The sophisticated solenoid: rethinking the humble device for new efficiencies.
"We started with a clean slate approach," recalls Dennis Maller, Trombetta's vice-president of engineering. Although his company had made industrial solenoids, relays, and other electrical components for years, their new Equalizer would be designed from scratch. "The only part of the past we clung to was the acquired knowledge we had in this area."
The Trombetta engineers sought to eliminate common problems they regularly see with work solenoids on industrial equipment, as well as simplify existing systems consisting of solenoids, controls, and the wire harnesses connecting them. "We strive for elegant solutions," says Maller. "This undertaking was handled with an awareness and acquired knowledge of both currently available electronic technologies, and trends in available component costs-versus-performance."
Most solenoids on the market are straightforward devices, consisting of a cylindrical coil that, when electrified, produces a magnetic field that causes a plunger shaft or arm to shift position. Trombetta engineers set out with a goal to compensate for the factors that would normally weaken solenoid performance in real-world applications and drive up installed cost. For example, the pull force of these types of work solenoids generally weakens in direct proportion to any reduction in voltage at the coil. "It is not uncommon to experience a 20% reduction in voltage due to wiring losses," says Maller. He adds that the force is further reduced as temperatures rise. "If the ambient temperature reaches 100[degrees]C and the solenoid's internal losses result in a 50[degrees]C rise above ambient, then the solenoid will lose about 1/3 of its pull force from the heat increase alone," he indicates. "When the two factors are combined, its not uncommon to see work solenoids loose about 50% of their force."
Maller believes that if solenoids are selected based on their published specifications, which typically only tell how they perform with rated voltage applied and with the coils at 25[degrees]C, then there is a good chance for problems unless very large safety margins were applied during the selection process. "Even choosing a safety margin of 30%, which might seem conservative at the time, can get you in trouble," warns Maller.
Additionally, Trombetta engineers saw that designers of compact, high-force solenoids commonly use dual winding coils: one high-watt, intermittent duty winding for pull-in and a second, fine wire, low-wattage coil for continuous duty hold. "Out of the gates, the design is compromised be cause you get only about 70% of the coil working in pull mode and 30% during hold mode," explains Maller. He adds that the use of fine wire in the hold coil makes the winding and connections somewhat fragile when applied to some industrial environments.
With almost all the high-force work solenoids available today, the OEM installation process involves running wires capable of carrying 40A or more, as needed by the pull coil, across long distances to connect to some type of control device that, in turn, must be rated for high currents. A primary design intent was to eliminate the performance guesswork by virtually eliminating variations normally associated with voltage and temperature while simplifying installation.
Trombetta's desire to improve the products for the OEMs in the mobile equipment industry drove them to look hard at the possibility of incorporating the solenoid and the control module, not just adding a control to a solenoid.
"What's really different about this new solenoid is that the control unit is integrated inside of the solenoid. The two were designed concurrently to work optimally together," says Maller. "It's not a solenoid with a pull coil timer inside. It's an integral electronic power management system that uses pulse width modulation." The Equalizer's control is located under the solenoid's end cap and connector shell, the end opposite the plunger.
The engineering challenges of embedding the controller in the solenoid were numerous. "The controller and solenoid were both designed from concept forward to work together," explains Maller. "It was an iterative process at first--we made early estimates about what power we could get from a control unit, designed a concept solenoid, relying heavily on finite element analysis (FEA) to determine if the performance goals could be met."
After the initial concept development, Trombetta engineers quickly determined that they were close and that the concept was feasible. "We then went back and forth tuning both ends of the system," says Maller. "We used a lot of magnetic FEA modeling time and a lot of lab time in controller tuning and verification."
Other engineering challenges included size constraints, controlling rises in temperature, selecting components compatible with the inevitable heat, managing shock and impact self-induced by the plunger striking the internal stop, and achieving a robust interconnection between the control and the coil.
Making the Equalizer uncomplicated to manufacture was also a design challenge. "Because it's a small package, it took a great deal of cooperative effort between product engineering and manufacturing engineering, using rapid prototype parts to test ideas," explains Maller.
Brains behind the control
Pulse width modulation (PWM) is commonly used in electro-hydraulics, but not with industrial work solenoids, according to Maller. PWM is a method for very efficiently varying the average voltage applied to the solenoid coil, in response to control parameters. Because of the varying coil voltage, Trombetta engineers were able to use a single coil and use 100% of the winding at all times for improved performance as well as efficiency, with power consumption and temperature rise about half that of similar solenoids.
