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INSIDE AN ELECTRONIC WASHING MACHINE: There’s much more than washing! These days there is barely a device plugged into mains power that isn’t chockablock full of electronics. There are PC boards inside TVs, VCRs, computers, clock radios, telephones, sound systems, washing machines… By Julian Edgar Washing machines?! Surely not! Yes, if you have bought a new washing machine in the last few years it will probably have a digital display and pushbuttons. But isn’t that just for the sake of cosmetics? Isn’t the control system inside as it always has been? The answer is a definite ‘no’. The old way In the good ol’ days, the “brain” of every automatic washing machine was its timer – an electro-mechanical Fig.1: a typical modern washing machine control system, where the electromechanical timer of previous models has been replaced by the electronic control system. 24  Silicon Chip device powered by a tiny electric motor. The timer motor turned a series of gears that in turn moved cams to activate switches. The switches controlled the various functions – wash, spin and rinse, and so on. While there was some control over the length of each stage, generally the sequence and duration of each event was fixed. A pressure switch sensed the level of water within the bowl. A very sensitive device with a large diaphragm, the pressure switch connected to a chamber whose air pressure changed as the washbowl filled. The ‘water level’ control simply placed a variable mechanical preload on the switch. The temperature dial was also mechanical in action, controlling the position of a water mixing valve. Other controls included a power on/off switch, lid switch (preventing operation of the machine with the lid up) and an out-of-balance switch that stopped a spin cycle if the washbowl began to rock too badly. Mechanically, the washing machine consisted of a stainless or vitreous enamel coated steel perforated drum, an agitator (a finned device rising from the floor of the drum), an electric motor (either a universal or brushless induction design) and a gearbox. The main function of the latter was to convert the rotary motion of the motor shaft into the back-and-forth motion of the agitator. It also allowed the washbowl to spin at high speed, to remove excess water from the clothes. So that was then – how about now? Component Layout Many modern washing machines run to full microcontrollers, error messages, self diagnosis, timed starts and other sophisticated features! Fig.1 shows a block diagram of a current Simpson washing machine. The range of Simpson washing machines is designed and manufactured in Australia by the Email Washing Products Group. Kym Mahlo, Appliance Controls Design Engineer for washing machines, was kind enough to give us an extensive tour of both the R&D lab and the insides of his favourite Simpson models. Electro-mechanical machines are still the majority of Email’s manufacturing base (approximately 80% electromechanical, 20% fully electronic). However, even the electro-mechanical machines contain an electronic Agitation Controller which controls the agitation and spin processes. The electric motor used in the Simpson machines is a 1500 RPM, induction design manufactured inhouse. It is connected via belt and pulleys to a gearbox that slows its speed for agitation and also allows the agitator and the washbowl to be locked together for the high-speed spin cycle (ie, bypassing the gearbox). The motor can be run in either direction, depending on how the windings are energised. During agitation, the motor typically runs 0.8 seconds forward, 0.5 seconds off, 0.8 seconds reverse, 0.5 seconds off, 0.8 seconds forward, and so on. The agitator rotates at 100 RPM. However, the Simpson machines have 40 In this photo taken from directly underneath the washbowl, the induction motor is at the top, driving the different “agitator gearbox through a reduction belt drive. The brake motor profiles” stored in is at bottom right. the microcontroller memory, so The motor speed is varied through its this sequence and speed is variable. current supply being pulsed on and During a spin cycle, the washbowl is off. For example, when the required rotated at up to 800 RPM; woollens speed is a nominal 400 RPM, the are spun at 400 RPM and delicates power to the motor is switched off at 600 RPM. until the speed drops to 300 RPM. The speed of the motor is moni- This means that the actual speed of tored by a Hall Effect sensor, working the motor varies within a 100 RPM in conjunction with an 8-pole ring band. This approach to motor speed magnet mounted on the motor shaft. control is taken because it achieves (Right): A cutaway view of a Simpson model a few years old shows the general layout of parts. The motor and gearbox are at the base, with the perforated stainless steel washbowl above. Behind the control panel is the PC board for the con­ trol system. (Below): At the bottom left is the induction motor, with the gearbox to its right. The main shaft (supported on hefty roller bearings) rises vertically, ending inside the base of the agitator. The brake motor is just to the right of the gearbox. March 2000  25 (Right): The front side of the PC board in a first generation Simpson design. Directly below the PC board are (from far left) an inductor-type pressure sensor, the hot and cold water solenoids, and the motor run capacitor. (Below): The exposed side of the PC board is covered in a bright orange “conformal” coating, designed to repel water. Much of the PC board is at 240VAC potential. diverter valve is also used during the water save function, directing water into the laundry trough rather than into the waste water system. Incidentally, the diverter valve is a slow-acting device that relies on the melting of a wax pellet to move its internals. The hot and cold water valves are 240VAC solenoid actuated, controlled by Triacs. Directly switching the 240VAC is cheaper than taking other approaches. However, it does mean that you shouldn’t lift off the covers and go fishing around behind the washing machine control the lowest energy consumption. As with each of the panel with the power on . . . eight electronic control system outputs, Triacs are used The final output of the control system is the brake moto perform this motor switching function. tor, used to slow the washbowl at the end of the spinning The pump (out) and pump (in) are respectively used cycle. It consists of a small induction motor and gearbox, for emptying and filling the washbowl. The filling pump to which a stainless steel wire is attached. When switched is used only when the “water save” function is activated. on, the wire is gradually pulled out of the gearbox casing, This is where either the sudsy wash water or non sudsy causing a pawl to engage the brake band. deep rinse water is stored in a laundry trough and pumped The lid microswitch has two purposes: it goes open back into the machine for the next washing