Detecting Out-of-Balance Conditions with
Christophe Lemaire, Analog Devices, Inc.
In an environmentally conscious world, domestic appliances are easy targets for efficiency improvements. Clothes dryers are high on this list, consuming up to 3.5 × more power than washers. One of the most effective ways to cut down overall power consumption is to reduce the time it takes to dry the laundry. Higher spin speeds in washers mean a drier wash, shorter dryer cycles, and lower power consumption. The demand for fast spins, coupled with a need to reduce machine weight and operating noise, is driving research into better out-of-balance detection and improved vibration control.
Today’s Washing Machines
Detecting Out-of-Balance Conditions
At the end of the wash/rinse cycle, just before the spin cycle, the water is usually drained, and the machine enters a cycle of slow and constant rotation during which the signals are monitored. If the ripple exceeds a set threshold, the cycle is interrupted and the rotational speed is reduced or varied to promote repositioning of the load inside the drum. This method is most effective only at low spin rates (<100 rpm). Even when algorithms are used to extrapolate the machine’s behavior at higher speeds, the accuracy of this technique is limited and the effect of an unbalanced load at higher speeds is unpredictable. A load that is well balanced at low speeds can become unbalanced at higher rates because different fabrics allow different degrees of water extraction.
In addition, when monitoring currents and voltages to sense fluctuations in drawn power, measurement accuracy is limited by the tolerances of the associated circuitry’s passive components. This limitation is acceptable in many cases, but as machines are made to spin faster and faster, improvement in accuracy is desirable.
Manufacturers have used a variety of other ways to detect and eliminate unbalanced loads. A European manufacturer combined motor current sensing with drum displacement sensing; another used several sensors, including a piezofilm accelerometer, together with mechanical damping to detect and control unwanted vibrations. Yet an- other’s use of a vibration sensor eliminated one of the three suspension springs and downsized the shock absorbers.
None of these solutions can handle vibration detection over the entire rate range of high-speed washing machines. The only accurate and effective way to identify a potential balance problem might be to directly measure the accelerational forces exerted on the drum/tub assembly throughout the spin cycle. But this technology was slow to arrive because:
• There were no sensors available that addressed the price/performance requirements of extremely cost-sensitive appliance manufacturers.
• Measuring out-of-balance conditions by monitoring the tach signal for velocity ripple worked fine for most washers because their spins rarely exceeded 1000 rpm.
Direct Sensing with MEMS Accelerometers
It requires minimal external passive components and can be mounted on a small PCB affixed to the tub and directly connected to the microcontroller in the machine’s controller unit. The dual-axis configuration allows both rotational and axial vibration sensing.
The original sensor was introduced in 1999, but when a lower cost, small leadless chip carrier package version became available in 2002, the accelerometer became the first commercially available MEMS product that could both satisfy out-of-balance applications and approach the target costs set by appliance OEMs.
In most vibration sensing applications there is no need to detect static acceleration signals, so the ADXL210E can be AC coupled to the microcontroller and thus eliminate the need for calibration. Adding a capacitor in series with the analog outputs will effectively achieve AC coupling. When using the digital outputs, the same result can be achieved by comparing one group of samples to a previous group and thus determine whether any signal change is the result of measured acceleration or offset drift. Any change slow enough to be unrelated to vibration-induced acceleration can be ignored and eliminated by resetting the
If need be, users can maximize the accler- ometer’s resolution and dynamic range by adding a small capacitor between the analog outputs and ground. The combination of this capacitor with an onchip resistor forms an RC filter, which can be set to limit the bandwidth to the frequency of interest. This will effectively limit the amount of noise generated by the device.
MEMS Sensor Evolution
Economies of scale achieved in the production of tens of millions MEMS automotive accelerometers have contributed to significant reductions in manufacturing costs, encouraging their use in cost-sensitive consumer products. Despite the appliance industry’s traditionally conservative stance on adopting new technology, MEMS sensors have found their way into washing machines for direct, more accu rate, out-of-balance condition detection. In the near future, new MEMS sensors are expected to become available at a price/performance point that will lead to acceptance across the industry.
Christophe Lemaire is Marketing Manager, Consumer and Industrial Markets, Micromachined Products Division, Analog Devices, Inc., Cambridge, MA.
For more information, contact Analog Devices, Inc., 617-761-7000.