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create_sys
Level 1
Level 1
10 sign-ins 5 replies posted 5 questions asked

Hello,

We are trying to develop a cap sense proximity-based sensor for human detection. We want to detect the presence of the human body at a distance of 40mm.  Our hardware contains a plastic frame which is covering a lot of metal components ( all are grounded ).  We are keeping a gap of 20mm in between the proximity sensor PCB attached to the plastic cover & the metallic frame which is carrying metallic components.  

Now, when we reduce down the area of proximity sensor PCB our sensing range drops down to almost 1-2mm but there is not back sensing of metal objects. But this is not useful to us as we need a minimum of 40mm of sensing range. 

Now, if we increase the sensor area, then there is continuous detection of back-side placed metal objects.

We cant remove the metallic objects as it's the primary hardware. We cant increase the gap between sensor & metal objects due to mechanical constraints.  

Please suggest what care can be taken to get rid of this issue. Also, suggest the best configuration of cap sense proximity to achieve the goal.

I have added a video clip that shows the problem. 

 

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1 Solution
Hari
Moderator
Moderator
Moderator
750 replies posted 500 replies posted 250 solutions authored

Hi @create_sys 

 

The reason the metal plate contributes to the signal is that the physical capacitance of the sensor is increasing in the presence of this conductive object. There are two approaches - 

 

1. Without modification of the hardware

Please reset the device once the entire assembly is in place. The CapSense system resets the baseline upon reset and this will fix the sensor always being on while the metal housing is placed. But please note that the sensing distance might be reduced on the bottom side, but since you were initially getting over 5cm, we might get 4cm with the housing. 
As a general rule of thumb, it is recommended to tune the CapSense sensors with the whole assembly.

 

2. With hardware modification - In case there is a requirement for greater sensing distance, a hardware modification would be required. We recommend placing a hatched shield on the layer below the sensor layer so that the sensor is guarded against any external sources (such as the metal housing in this case). You can find more details about shield in our Proximity sensing with CapSense AN - https://www.cypress.com/file/46741/download

 

It would also be great if you can run the CapSense tuner on your PC and check the performance of the sensor. Note that you can provide a lower value for the proximity threshold, thereby allowing you to sense a greater distance, if the noise level in the system is low. We recommend having an SNR of 5:1 at the required sensing distance. You can check the SNR directly from the tuner. 

 

Best regards, 
Hari

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1 Reply
Hari
Moderator
Moderator
Moderator
750 replies posted 500 replies posted 250 solutions authored

Hi @create_sys 

 

The reason the metal plate contributes to the signal is that the physical capacitance of the sensor is increasing in the presence of this conductive object. There are two approaches - 

 

1. Without modification of the hardware

Please reset the device once the entire assembly is in place. The CapSense system resets the baseline upon reset and this will fix the sensor always being on while the metal housing is placed. But please note that the sensing distance might be reduced on the bottom side, but since you were initially getting over 5cm, we might get 4cm with the housing. 
As a general rule of thumb, it is recommended to tune the CapSense sensors with the whole assembly.

 

2. With hardware modification - In case there is a requirement for greater sensing distance, a hardware modification would be required. We recommend placing a hatched shield on the layer below the sensor layer so that the sensor is guarded against any external sources (such as the metal housing in this case). You can find more details about shield in our Proximity sensing with CapSense AN - https://www.cypress.com/file/46741/download

 

It would also be great if you can run the CapSense tuner on your PC and check the performance of the sensor. Note that you can provide a lower value for the proximity threshold, thereby allowing you to sense a greater distance, if the noise level in the system is low. We recommend having an SNR of 5:1 at the required sensing distance. You can check the SNR directly from the tuner. 

 

Best regards, 
Hari

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