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Sensing and Control

Turbidity Sensor for
Electromechanical Dishwasher Control

Mr. Gary O'Brien
Honeywell, MICRO SWITCH Division
11 West Spring Street
Freeport, IL 61032
U.S.A.
815/235-6760
815/235-5526 FAX

Contents:


Introduction

Today there are several dishwashers in the market which achieve enhanced performance in energy savings and washability by employing electronic sensors and controls. Perhaps the most important sensor included in this type of system measures turbidity, which is an indication of the amount of particulate matter in the wash solution. Electronically controlled machines have the ability to acquire turbidity data at virtually any time in the cycle and have extreme flexibility in using this information to alter the wash parameters such as time, number of fills and water temperature. The cost of these turbidity sensors combined with the overhead associated with electronic controls has positioned this class of dishwasher in the mid to upper price points. This paper will discuss the design of a simple and lower cost turbidity sensor which could be incorporated into an electromechanical timer controlled machine to provide possible improvements in efficiency and washability. This could be accomplished by integrating the sensor in such a way that the water from one wash could be carried over into the next wash provided that it is clean enough to continue the removal of soil.

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TURBIDITY IN THE DISHWASHER

Background

Simply stated, turbidity is a measurement of the "dirtiness" of a liquid solution, due to the presence of suspended particulate matter. Turbidity does not represent the direct measurement of the amount of this particulate matter, instead it measures the optical effects that the particles have on light which is directed through the liquid. Consider a container of pure water. If we neglect refraction at the boundaries, a beam of light essentially passes straight through such a sample. As a particulate is added to the water, some of the light rays are scattered, absorbed, or reflected. The interference that the particles produce depends on a variety of factors, including particle size, composition and shape, as well as light wavelength and polarization. In general, the greater the concentration of particles, the more interference there will be with the light. There are many ways that electronic sensors can be used to measure this interference, and thus indicate the turbidity value of the solution under examination. The units of turbidity measurement are called NTU's or Nephelometric Turbidity Units.

The information provided by turbidity sensing can give valuable insight into the wash process that occurs in a dishwashing machine. In such an application, turbidity levels indicate the amount of food soil that has been removed from the dishes. In addition, analysis of the signal over time, can indicate the rate of soil removal, the type of soil being removed and the point at which further washing will not remove additional soil. Knowledge about these parameters allow a controller to identify the type of load being washed so that it can conserve water, energy, and time, without sacrificing the cleanliness of the dishes. Whereas conventional dishwashers use a feed-forward approach to set wash parameters based on user input and cycle selection, turbidity sensing provides a system of direct feedback to allow more accurate, dynamic control of the wash process.

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Conventional Turbidity Sensing

In recent years, several manufacturers have applied turbidity sensing to their dishwashers to take advantage of the efficiency and performance enhancements it provides. There are a variety of sensor configurations ranging from high end ratiometric sensor subsystems, to single axis unconditioned transducers.

The high performance turbidity sensors are ratiometric in design, comprised of a light source with two detectors: a transmitted detector to receive light directly from the source through the media, and a scattered detector to receive light which has been scattered by the particles suspended in the media. Turbidity is measured as the ratio of scattered light to transmitted light. In a clean solution, the light from the source travels unimpeded to the transmitted detector, producing a low ratio value (numerator < denominator). In a dirty solution, much of the light is scattered to away from the transmitted detector toward the scattered detector, producing a high ratio value (numerator > denominator)

One strength of the ratiometric configuration is the rejection of common mode variations in light source output and photodiode sensitivity, due to aging and temperature dependence. The ratiometric design also makes the sensor resistant to inaccuracies caused by small amounts of common mode build-up on optical surfaces. Sensors of this design exhibit good sensitivity over a broad range of turbidity levels from virtually clear solutions to those that are essentially opaque. High performance ratiometric sensors can contain onboard signal conditioning to convert raw sensor signals to digital values for transmission to the host controller. Given the magnitude of typical scattered photocurrents (<1uA), some form of local conditioning is usually required. Mechanically, a sensor in this class can be configured as either a flow through device or an immersable probe.

