Capacitive Sensors To Build A Better Water Cooler Control
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Capacitive Sensing Builds a Better Water-Cooler Control By (Dave Van Ess, Applications Engineer and Member of Technical Staff, Cypress Semiconductor Corp.)
Executive Summary Capacitive sensing offers developers a new way to interact with users that overcome the traditional problems associated with mechanical levels or push button switches that engage a solenoid controlled value. Exploring the use of capacitive sensing in a water cooler illustrates not only how capacitive sensing can make devices more reliable but also how the controller managing capacitive sensing can take on additional functions to add further value to customers as well as reduce maintenance expenses.
A big problem with mechanical spigots is that they can be forced on, or even broken off, causing all the water to be dispensed. It is also easy to override a push button taping it pressed on or by jamming some object into its housing to force it continuously on. Mechanical switches wear out and also must penetrate their product’s case, allowing contamination to in crevasses or crannies. A capacitive sensor does not penetrate the case so they have no crevasses to trap gunk. They also do no wear out. This makes them the ideal switch from a product that dispenses food or food grade products. Shown in figure 1, a capacitive switch is essentially a capacitor formed from two adjacent traces. Physical laws determine what capacitance exists between them. If a conductive object, such as a finger, is brought in close proximity to these plates, a parallel capacitance couples with this sensor. Place a finger on the capacitive sensor, and the capacitance increases. Remove the finger, and the capacitance decreases. Measure this capacitance and you can determine the presence or absence of a finger. Figure 1: Basic Capacitive Sensor
All that is needed to make a capacitive sensor is a trace, a space, and a trace. These traces can be made part of a circuit board with an insulated overlay placed directly over them. They can be made to conform to a curved surface.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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To construct the capacitive switch one needs: • A capacitor • Capacitance measuring circuitry • Local intelligence to translate this capacitance values to a sense state. A typical capacitive sensor has a value of 10 to 30pF. Typical finger coupling capacitance to the sensor through 1mm of insulating overlay is in the range of 1 to 2pF. For thicker overlays the coupling capacitance decreases. To sense the presence or absence of a finger, it is necessary to implement capacitance sensing circuitry that can resolve better than 1 part in a 100 capacitance change. A delta sigma modulator is an effective and simple circuit for measuring capacitance. A typical topology is shown below. Figure 2: Delta Sigma Modulator Topology for Measuring Capacitance
Phased switches cause the sensor capacitor to inject a charge into the integrating capacitor. This voltage increases until it is greater than the reference voltage. The comparator goes high causing the discharge resistor to be engaged. This resistor is removed when the integrating voltage falls below the reference voltage. The comparator is supplying negative feedback needed to make the integrator voltage and reference voltage match.
Sensor Charge Current During f1 the sense capacitor (Csensor) is charged to the supply voltage. During f2 the charge is transferred to the integrating capacitor (Cint). Feedback is holding its value to the reference voltage (k*Vdd). Each time this switch combination is actuated, a quanta of charge is transferred. These quanta are transferred at the rate of the switch clock (fc) for a charge current shown in the equation below.
I c = qc ⋅ f c = (Vdd − k ⋅ Vdd ) ⋅ C sensor
Discharge Current The discharge current is implemented with a resistor. When the comparator is high it engages a switch to connect the discharge resistor. The comparator will cycle high and low in some ratio to attempt to keep the integrating capacitor voltage equal to the reference voltage. The percentage that this comparator is high is defined as its “DensityOut”. The charge is only removed this percentage of the time. The current is shown in the equation below.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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ID =
k ⋅ Vdd ⋅ DensityOut Rdis
In steady state the charge and discharge current must match. Setting IC to ID results in the equation below.
C sensor = DensityOut ⋅
k 1 1 ⋅ ⋅ 1 − k Rdis f s
The sensor capacitor is proportional to the density. The sample frequency, discharge resistance, and reference value (Vdd·k) are known. Measure the density and the sensor capacitance can be calculated. The reference voltage was made proportional to the supply voltage so that the supply voltage would fall out of the capacitance/density equation. This makes the circuitry tolerant to power supply fluctuations. Digital circuitry is used to measure this density. One such circuit is shown below. Figure 3: Digitally Measuring Density
The PWM gates the density input to the enable gate of a counter. This allows “m” cycles to be counted. Suppose the counter accumulated “n” sample during this period, then the density would be n/m. Running this for 100 cycles results in a resolution of 1 part in 100. Running 10 times longer results in a resolution of 1 part in 10,000. The greater the number of cycles observed, the better the resolution.