The modulation affects pulse duration (or width of pulses), which is changed in response to the information being interpreted by the control's microprocessor. Unlike other solenoids, the Equalizer uses PMW to precisely control coil power by changing the duty cycle (coil on time/ (on + off time)) in response to voltage and temperature conditions inside the solenoid.
"It's all in the micro-code," explains Maller. He describes the code as the brains and personality of the control. "There is a fixed repetition rate (frequency) for the on-and-off switching of power to the coil. The micro-code monitors voltage and temperature during each cycle to predetermine the on-and-off time ratio (duty cycle) for the next cycle." Integrating the control with the solenoid is the only way to make this precise control practical." Maller adds that, because of the design, the solenoid's performance is virtually unaffected by the fluctuations in voltage and temperature that are inevitable with mobile equipment.
Trombetta engineers designed the Equalizer to operate at ambient temperatures between -40 to 105[degrees]C. Continuous operation in ambient temperatures as high as 105[degrees]C still produces the rated force of 12 lb, even with voltage at the solenoid as low as 10V. With ambient temperatures lower than 105[degrees]C, the Equalizer will produce its rated force at voltages even less than 10 volts.
Maller warns that, although the performance of many solenoids may sound good using laboratory results obtained under ideal conditions, the Equalizer is designed for operating conditions that tend to vary widely. "Some solenoids may have specifications that look good at nominal ratings, but in the real world, things are affected by temperature and voltage fluctuations," he says. "Specifications that look only at nominal voltage and room temperature are oversimplified."
Equalizer's full voltage range is from 6 to 15 VDC when operating continuously, but can increase to 16 VDC for 30 minutes or less. Its ratings also allow it to be subject to 24 volts during jump starts of 5 minutes or less.
Elegant design solutions
By designing the Equalizer with PWM drive and a single robust coil, engineers eliminated the fine-wire winding and fragile interconnect wiring. Because the full coil winding is always in use, the device achieves a 50% reduction in the continuous operating power, so there is less energy drain on the power source and less heat to dissipate during operation.
Maller says that there are numerous ways to make dual coils work, but they are all inefficient and they all have compromises. "For example, one approach uses a mechanical switch in the back of the solenoid, but if the solenoid is not properly adjusted, such that it can't pull in all the way, it will fail to open the switch. When that happens, it is not apparent for the first minute or so, and therefore the adjustment error may escape detection during a short run-up test. Once in field use, left on for several minutes, the solenoid will overheat and maybe blow a fuse, if you are lucky. If you are not lucky, it will ruin the solenoid and maybe the wiring harness with it," he says.
There are also add-on controls that can be wired to the dual-winding solenoids. When powered up to retract the solenoid, these controls (either solid state or electromechanical) energize the high-wattage pull winding for a short period, and then shut it off. Such controls still result in the inefficiency of running continuous on only about 30% of the windings during hold, as well as the mechanics of mounting and connecting a second device.
Package and power
Integrating the control module into the solenoid has several other advantages for design engineers. First, the integrated design keeps the size of the solenoid down to a minimum, so the unit can fit in a relatively small envelope.
The Equalizer uses a three-pin Packard Electric Metri-Pack series 150 male receptacle integral to the solenoid body. Two wires are for the typical battery + and - to the solenoid, while the third wire is the command signal line. The control line draws only a few milliamps of current, so the control wiring and external switching device can be sized accordingly small.
Battery power is routed directly to the solenoid to minimize wiring expense and wiring losses. This is an advantage on bucket lifts and other equipment where the operator's point of control is a significant distance from the solenoid; for example, on a piece of mobile equipment such as a utility truck with a bucket on the end of a boom or a high-reach platform with operators controls on the platform. Only the low current command signal needs to be wired to the operator's station. Because the solenoid uses a consistent amount of power under broadly varying conditions, it eliminates most of the headaches usually encountered in selecting a protective fuse that will adequately protect equipment without the nuisance failures; for example, when the solenoid is cold.
Other suitable applications include equipment with distributed actuation of control; differential locks; and distributed stop, release, lock, and unlock capabilities. For example, in differential lock applications in a mobile application, the Equalizer can be activated on from a convenient location next to the driver to engage a mechanical lock inside the differential, in turn engaging both wheels to operate at the same speed for better traction.
Typical Equalizer pull force is 12 lb minimum at 10 to 15V, even when operating continuously at 105[degrees]C. Hold force is 25 lb at 6 VDC when operating through the full temperature range. The device is offered in side- or flangemount, or customer-specified configurations.
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