proximity sensor for liquid go/no-go sensing

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HeGi_2497906
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We are starting a project for detection of liquid in a large plastic (polypropylene) tank, the need is to have 4 - 5 specific levels that are "seen" as having liquid or not.  The concept is to have a reference sensor covered with the same material that always "sees" no liquid and use that to provide temperature compensation and a reference as to what no liquid capacitance should be.  The first concept is to have sensors that will be connected to the main PCB, which will have one sensor and the reference sensor, with 22 ga solid core wires approximately 6 - 10 inches in length.  The sensors will be in contact with the outer wall, attached via adhesives.

We have several questions, about the connections and sensors.

Note: the design is for holding tanks on RVs, therefore one of the tanks will have human effluent, will this cause sensing issues? Again we are looking for only Go/No-Go response, not levels of fullness or distinguishing between types of fluids.

1. Will the proximity sensor work over the wires, or should we put a small PSOC4 on each sensor? If can sense over the wires, is shielded wires recommended?

2. The thickness of the tank is 7mm - 9.5mm, I am considering a loop sensor, is this the best choice, will this work through the tank wall? Can you advise on loop size and gap sizes for that thickness?

3. We will drive the shield and guard, to eliminate condensation issues and minimize CP, is this a good idea.

Tank concept:

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1 Solution
BragadeeshV
Moderator
Moderator
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First question asked 1000 replies posted 750 replies posted

Hi HeGi_2497906​,

1. Will the proximity sensor work over the wires, or should we put a small PSOC4 on each sensor? If can sense over the wires, is shielded wires recommended?

Cypress: It is recommended to use a solid sensor pad instead of a proximity loop. You can refer to the app note "AN202478 - PSoC® 4 - Capacitive Liquid-Level Sensing" for information on sensor patterns and layout guidelines. Since you want only ON/ OFF detection, you can have sensor pads on the specific heights.

What is the height/ length/ width of your tank? If you are using CSD sensing method then the Cp should be within limits (5pF to 200 pf for 4th Gen CapSense devices, 5 -65 pF for 3rd gen CapSense devices).

Note Sensor pads needs to be within the area 25 sq mm to 3,000 sq mm.

2. The thickness of the tank is 7mm - 9.5mm, I am considering a loop sensor, is this the best choice, will this work through the tank wall? Can you advise on loop size and gap sizes for that thickness?

Cypress: Please find the dielectric constant of the material. We think the thickness of the tank is fine for your producing enough signal. For sensor pattern refer previous recommendation

3. . We will drive the shield and guard, to eliminate condensation issues and minimize CP, is this a good idea

Cypress: Guard is not required in this case. Guard is required only when the liquid makes contact with the sensors (since this is not your case, guard is not required) Shield is a good option to reduce error Cp of the sensor.

4. The first concept is to have sensors that will be connected to the main PCB, which will have one sensor and the reference sensor, with 22 ga solid core wires approximately 6 - 10 inches in length.  The sensors will be in contact with the outer wall, attached via adhesives.

Cypress: The trace (sensor to PSoC pin) width should not be more than 7 mil. Can you please let me know what wire will have 22 ga dia?

Summary;

1. CSD + Shield enabled

2. PSoC 4S devices that has latest Generation CapSense

3. Layout guidelines from "AN202478 - PSoC® 4 - Capacitive Liquid-Level Sensing"

Regards,

Bragadeesh

Regards,
Bragadeesh

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9 Replies
BragadeeshV
Moderator
Moderator
Moderator
First question asked 1000 replies posted 750 replies posted

Hi HeGi_2497906​,

1. Will the proximity sensor work over the wires, or should we put a small PSOC4 on each sensor? If can sense over the wires, is shielded wires recommended?

Cypress: It is recommended to use a solid sensor pad instead of a proximity loop. You can refer to the app note "AN202478 - PSoC® 4 - Capacitive Liquid-Level Sensing" for information on sensor patterns and layout guidelines. Since you want only ON/ OFF detection, you can have sensor pads on the specific heights.

What is the height/ length/ width of your tank? If you are using CSD sensing method then the Cp should be within limits (5pF to 200 pf for 4th Gen CapSense devices, 5 -65 pF for 3rd gen CapSense devices).

