27 February 2001
Motion Control and Vibration: Integrating System Design
A nontraditional approach to a common design problem may open doors to further opportunities.
Increasingly, as motion control systems become more refined, vibration becomes more important to a system's overall design and functionality. Traditionally, however, engineers have looked at motion control and vibration as separate issues. Motion control, it might be said, presents familiar design engineering problems, while vibration suggests more subtle problems. Few design engineers possess the experience or training to address both sets of problems in a single design solution.
Many engineers faced with this challenge arrive at solutions that offer, at best, a compromise in terms of optimum vibration control, overall system performance, energy efficiency, and cost. Consequently, many system designers have come to accept such compromises as givens. To arrive at meaningful design improvements, however, engineers might begin by first questioning some "givens" of design.
An area presenting challenges to many design givens is in the field of semiactive control using magneto-rheological (MR) fluids. A variety of robust, high-strength MR fluids and devices that enable the benefits of controllable fluid technology are now commercially available. Commercialized, mass-produced devices currently include rotary brakes for use in pneumatic systems, complete semiactive damper systems for heavy-duty truck seat suspensions, and adjustable linear shock absorbers for racing cars. MR fluid dampers that enable semiactive primary suspension in passenger cars have recently been announced. In addition, very large MR fluid dampers appropriate for seismic or wind damage mitigation in civil engineering structures, such as buildings and bridges, are under commercial development. The key to success in all of these implementations is MR fluid's ability to rapidly change its rheological properties upon exposure to an applied magnetic field.
A new way of using MR fluids has been developed in which the fluid is contained in an absorbent matrix. Such MR fluid sponge devices (Figure 1) enable the benefits of semiactive control to be realized in cost-sensitive applications (e.g., domestic washing machines). Moreover, it's in these simple applications that one might examine the issues of vibration and motion control in one design solution.
What Comes Out in the Wash
The common household washing machine represents the traditional compromise between controlling vibration associated with the spin cycle and optimum system performance and efficiency. The tub in a conventional machine is suspended by a number of coil springs, which provide both mechanical support and vibration isolation at high frequency. To prevent potentially damaging vibratory excursions when the drum speed approaches resonance during spin cycles, static vibration dampers are added to the suspension.
This system represents a compromise because while conventional dampers easily control the tub's motion at resonance, the system significantly degrades high-speed vibration isolation. This compromise in system design limits not only the tub's size but also the dimensions of the housing that accommodates the tub's overall motion.
In terms of the machine's overall performance, static damping draws energy from the motor that might otherwise go toward maximizing spin speeds for optimum water removal from clothes (thus shortening drying time). So how does one improve vibration control, performance, and energy efficiency without pricing the appliance beyond the reach of consumers?
What's Coming 'Over There'
European manufacturers of horizontal axis, front-loading washing machines are pushing for such increased performance and efficiency. Smaller in load capacity and typically installed in apartment household kitchens, these machines are designed for maximum energy efficiency and quiet operation (in small spaces and high-occupancy buildings, neighbors are never far away).
Often, households will have only a washing machine, not a dryer. For that reason, tub speeds are reaching 2,000 revolutions per minute, effectively becoming centrifuges in which almost all of the water is removed. In fact, manufacturers have had to reduce the size of the drain holes in tubs to prevent extrusion of clothes during the spin cycle.
To achieve this level of performance, however, manufacturers have incorporated a controllable damping system designed around MR fluid (Figure 2). Controllable dampers provide a simple, cost-effective solution to the dilemma of providing high-level damping at resonance because they can simply be turned off at high spin speeds for a high degree of vibration isolation.
The MR fluid sponge damper requires no seals or bearings and uses the same inexpensive components found in existing passive dampers, but with a few important modifications. The damper consists of a layer of open-celled polyurethane foam, or other suitable absorbent matrix materials, saturated with about 3 milliliters of MR fluid surrounding a steel bobbin and coil. Together, these elements form a piston on the end of a shaft. The piston is free to move axially inside a steel housing that provides the magnetic flux return path. Damping force is proportional to the sponge's active area.
The application of a magnetic field causes the MR fluid in the matrix to develop a yield strength and resist shear motion. The amount of force produced is proportional to the area of active MR sponge that's exposed to the magnetic field. This arrangement can be applied in both linear and rotary configurations that call for a direct shear mode of operation.
While passing through resonance, these controllable dampers may be energized to provide a high level of damping that totally controls the tub's excursions. At high speed, the MR sponge dampers are turned off, enabling a high level of vibration isolation, as shown in Figure 3.
With this enhanced vibration control, the drum may be made larger (or the housing made smaller) because less overall tub motion must be accommodated. Ideally, a pair of controllable dampers provides 50–150 N of damping force when energized and a low residual force of <5 N when turned off.
Controllable MR fluid dampers have low power requirements—so low, in fact, that a net energy savings can be realized. Effective resonance control typically requires about 10 watts of input power to the MR dampers for about 5 to 10 seconds, as the drum speed ramps through criticality. This power is readily available from existing onboard electronics in a standard machine. Only a low-cost relay, associated with the wash cycle timer, is required. Removing damping during the machine's high-speed spin phase decreases the motor's power consumption. This is a significant improvement over conventional passive dampers, which require the motor to work against their force to achieve the same spin speeds.
- Enables enhanced energy efficiency because clothes come out drier
- With heightened vibration control, allows tubs to be larger, housings smaller
- Achieves greater performance in terms of higher spin speeds without increased power consumption of more powerful motors
- Enables machines to accurately weigh loads, control water/detergent usage
- Uses onboard electronics
- Requires no additional operator control
- MR provides real-time controllability
- Dampers designed using existing materials, life of the machine
- When using existing materials, slightly increases materials cost but improves energy efficiency. Result: greater functionality and improved energy efficiency at roughly the same cost.
- Easily adapts additional electronic controls to extant machine electronics footprint.
The elegant simplicity of this nontraditional approach to a common design problem opens the doors to additional, and perhaps unanticipated, opportunities. For example, in the quest for maximized energy efficiency, washing machines are being designed—and rated by their ability—to use only the amount of water and detergent necessary to clean a load of clothes and no more (Figure 4). The goal is to weigh and wash a single silk shirt efficiently.
Conventional, passive dampers create barriers that prevent a machine from accurately weighing loads before washing; however, controllable dampers remove that barrier with no added costs. MC
Figures and Graphics
- Figure 1. Simple MR fluid sponge damper.
- Figure 2. A washing machine controlled by MR dampers.
- Figure 3. On/Off damping to control resonance and isolation.
- Figure 4. Damper force variation with current, displacement, speed, and life.
- Figure A. Elements of MR sponge damping.
- PDF of "Motion Control and Vibration: Integrating System Design"
J. David Carlson received his B.S. in physics from Case Western Reserve University and his Ph.D. in physics from the University of Colorado. He was a Postdoctoral Research Fellow at Ohio University, part of the research faculty at UC-Davis's Department of Physics, and an adjunct professor at both Gannon and Penn State Universities. Dr. Carlson has authored more than 90 technical papers and received 38 U.S. patents relating to controllable fluids and devices. Since 1983, he's been involved with the research and development of electro-rheological (ER) and magneto-rheological (MR) fluids. He is one of the founders of the International ER and MR Fluid Conference Series and a member of APS, ACS, IEEE, MRS, SPIE, and SAE. Contact him at 111 Lord Drive, Cary, NC 27512-8012; tel: (919) 468-5979; fax: (919) 481-0349; www.lordcorp.com or www.mrfluid.com.