Electric cooktops with a twist

Nils Platt
Manager, design engineering
BSH Home Appliances Corp.
Huntington Beach, Calif.

No one likes to clean food crud out of the nooks and crannies of a cooktop. This was one of the motivations for developing a new kind of control dial that works with glass ceramic versions. The basic idea was to design the dial so the control didn't break the outside surface of the cooktop. The approach was to magnetically couple the dial to the control through the cooktop surface. The knob gets held in place by a powerful magnet. It repositions itself when placed back on the cooktop after cleaning. For safety's sake, all the heating elements turn off and lock out when the dial comes off.

This development is interesting in that numerous applications can benefit from having controls that don't compromise the integrity of the operator-panel surface. Of course, capacitive-touch switches can accomplish the same thing and have been around for years. But they have several drawbacks, particularly when applied in cooktops.

For example, they sometimes don't respond readily to touch because of their capacitive nature. The biggest difficulty in a cooktop, however, is moisture. As elements heat up, moisture tries to move to the coolest place inside the cooktop: the electronic-control area. This area must stay relatively cool (below about 105°C) because it contains critical components such as the capacitor, transformer, and relays or triac. That compares to the roughly 300°C usually present at the heater housing. With electric cooktops, moisture usually condenses directly under the glass surface on the sensors. And depending on climate and humidity, the amount of moisture can be very high. Condensing moisture on capacitive or optical sensors influences the sensor signal and confuses the electronics. The more sensors, the more debilitating the influence on the signal.

Designers deal with these problems in several ways. One is to seal and enclose controls in a plastic housing. Finding the right UL-listed plastic material (it must be stable to at least 105°C) isn't easy. Another moisture-related problem is common to controls driven by triac instead of relays. The fan that cools the triac blows moisture directly into the control area.

Magnets, the key

The new magnetive control, called the mTwist, provides all the conveniences of rotary-control knobs without the potential drawbacks, such as drilled holes that must be sealed for a switch shaft, for instance. The mTwist removable knob operates electronically-magnetically. The control knob is magnetically centered and a magnetic field sensor in the cooktop tracks the knob's every turn. The actual setting can be read from a seven-segment display. The removable rotary knob is simple to operate. When positioned on a particular setting, the knob activates the control system and simultaneously switches on the cooking stage indicator to standby mode. As is typical, a clockwise turn boosts heat to the highest stage and a counterclockwise turn lowers the heat back down to zero. Simply removing the magnetic knob from the cooktop switches off the control system and extinguishes the indicator. The obvious benefits here are child safety and easy cleaning, as no awkward controls get in the way. The mTwist magnetic system also resists moisture.

The mTwist control is basically a two-circuit-board design. On the sensor board, which is the base for the south-side part of the magnetic system, are the capacitive sensors, processor, LEDs, and seven-segment displays, as well as the low-voltage area. The relays, transformer, and capacitors sit on a 120-Vac power board. The PCBs are held together with specially designed plastic standoffs and the entire control is spring-forced against the glass-panel underside. This guarantees an absolute even position of the sensors and magnetic system to the glass.

The south-side part on the sensor board includes a stainless-steel bracket, three Hall sensors, one magnet that sits in the center of the bracket, and a plastic housing. The dial is magnetically centered on top of the glass on the north side of the magnetic system. One magnet is in the knob and one is under the glass. A star-shaped stainless-steel metal bracket attaches to the magnet on both the knob and underneath the glass. The South Pole underneath the glass and the North Pole in the knob split into the tips of the metal bracket. When the knob sits on top of the glass, the two magnets attract and align, thus setting the initial Off position. Turning the knob moves the fingers of the star-shaped bracket like wheels over the sensors, which detect the fingers crossing the magnetic field.

Design challenges

The mTwist magnetic system uses neodymium-iron-boron permanent magnets. Magnets made from this material offer high densities and excellent magnetic properties. This is thanks to the strongly magnetic matrix phase Nd₂Fe₁₄B, which features high saturation polarization and high magnetic anisotropy. A ductile neodymium-rich bonding phase at the grain boundaries gives these magnets good mechanical properties.

Neodymium-iron-boron rare earths feature especially high coercivities with simultaneous high saturation and excellent temperature stability. They work well in normal conditions, such as room temperature, humidity levels to 50%, without condensation, despite having no special surface protection. However, for the mTwist, the magnet surface needed a coating because otherwise high humidity and dew formation could cause corrosion. The magnets are protected by a double coating of nickel and tin.

In designing the electronic controls, BSH engineers tested several sensing schemes, including optical, induction, mechanical-magnetic switch, and Reed-contact systems, before settling on the Hall sensor and five-finger bracket design. (As a quick review, the working principle of Hall sensors is this: A voltage is generated transversely to the current-flow direction in an electric conductor, if a magnetic field is applied perpendicularly to the conductor.)

A sensing element detects the components of the magnetic flux perpendicular to the surface of the chip and emits a proportional electrical signal, which is processed in evaluation circuits integrated on the sensor chip.

Designing ceramic cooktops with electronic controls is tricky business. The completed design must withstand severe impact tests. If the entire control system isn't properly spring-forced against the glass, the impact will immediately break the surface.

Assembly tolerances also require a final function test on the end of the assembly line. A 1-mm-thick plastic gage is used to simulate an air gap. The control dial must continue functioning and if it doesn't, the assembly must be rechecked. The mTwist design has passed such rigorous testing and now awaits any punishment home chefs can dish out.

BSH Home Appliances Corp. is a subsidiary of Bosch and Siemens Hausgeraete GmbH. The mTwist magnetic system is a Bosch exclusive.