How a Washing Machine Senses Load Size and Imbalance
Many washing machines judge how heavy the laundry is and whether it is lopsided by watching the drum motor's electrical power, not by weighing it.
In this article
A modern washing machine has to answer two questions before it spins fast: how much laundry is in the drum, and is that laundry spread evenly? Many machines answer both by watching the electrical power that the drum motor draws, rather than by using a scale. This article explains the idea, using a published patent as an example, and why the machine sometimes stops to ask for help.
Why balance matters
When the drum spins, wet clothes are pressed against its wall. If they bunch up on one side, the drum is out of balance. The heavy side swings round the axis, which shakes the cabinet, makes noise and puts strain on bearings and springs. At high spin speeds the effect grows quickly, so the machine needs to check balance at low speed first.
Manufacturers also build in passive defences. According to Wikipedia's article on washing machines, balance rings filled with a salt-water solution (or oil with small balls) can counter an uneven load, and many front-loaders carry a concrete block of roughly 25 kg to damp vibration. Other designs use water-filled counterweights or hydraulic suspension. These parts reduce shaking, but they do not tell the controller what is happening inside the drum.
Reading the motor instead of weighing the load
A drum full of wet laundry is heavy and resists changes in speed. A motor that has to speed it up draws more power than one turning an empty drum. A lopsided load adds a second effect: as the heavy patch rises on one side and falls on the other, the motor works harder, then easier, once per turn. The power therefore rises and falls in step with the rotation.
A US patent for a washer controller (US 7,958,585, listed on Google Patents) describes this approach. The controller measures voltage and current on the inverter that drives the motor, takes many samples each second, and computes power. It does not need a separate sensor for the drum. The patent presents one example design; other makers use their own methods, and many do not publish them.
A short test routine
In the patent's example, the drum is run through a short sequence of speeds, and each stage gives a different piece of information.
- At a modest speed (about 100 RPM in the example), the controller averages the power to get a baseline.
- It then sums how far the power rises above and falls below that baseline. This total grows with the size of the imbalance, but it also depends on how heavy the load is.
- The drum is ramped up and then down between roughly 85 and 150 RPM at the same rate. Friction affects both ramps in a similar way and largely cancels out, so the difference between them reflects the inertia of the load, which tracks its size.
- The controller combines the two results, using calibration constants set for that machine, to estimate the imbalance in kilograms.
The patent also describes a check just below the drum's lowest resonance speed, where an uneven load makes the drum vibrate more and the motor works harder. The controller uses that extra power to tell a single heavy lump from a pair of lumps on opposite sides.
What the machine does with the result
If the measurements are within limits, the program carries on to a suitable spin speed. If any value exceeds its maximum, the machine stops. In the patent's example it then either asks the user to redistribute the clothes or tries to redistribute them itself, with a limit on the number of retries. That is why a washer may pause and start a slow tumble again before the final spin, and why a single heavy item such as a towel or a duvet can cause an early stop.
Load size also shapes the wash
Knowing the load size has a second use. Wikipedia notes that Fisher & Paykel's SmartDrive line, introduced in 1991, used a computer-controlled system to determine the load size and adjust the wash cycle. In general, a machine that knows the load can set the water level and the time more closely to what the laundry needs. Front-loaders, the same article says, fill to the same low level each time and refill if the clothes soak up water, while top-loaders fill until the clothes can float freely.
Spin speed is limited by the same checks. Wikipedia gives typical home spin speeds of about 1,000 to 1,400 RPM for front-loaders, with some models able to reach 2,000 RPM. A faster spin removes more water but demands a better-balanced drum.
The motor matters too
Reading power from the motor works best when the controller can drive the motor precisely. Wikipedia says most newer front-loaders use a brushless DC direct-drive motor, which removes the belt and pulley, and that Fisher & Paykel introduced this arrangement in 1991. Electronic speed control has been common on more expensive models since the 1970s, and microcontrollers began to replace electromechanical timers in cheaper machines after appearing in upmarket models in the early 1990s.
Limits of the method
The patent text itself notes that judging imbalance from motor current alone can be inaccurate, because the motor also carries other loads. That is the reason for the calibration steps and the filters described above. Different washers, from different brands and with different drums, will behave differently, so the figures in this article are an example rather than a rule for every machine.
Sources
Share
Keep reading
Related articles
How a Drone Stays Level: Gyros, Motors and PID Loops
A quadcopter is naturally unstable. See how four motors, motion sensors and a fast feedback loop called PID let it hover and hold its angle.
How a Smartwatch Measures Your Heart Rate With Light
Smartwatches shine green light into your wrist and read how much blood absorbs. Here is how the method, called photoplethysmography, works and where it struggles.
How a Fuel Nozzle Knows When the Tank Is Full
The click that stops a petrol pump uses no electronics: a tiny sensing hole, a venturi and a diaphragm. Here is how the shut-off works and why it trips early.