The other primary type of turbidity sensor used in dishwasher control today is somewhat simpler and lower in cost than the ratiometric design. Sensors in this category can be classified as transmissive only devices, because they determine turbidity levels by measuring the change in the transmission of light only on a single axis through the media. In this configuration, turbidity is measured at the output of the transmitted detector. The output level of the transmitted detector is inversely proportional to the soil level in solution, since more soil scatters more light away from the detector. There is no direct measurement of the scattered component in this configuration. This results in a slight loss of sensitivity at the lower turbidity levels when the solution is nearly clear. These types of sensors also are effected by conditions such as buildup on the optical surfaces, light source output drift, and photodiode sensitivity drift. Compensation techniques can be used to lessen some of these effects. For example, zeroing out the sensor's null value at a point in the cycle when it is known that there is clean water can eliminate the effects of light source drift on sensor accuracy. This however does require a microcontroller with non-volatile memory to store the offset value. Transmitted only sensors don't necessarily need onboard signal conditioning since the photocurrents of a transmitted photodiode are large enough to send to the main controller for processing. Some photodetectors use integrated current to frequency converters to perform this conditioning right in the detector package. As with higher end ratiometric sensors, transmissive only devices can be configured as flow through or immersion type sensors.

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ELECTROMECHANICAL SINGLE POINT TURBIDITY SENSING

Design Objectives

Given the scope of existing turbidity sensors used in the dishwasher, there is one commonality between them all; their dependence on a host electronic controller. Such a controller is necessary to power the sensor, and to acquire, process and act upon the sensor output. This need for electronic controls precluded turbidity sensing from being applied to an electromechanical timer based dishwasher. This presented a challenge to design a turbidity sensor that could be used on an electromechanical machine. Such a sensor would need to be applied with little modification to the existing machine. In addition it would have to be low in cost to maintain the price benefits of the electromechanical over the electronic machine. Finally, the total solution would need to deliver tangible and reliable performance enhancements over the standard dishwasher. The electromechanical single point turbidity sensor meets these objectives.

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Sensor Application
Before considering the actual design of the single point turbidity sensor, first consider the application of such a device in the dishwasher. The basic goal of the turbidity sensor in the application is to make a judgment with respect to the water's ability to continue removing soil. The cleanliness of the water is proportional to its ability to remove soil from the dishes. An electromechanical timer controls a dishwasher by stepping through a predetermined sequence of events. The fundamental sequence is a fill, wash, and drain. This sequence is repeated a number of times depending on the cycle selected, i.e. the start position of the timer. Each drain is an opportunity to flush the soil that has been removed from the system in order to bring in clean water to continue washing most effectively. There are times, however, when the water is clean enough at the end of a wash so that it could be carried over to the next wash step and save water while still cleaning the dishes. This can be executed in a manner which does not interfere with the progression of the timer. Figure 1
Figure 1
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General Sensor Characteristics

Figure 2 With a basic understanding of the application in hand we are now ready to look at the general characteristics of the sensor itself, including its packaging, sensing arrangement, and machine interface. Figure 2 shows a CAD drawing of the single point turbidity sensor. The sensor is designed as a probe type device which must be submerged in the wash solution in order to take a measurement. The probe design allows for easy installation and replacement of the sensor without necessitating major modifications to the dishwasher. A simple round opening with a small radial sealing surface is all that must be added to the container in order to accept the probe. The sensor is inserted from the dry side of a container with only a small portion actually exposed to the harsh wash media. The probe must be mounted at a low point within the dishwasher so that some carryover water from the previous cycle keeps the sensor submerged at all times. This prevents routine wetting and drying events which can quickly lead to the buildup of opaque calcium deposits on the optical surfaces due to hard water conditions.

Optically speaking, the sensor is based upon a transmissive only arrangement such as that described in the previous section entitled Conventional Turbidity Sensing. The emitter is a tight beam, sidelooking Infrared Emitting Diode or IRED, which is coupled to a photoreceiver through the liquid solution being measured. The receiver is a sidelooking silicon photodiode with matched sensitivity characteristics to the wavelength of the emitter. The transmissive design provides adequate sensitivity in the turbidity range which is necessary to make the binary "clean" or "dirty" decision. In certain dishwashers this range is the neighborhood of 400 to 800 NTUs. In order to enhance performance, each sensor is calibrated in production to set the turbidity threshold point to an exact NTU level dictated by the application. This calibration compensates for optical misalignment and electrical component tolerances. It also makes the design flexible, as the set point can be adjusted within a wide range of NTU levels simply by modifying the media used in the production tester. Calibration helps to narrow the performance gap between transmissive and ratiometric sensors without significantly affecting the cost savings of a transmissive design.