Note Sensor pads needs to be within the area 25 sq mm to 3,000 sq mm.

2. The thickness of the tank is 7mm - 9.5mm, I am considering a loop sensor, is this the best choice, will this work through the tank wall? Can you advise on loop size and gap sizes for that thickness?

Cypress: Please find the dielectric constant of the material. We think the thickness of the tank is fine for your producing enough signal. For sensor pattern refer previous recommendation

3. . We will drive the shield and guard, to eliminate condensation issues and minimize CP, is this a good idea

Cypress: Guard is not required in this case. Guard is required only when the liquid makes contact with the sensors (since this is not your case, guard is not required) Shield is a good option to reduce error Cp of the sensor.

4. The first concept is to have sensors that will be connected to the main PCB, which will have one sensor and the reference sensor, with 22 ga solid core wires approximately 6 - 10 inches in length.  The sensors will be in contact with the outer wall, attached via adhesives.

Cypress: The trace (sensor to PSoC pin) width should not be more than 7 mil. Can you please let me know what wire will have 22 ga dia?

Summary;

1. CSD + Shield enabled

2. PSoC 4S devices that has latest Generation CapSense

3. Layout guidelines from "AN202478 - PSoC® 4 - Capacitive Liquid-Level Sensing"

Regards,

Bragadeesh

Regards,
Bragadeesh
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Bragadeesh, thanks for your advice, here is the plan, I will make two test PCBs, one that wires directly to a main PCB with the capsense module, a second where there is a capsense MCU right on the PCB, a CY8C4014 8pin part.

Here is the wire only design, the PCB is 0.5inx0.5in

sensor back.PNGsensor front.PNG

Here is the PCB with the MCU, it is 1"x.05", ignore the size difference, cut and paste issue

sensor with MCU back.PNGsensor with MCU front.PNG

Refer to https://www.cypress.com/documentation/code-examples/ce202479-psoc-4-capacitive-liquid-level-sensing  for an example project on liquid level sensing.

We recommend you to evaluate PSoC 4000S device since it has latest generation of CapSense in it.

Regards,

Bragadeesh

Regards,
Bragadeesh
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My sensor is 120mm^2, at 7mm dia, with a 1.5mm gap between, single point ground/shield, 7mil trace, as short as possible, datasheet specified hatching front and rear.  I can make it larger, should I?

Agreed, the 4000s has a better cap sensor, and I plan to use that for the wired sensor, but for the sensor with the MCU directly next to the sensor as shown will the 4000 do the job?  Cost and size are factors.. 

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Since your wall thickness is high, you might get less Cf (capacitance added by the liquid). This needs to be tuned accordingly.

4000 will be sufficient for this application if cp is less.

Regards,

Bragadeesh

Regards,
Bragadeesh
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I agree that the MCU right on top of the sensor is best.

How do I tune for a lower Cf, and thicker overlay.  On tuning, are your referring to sw tuning in the component? or tuning of the sensor design. If the sensor design, please help with some specifics, Larger sensor?  Larger shield? Bigger sensor/shield gap?  I want to make a very reliable sensor and my measurement times are in seconds, so I can filter out errors based on time,  Help me understand your comment on tuning please, and thanks for your great help.

Herb

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Hi HeGi_2497906​,

By tuning I meant the hardware (Resolution, Modulator frequency, Sense clock frequency) and software parameters (finger threshold etc) to get reliable signal from the sensors.

The shield electrode should be within 1 cm spread from the sensors and not more than that. Please refer to PSOC 4 Capsense design guide for information on shield electrode.

Regards,

Bragadeesh

Regards,
Bragadeesh
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My design looks just like this one, and the shield is not more than 1cm in any direction.  My final sensor is 15mm dia disc with a 2mm gap, so a radius of 9.5mm, and then the shield ends in a 25.4 mm square.  On the corners, the distance it 18 mm, minus the 9.5mm, so my shield can only be 8.mm max from the edge of the gap ring.

Key question, is the hatching on the non sensor side part of the shield circuit or is it ground?

Herb

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It should be shield.

Regards,
Bragadeesh
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