasing productivity. By using grip-mounted switches to accomplish high-frequency tasks, equipment operators no longer must avert their attention or reposition their body to activate a switch that previously was difficult to reach.
Electric joysticks are specifically designed with hollow input shafts to provide wire routing from the handgrip down through the joystick. Joysticks are manufactured as single, dual or 3-axis control ( X, Y and twist). The handgrip can provide a platform for additional proportional axes by installing a thumb wheel pot, analog rocker or thumb-operated miniature joystick.
With the wide variety of joysticks available, there is sure to be at least one for virtually any application. A discussion of your specific application with the joystick manufacturer will ensure the best joystick choice. Find a company that manufactures a full range of joysticks and is willing to spend the time to suggest a solution.
|Large, hand controls||Small, finger-operated|