Replacing Spigots With Solenoids In a typical water cooler the water is dispensed from a mechanical spigot. The level must be close to the nozzle. Using capacitive sensing, the lever is replaced with a solenoid value. The switch can be placed for the ergometric convenience of the user. The CPU can also time the length switch is pressed so to not allow vandals to force the value continuously on. This vandal protection can be as simple or as complicated as you desired. This project is implemented with a Cypress CY24x94 PSoC device. One pin will be used for a sensor, one for the discharge resistor, and one for integration cap for a total of three pins. One output is used to drive the water value. A block diagram is shown below.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 4: Capacitive Sensing Controlled Water Valve
Capacitive Sensing Plus Temperature A convention water cooler consists of: ·
A water tank
·
A refrigeration compressor
·
A thermal relay.
The thermal relay monitors the temperature of the water in the tank. When the tank goes above a specific temperature, the thermal engages, causing the compressor to run. Adjusting the water temperature requires adjusting a screw on the relay. It is an open loop, hit-or-miss operation. Instead of using a thermal relay, the same controller managing the capacitive sensor can be used to measure the temperature and then control the power to the compressor. Rather than require a second controller, the first can be reconfigured to also take on the task of measuring temperature. Temperature can easily be measured using a thermistor. A thermistor is a semiconductor device that becomes less resistive as the temperature increases. Measure the resistance and the temperature can be calculated. The figure below shows a circuit for measuring resistance.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 5: Resistance Measuring Hardware
By measuring the voltages across the thermistor and the reference resister it is possible to determine the thermistor’s resistance.
Rther = Rref ⋅
Vin − Vref − Vref + − Vin
The same hardware used to sense capacitance can be reconfigured to allow the temperature to be measured. When converted back to a temperature this value is used to determine if the refrigeration compressor should be turned on. Extra thermistors can be provided to measure the room temperature and compressor temperature as an overheated compressor can cause a premature failure. The sensing controller can disable its operation when a problem is detected, flag the user that the unit has malfunctioned, and wait for the unit to be repaired.
Capacitive Sensing Plus Multimeter So the compressor is running hot. One of the first troubleshooting suggestions is to measure the input voltage. This is a diagnostic that can easily be accomplished with dynamic reconfiguration. Reconfiguring the controller to be a voltmeter enables measurement of the main voltage. Other system voltages can also be measured. The figure below shows an expanded block diagram with all these extra features.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 6: Improved System with Water Temperature Control.
Capacitive Sensing Plus the Display With temperature being easy to measure, it would be ideal if the user could also set the desired temperature. This requires a keypad and display. The keypad is simple as it can be built of capacitive sensors that use the capacitive sensing user module already placed. The controller can also control a LCD driver chip using a standard industry protocol. It is now possible for the user to set the desired water temperature and be able to see it displayed. 16 inputs will be reserved for a user interface.
Capacitive Sensing Plus Time With the addition a clock crystal, the capacitive sensing controller can keep accurate time. This as the advantage that the cooler can be turned off or the operation temperature set point increased when it is traditionally not used. The figure below shows an expanded block diagram with all these extra features.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 7: Complete Water Cooler Block Diagram.
Capacitive Sensing Plus USB A major cost to ownership of a water cooler is the repair service calls. If the capacitive sensing controller also has a USB interface, this could be used for a diagnostic port. When the repair technician visits, troubleshooting begins by plugging a laptop into the service port. It would also be possible that the owner’s PC be connected to the port and a remote technician could determine the problem.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 8: Complete Water Cooler Block Diagram with USB Diagnostic Port.
Capacitive Sensing Plus Whatever The large number of I/O pins and dynamic reconfigurable of a capacitive sensing controller, there are endless features that could be added. The addition of a stress gauge to measure the weight of the remaining water in the bottle or a wireless interface to allow even easier diagnostics are just a couple of possibilities. With no mechanical parts and easily conforming to curved surfaces, touch sense capacitor switches can be an ideal technology for today’s product applications. With dynamic reconfiguration it is possible to reuse hardware to perform additional system functions with no additional cost.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 8 of 9 November 2007
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References
Cypress Semiconductor 198 Champion Court San Jose, CA 95134-1709 Phone: 408-943-2600 Fax: 408-943-4730 http://www.cypress.com © Cypress Semiconductor Corporation, 2007. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. PSoC Designer™, Programmable System-on-Chip™, and PSoC Express™ are trademarks and PSoC® is a registered trademark of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are property of the respective corporations. This Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress. Disclaimer: CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress’ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. Use may be limited by and subject to the applicable Cypress software license agreement.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Executive Summary Capacitive sensing offers developers a new way to interact with users that overcome the traditional problems associated with mechanical levels or push button switches that engage a solenoid controlled value. Exploring the use of capacitive sensing in a water cooler illustrates not only how capacitive sensing can make devices more reliable but also how the controller managing capacitive sensing can take on additional functions to add further value to customers as well as reduce maintenance expenses.