load. Normally, circuit when the lid is lifted, to stop the machine when mains water pressure is used to fill the washbowl. The the lid is raised; it also functions as an out-of-balance shut-off, being reset when the lid is opened. The cost-saving Fig.2: the various inputs and outputs effected by using the switch for to the microcontroller. Compared to both purposes is important: older models, virtually all functions again and again Kym Mahlo are now variable. stressed that even a saving of a few cents was vital in this very competitive market. Two of the input sensors can also be seen in Fig.1. As with old machines, the level of water within the washbowl is sensed by air pressure but instead of a switch, a Motorola solid state sensor is used. It has a 0-5V output and is calibrated over the range of 0-400mm of water. The use of an analog sensor rather than the old on/off switch allows the micro to sense the speed with which the washbowl is being filled, in addition to the 26  Silicon Chip The brake motor slows the washbowl at the end of the spinning cycle. It consists of a small induction motor and gearbox, to which a stainless steel wire is attached. When switched on, the wire is gradually pulled out of the gearbox casing, causing a pawl to engage the brake band. water level itself. In fact, another approach was used in the model prior to this machine. That design used a sensor whose inductance varied with pressure. The sensor was used to change the frequency of an oscillating circuit, with the frequency then being roughly proportional to the water level. Temperature sensing is carried out with an LM335 solid state sensor which is embedded in a mixing chamber through which the water passes before entering the bowl. The Microcontroller Two different micros have been used in the Simpson washing machines, an SGS Thomson ST9 or a Toshiba TMP870. Both of these controllers are designed for appliance appliFig.3: an excerpt from cations. Both have interrupt inputs, allowing part of the software logic. synchronisation of the microcontroller (and The software is written in so Triac operations) with the mains. These C and the microconcontrollers use an 8MHz external oscillator, troller program length have analog and digital inputs and digital varies from 5 to 30KB, outputs for driving the LEDs and Triacs via depending on the higher current buffers or transistors. “A micromachine in which the controller is a one chip solution” said Kym, control system is being “It’s the most appropriate technology at the used. lowest cost.” A large inductor is fitted to the PC board at the motor drive outputs, to protect the motor drive Triacs fail to switch off, resulting in a flood! It can be seen that against noise which could cause the micro to turn both the prevention of noise from disrupting the micro is very the forward and reverse Triacs on together. In fact in the important. A 5VA 5V power supply is used to supply the microconarea of EMC, Kym commented that it was the immunity of the washing machine control system from external troller, sensors and their signal conditioners and the LEDs. noise - rather than preventing the emission of EMI - that A buzzer is also mounted on the PC board, giving audible indication that the buttons have been pressed, signalling was the more important design requirement. Another potential disaster is where the water solenoids the end of the washing cycle, and also indicating errors. March 2000  27 A test bench system is used to debug the software. It consists of a modified washing machine control panel, EPROM emulator and PC. The potentiometer inputs for water level and motor speed can be seen, along with the toggle switch that simulates the operation of the lid microswitch. The LED display is able to indicate more than 16 codes, displaying cycle times, machine status (eg ‘SP’ - start/ pause) and error codes (eg ‘PU’ – drain hose blocked). All fault codes are stored in an EEPROM that – depending on which of the micros is being used – is either internal or external to the microcontroller. In addition to storing error codes, the EEPROM is also used to store the information for the user’s “favourite wash” program. This can be set by the user to provide their favourite cycle, load and water temp parameters. Another feature possible is delayed start, where the washing machine can be programmed to start its operation after up to 23 hours. Finally, the EEPROM is used to configure the control system to the washing machine model in which it is being used. Fig.2 shows the inputs and outputs to the microcontroller. The PC board tracks are entirely covered with a bright orange “conformal” coating which repels water. This is applied so that water cannot come in contact with the board, much of which is working at 240VAC potential. Part of the Email test sequence is to pour a bucket of water over the top of the working machine, a behaviour apparently not unknown in customers… In fact, in the R&D lab, a number of washing machines are set up to allow wet testing. Monitoring equipment displays factors such as the ‘on’ and ‘off’ time of the agitator, hot and cold water flows, and the temperature of the hot water, cold water or washbowl water. The software writing and debugging is carried out entirely in-house. 28  Silicon Chip Written in C, the microcontroller program length varies from 5 to 30KB, depending on the machine in which the control system is being used. Fig.3 is an excerpt from part of the software logic. Laying out the complete program in this way would require literally hundreds of pages, Kym suggested. For example, he made the point that the second box in Fig.3 (“Turn hot and cold on in a ratio according to temperature selection”) is a very simplistic representation. This process in fact uses the feedback from both the temperature and pressure sensors to modify the ‘on’ times of each of the solenoids to achieve the required water temperature. The monitoring gear measures and displays factors such as the ‘on’ and ‘off’ time of the agitator, hot and cold water flows and water temperatures. In order that the program can be debugged and the effect of software changes easily studied, a control system test bed is used. This consists of a microcontroller emulator working in conjunction with a PC. It is connected to a modified washing machine control panel that incorporates the normal LEDs, electronic control board and buttons. In addition, other LEDs have been fitted to indicate the status of each of the outputs. Potentiometers are used to simulate the input of water level and motor speed, while a toggle switch replaces the lid microswitch. So as you can see, electronics is making major inroads into all consumer goods – even the humble SC washing machine! A wet washing machine test area is set up in the Email R&D lab. It allows the testing of a wide variety of parameters, from washing efficacy to the temperature and flows of the water. And yes, there was a basket of washing just out of the shot!