The sensor interface to the dishwasher is described by series of five pins: Power Line, Power Neutral, IRED Neutral, Relay Contact 1, and Relay Contact 2. Since the electromechanical dishwasher is without a low voltage DC supply, the single point turbidity sensor is designed to operate from standard 60 hertz, 120 volts AC. This accounts for the Power Line and the Power Neutral pins. The sensor is activated by applying power and neutral to these pins using external switching. The power supply internal to the sensor uses neutral as the reference for the voltage used by the sensor electronics. The IRED of the sensor controls when an actual measurement of the wash solution is performed. This IRED is toggled on and off using the IRED Neutral pin. When an external switch connects this pin to neutral, the IRED is turned on and the sensor is able to make a determination on whether or not the water is "dirty" or "clean". That is of course provided that there is power applied to the sensor so that it is active. Contacts on the electromechanical timer can serve to activate the sensor and toggle the IRED in order to take a measurement. Finally, the output of the sensor is applied to the dishwasher via the two Relay Contact pins. The relay was chosen to statically handle the loads commonly found in dishwashing machines which could be used to inhibit drains and fills, mainly motors and solenoid valves.

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Electrical Design

Figure 3 illustrates the functional block diagram of the electronics inside the single point turbidity sensor. The sensor uses a drop resistor power supply to convert the 60 hertz, 120 volts AC to 12 volts DC. The large input impedance inherent to this power supply helps to make the sensor very resistant to potentially damaging voltage surges on the power line. The drop resistor configuration is also more cost efficient compared to alternatives such as drop cap and transformer based power supplies.

Figure 3
Figure 3
The heart of the sensor is the transmissive optical pair comprised of an Infrared Emitting Diode and matched silicon photodiode. Although less sensitive to food particles than the visible spectrum, an 880 nm, AlGaAs IRED was chosen because of some key advantages it provided in the system. The IRED has a much higher ratio of output power to drive current than cost comparable visible emitters. This is compounded by the increased sensitivity of the photodiode to the infrared spectrum over the visible spectrum. This increase in the efficiency of the optical circuit means that the sensor draws significantly less current than a visible light version which is important to keep costs low in the power supply circuit. These advantages justify the loss of some sensitivity to the food particles being measured. Testing shows that for the application, adequate sensitivity is achieved with IR radiation. The final component to the optical circuit is the IRED adjustment block. In order to increase the accuracy and the sensor to sensor repeatability, each sensor is calibrated to set the turbidity threshold for the "clean" / "dirty" decision to a specified NTU value. This is done by trimming the drive current of the IRED. The NTU trip point of each sensor is proportional to the output power of the IRED.

The silicon photodiode of the optical circuit produces a photocurrent that is inversely proportional to the turbidity level of the wash media. By processing this photocurrent the sensor makes the distinction between clean and dirty liquid media. Figure 3 shows that first step of this processing is performed by an amplifier which converts the photocurrent to a voltage with a gain of approximately 2 mega-volts/amp. Such a high gain is necessary given the magnitude of photocurrent that is produced. This extreme amount of gain pushes the reasonable limits for low cost circuit design, providing even more support for the use of a high efficiency IR optical circuit. The voltage output of the amplifier is then compared to a fixed reference voltage, which sets the turbidity level at which the "clean" / "dirty" determination is made. If the output of the comparator is low, then the wash media is cleaner than the reference turbidity level. If the output of the comparator is high, then the wash media is dirtier than the reference media. Strictly speaking the reference voltage is not fixed, it does change with temperature. An NTC thermistor is used to adjust the reference value in such a way so that it compensates for the changes in IRED output power over temperature. The IRED is responsible for the majority of the variation of the sensor over temperature, and therefore is the prime target for this compensation. The output of the comparator is a digital signal, either +12 or 0 volts. This signal is passed through a substantial low pass filter to help reject false triggering signals caused by noise in the system. From the filtering stage, the signal is then feed into a latching relay driver which actuates the relay when the voltage is high. The latching nature is such that once the relay is triggered, it stays triggered no matter what happens to the optical signal. The only condition that allows the relay to reset is a complete power down of the sensor. This latching prevents the sensor from cycling a substantial electrical load such as a drain motor during the time that is has the ability to enable or disable the drain.
Figure 4
Figure 4
A more detailed example of how the sensor can make its decision and execute its control within a wash cycle is shown in Figure 4. After a wash sequence is completed the timer shuts off its wash motor and applies power to the sensor. A short pause is then taken at this point for two reasons: One is to allow interfering air bubbles in the system to rise up and away from the turbidity sensor optics. The other is to allow the sensor's onboard power supply time to stabilize. After this pause, the IRED is activated and the sensor measures the turbidity over a fixed time interval. At the end of this interval the sensor has made its decision and the relay is latched open or it remains closed. Before the timer applies power to the drain operation, the IRED is shut off so that no future triggering of the relay is possible. The latching circuit insures that if the relay was triggered that it will not release. The timer then applies drain motor power, and, depending on the state of the sensor, a drain will or will not occur. The timer is oblivious as to whether or not the sensor disabled the drain. At the end of the drain attempt, power is removed from the drain motor and the timer attempts to fill the machine by actuating the fill valve. If the sensor caused the drain to be skipped, then it must also cause the subsequent fill to be skipped. For this reason the sensor remains powered up during this period and its output relay remains latched open. By wiring the fill valve power through the sensor output relay, the sensor can effectively cancel a fill just as it cancels the preceding drain. Finally, the fill attempt is concluded and power is removed from the fill valve and then from the sensor. The removal of sensor power resets the output relay which restores connection of the drain and fill loads to their normal sources. The particular switching sequence described here allows the sensor to override the drain and fill actions in such a way so that the loads are never switched while under power. This "dry switching" eases the burden on the relay, since extreme inductive spikes and contact arcing are avoided.
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Mechanical Design