A big problem with mechanical spigots is that they can be forced on, or even broken off, causing all the water to be dispensed. It is also easy to override a push button taping it pressed on or by jamming some object into its housing to force it continuously on. Mechanical switches wear out and also must penetrate their product’s case, allowing contamination to in crevasses or crannies. A capacitive sensor does not penetrate the case so they have no crevasses to trap gunk. They also do no wear out. This makes them the ideal switch from a product that dispenses food or food grade products. Shown in figure 1, a capacitive switch is essentially a capacitor formed from two adjacent traces. Physical laws determine what capacitance exists between them. If a conductive object, such as a finger, is brought in close proximity to these plates, a parallel capacitance couples with this sensor. Place a finger on the capacitive sensor, and the capacitance increases. Remove the finger, and the capacitance decreases. Measure this capacitance and you can determine the presence or absence of a finger. Figure 1: Basic Capacitive Sensor
All that is needed to make a capacitive sensor is a trace, a space, and a trace. These traces can be made part of a circuit board with an insulated overlay placed directly over them. They can be made to conform to a curved surface.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 1 of 9 November 2007
[+] Feedback
To construct the capacitive switch one needs: • A capacitor • Capacitance measuring circuitry • Local intelligence to translate this capacitance values to a sense state. A typical capacitive sensor has a value of 10 to 30pF. Typical finger coupling capacitance to the sensor through 1mm of insulating overlay is in the range of 1 to 2pF. For thicker overlays the coupling capacitance decreases. To sense the presence or absence of a finger, it is necessary to implement capacitance sensing circuitry that can resolve better than 1 part in a 100 capacitance change. A delta sigma modulator is an effective and simple circuit for measuring capacitance. A typical topology is shown below. Figure 2: Delta Sigma Modulator Topology for Measuring Capacitance
Phased switches cause the sensor capacitor to inject a charge into the integrating capacitor. This voltage increases until it is greater than the reference voltage. The comparator goes high causing the discharge resistor to be engaged. This resistor is removed when the integrating voltage falls below the reference voltage. The comparator is supplying negative feedback needed to make the integrator voltage and reference voltage match.
Sensor Charge Current During f1 the sense capacitor (Csensor) is charged to the supply voltage. During f2 the charge is transferred to the integrating capacitor (Cint). Feedback is holding its value to the reference voltage (k*Vdd). Each time this switch combination is actuated, a quanta of charge is transferred. These quanta are transferred at the rate of the switch clock (fc) for a charge current shown in the equation below.
I c = qc ⋅ f c = (Vdd − k ⋅ Vdd ) ⋅ C sensor
Discharge Current The discharge current is implemented with a resistor. When the comparator is high it engages a switch to connect the discharge resistor. The comparator will cycle high and low in some ratio to attempt to keep the integrating capacitor voltage equal to the reference voltage. The percentage that this comparator is high is defined as its “DensityOut”. The charge is only removed this percentage of the time. The current is shown in the equation below.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 2 of 9 November 2007
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ID =
k ⋅ Vdd ⋅ DensityOut Rdis
In steady state the charge and discharge current must match. Setting IC to ID results in the equation below.
C sensor = DensityOut ⋅
k 1 1 ⋅ ⋅ 1 − k Rdis f s
The sensor capacitor is proportional to the density. The sample frequency, discharge resistance, and reference value (Vdd·k) are known. Measure the density and the sensor capacitance can be calculated. The reference voltage was made proportional to the supply voltage so that the supply voltage would fall out of the capacitance/density equation. This makes the circuitry tolerant to power supply fluctuations. Digital circuitry is used to measure this density. One such circuit is shown below. Figure 3: Digitally Measuring Density
The PWM gates the density input to the enable gate of a counter. This allows “m” cycles to be counted. Suppose the counter accumulated “n” sample during this period, then the density would be n/m. Running this for 100 cycles results in a resolution of 1 part in 100. Running 10 times longer results in a resolution of 1 part in 10,000. The greater the number of cycles observed, the better the resolution.
Replacing Spigots With Solenoids In a typical water cooler the water is dispensed from a mechanical spigot. The level must be close to the nozzle. Using capacitive sensing, the lever is replaced with a solenoid value. The switch can be placed for the ergometric convenience of the user. The CPU can also time the length switch is pressed so to not allow vandals to force the value continuously on. This vandal protection can be as simple or as complicated as you desired. This project is implemented with a Cypress CY24x94 PSoC device. One pin will be used for a sensor, one for the discharge resistor, and one for integration cap for a total of three pins. One output is used to drive the water value. A block diagram is shown below.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 3 of 9 November 2007
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Figure 4: Capacitive Sensing Controlled Water Valve
Capacitive Sensing Plus Temperature A convention water cooler consists of: ·
A water tank
·
A refrigeration compressor
·
A thermal relay.
The thermal relay monitors the temperature of the water in the tank. When the tank goes above a specific temperature, the thermal engages, causing the compressor to run. Adjusting the water temperature requires adjusting a screw on the relay. It is an open loop, hit-or-miss operation. Instead of using a thermal relay, the same controller managing the capacitive sensor can be used to measure the temperature and then control the power to the compressor. Rather than require a second controller, the first can be reconfigured to also take on the task of measuring temperature. Temperature can easily be measured using a thermistor. A thermistor is a semiconductor device that becomes less resistive as the temperature increases. Measure the resistance and the temperature can be calculated. The figure below shows a circuit for measuring resistance.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 4 of 9 November 2007
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Figure 5: Resistance Measuring Hardware
By measuring the voltages across the thermistor and the reference resister it is possible to determine the thermistor’s resistance.
Rther = Rref ⋅
Vin − Vref − Vref + − Vin
The same hardware used to sense capacitance can be reconfigured to allow the temperature to be measured. When converted back to a temperature this value is used to determine if the refrigeration compressor should be turned on. Extra thermistors can be provided to measure the room temperature and compressor temperature as an overheated compressor can cause a premature failure. The sensing controller can disable its operation when a problem is detected, flag the user that the unit has malfunctioned, and wait for the unit to be repaired.
Capacitive Sensing Plus Multimeter So the compressor is running hot. One of the first troubleshooting suggestions is to measure the input voltage. This is a diagnostic that can easily be accomplished with dynamic reconfiguration. Reconfiguring the controller to be a voltmeter enables measurement of the main voltage. Other system voltages can also be measured. The figure below shows an expanded block diagram with all these extra features.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 5 of 9 November 2007
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Figure 6: Improved System with Water Temperature Control.
Capacitive Sensing Plus the Display With temperature being easy to measure, it would be ideal if the user could also set the desired temperature. This requires a keypad and display. The keypad is simple as it can be built of capacitive sensors that use the capacitive sensing user module already placed. The controller can also control a LCD driver chip using a standard industry protocol. It is now possible for the user to set the desired water temperature and be able to see it displayed. 16 inputs will be reserved for a user interface.
Capacitive Sensing Plus Time With the addition a clock crystal, the capacitive sensing controller can keep accurate time. This as the advantage that the cooler can be turned off or the operation temperature set point increased when it is traditionally not used. The figure below shows an expanded block diagram with all these extra features.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 6 of 9 November 2007
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Figure 7: Complete Water Cooler Block Diagram.
Capacitive Sensing Plus USB A major cost to ownership of a water cooler is the repair service calls. If the capacitive sensing controller also has a USB interface, this could be used for a diagnostic port. When the repair technician visits, troubleshooting begins by plugging a laptop into the service port. It would also be possible that the owner’s PC be connected to the port and a remote technician could determine the problem.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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Figure 8: Complete Water Cooler Block Diagram with USB Diagnostic Port.
Capacitive Sensing Plus Whatever The large number of I/O pins and dynamic reconfigurable of a capacitive sensing controller, there are endless features that could be added. The addition of a stress gauge to measure the weight of the remaining water in the bottle or a wireless interface to allow even easier diagnostics are just a couple of possibilities. With no mechanical parts and easily conforming to curved surfaces, touch sense capacitor switches can be an ideal technology for today’s product applications. With dynamic reconfiguration it is possible to reuse hardware to perform additional system functions with no additional cost.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
Page 8 of 9 November 2007
[+] Feedback
References
Cypress Semiconductor 198 Champion Court San Jose, CA 95134-1709 Phone: 408-943-2600 Fax: 408-943-4730 http://www.cypress.com © Cypress Semiconductor Corporation, 2007. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. PSoC Designer™, Programmable System-on-Chip™, and PSoC Express™ are trademarks and PSoC® is a registered trademark of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are property of the respective corporations. This Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress. Disclaimer: CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress’ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. Use may be limited by and subject to the applicable Cypress software license agreement.
Capacitive Sensing Builds a Better Water-Cooler Control Published in Planet Analog (http://www.planetanalog.com)
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