Some of the mechanical design goals for the single point turbidity sensor were to produce a package which provided robust protection for the electronics and ease of manufacturability and assembly of the sensor into the dishwasher. In order to effectively protect the electronics while presenting the optics to the liquid media, care was taken in the package design and material selection. As shown in figure 2, exposure to the liquid is limited by design to a small percentage of the overall housing. An "O" ring seal in radial compression against the container's opening is used to prevent leakage around the outside of the sensor housing, but only the plastic housing itself protects the electronics from the liquid. Because of this fact, material selection was critical to insure that the barrier between the liquid and the electronics would be durable in the dishwasher environment over the life of the appliance. A careful search led to the use of polyetherimide as the material used to mold the sensor's upper housing. This material demonstrated excellent chemical resistance and passed extensive accelerated life testing in the dishwasher media. In addition, optical clarity at the 880nm wavelength was more than adequate with a transmission coefficient of 85%. Finally, the polyetherimide when used in the proper thicknesses, met the standard UL94V0 flammability rating, which was necessary to implement this sensor into a dishwasher. Since only a small portion of the sensor housing is actually exposed to the liquid media, the dry side of the housing was designed to include ventilation for the electronics, while still protecting and isolating them from the other parts of the appliance. This vented housing prevents moisture from becoming trapped and allows good dissipation of the heat from the resistors used in the sensor's power supply.

The sensor housing was designed to facilitate assembly of the sensor itself and its installation into the appliance. The housing consists of an upper portion which supports and positions the printed circuit board using two channel details within which the board slides. A bottom cover piece is then used to secure the board in place and provide a feature to accept the mating connector. The bottom cover is attached to the upper portion of the housing using four snap details. These features make assembly of this sensor quite simple, and virtually eliminate improper construction of the sensor in production. Thought was also put into the design of the sensor to provide smooth installation into a dishwasher. Lead in and keying features were included on the upper housing and the bottom cover. These features assist the operator in the proper insertion of both the sensor into the container and the wire harness connector into the sensor, even in "blind" assembly operations. The amount of effort to seat the sensor into the container was also reduced by the selection of a low insertion force silicone "O" ring seal on the sensor. Two self tapping screws are used to hold the sensor in place in the final assembly operation of the sensor into the appliance. Incidentally, two mechanical modifications must be made to a container to accept the sensor. One is the addition of two mounting bosses to accept the screws just mentioned. The other is the addition of a small cylindrical opening with a radial sealing surface near the bottom of the container to actually allow the sensor access to the liquid media within the dishwasher.

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Conclusion

The single point turbidity sensor allows low cost electromechanical dishwashers to achieve enhanced performance that was formally confined to electronically controlled machines. The benefits of single point turbidity include water savings and improved washability. A dishwasher equipped with single point turbidity is adaptive to different loads and eliminates the burden on the consumer to accurately select the proper cycle for the load being washed. Single point turbidity allows a single start point on the timer to service all non-delicate loads. If the load is exceptionally dirty, the sensor does nothing, and the default cycle executes much as it would for a conventional pots and pans selection. For all other loads on down to the exceptionally clean and pre-rinsed, the sensor has the ability to skip the appropriate number of drains leading up to the main wash.

Single point turbidity is relatively easy to implement. There is no need for electronics development, software changes, or major cycle changes. Only minor rewiring and a few additional cams on the timer are required. Mold changes necessary to mount the sensor to the dishwasher are fairly simple. Finally, calibration in production provides extreme flexibility so that the sensor can be matched effectively with the specific dishwasher to which it is applied. Single point turbidity sensing is a cost effective approach to achieving enhanced performance and efficiency in the dishwasher. The concept is simple, the application is straightforward, and the results are substantial.

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