FDC2214EVM: FDC2214EVM

Hi Chris,

Many thanks for coming back to me.

As I said I have connected the sensor of size 3cm x 16cm via a wire (shown as red) in the following picture into sensor channel 0 of FDC2214 EVM:

The sensor is mounted on edge of flap or wing of a prototype security gate:

 As it can be seen the sensor is a copper strip on fibre glass used for making PCB and mounted using the electric tape on the edge of flap or wing of security gate.

The mechanical parts of gate are made of metal (stainless steel) covered in red plastic covering. This is demo prototype of gate where the corridor is made of wood.

The flap or wing is able to move via mechanical mechanism connected to an electric motor to open and close the gate as shown below:

My aim is to use this sensor mounted on the edge of flap (wing) so that it can detect the human body (or hand) to stop closing the gate before it hit the body of a person.

I was thinking of measuring the capacitance of sensor mounted on the edge of flap in a calibration phase for different angles of flap when there is no body at the gate which gives me a baseline profile of measure capacitance versus flap angle for case there is no body at the gate.

And then compare the capacitance measured from the sensor at each angle when flap is closing with baseline capacitance for each angle. By comparing the two capacitance I hoping to detect the human body at the gate as proximity sensor and stop closing the gate.

I am thinking of using FDC2214 for this purpose of measuring capacitance based on the demos I have seen on youtube for proximity detection.

My problem now is that when the sensor is mounted on the edge of flap, the sensitivity of sensor is decreased with respect to the case it is on my desk.

I am suspecting that metal parts behind the sensor absorbing the electric field from the sensor and causes decreased sensitivity.

I was wondering how could I increase the sensitivity of sensor given that I have limited amount of area on the edge of flap to increase the size of sensor?

Could I increase the sensitivity by some kind of shielding between the sensor and the other metal parts?

Could I increase the voltage of sensor to increase the electric field and hope that this would increase sensitivity?

I tried to connect to connect the ground on EVM to earth but it did not seem to increase sensitivity noticeably?

If I can get this sensor to detect the human body in 10 cm to 50 cm proximity, then we will be using FDC2214 in our product massively so any help would greatly be received and appreciated.

Thanks

Amin

Hi Amin,

Thanks! That was very clear. To maximize the sensitivity, when you construct your sensor, you should try to minimize any fixed sensor capacitance. This includes the sensor plate to ground capacitance:

So, you should try to increase d to reduce the fixed capacitance. For more effective shielding, try to increase the size of the ground plane; although this will slightly increase the fixed capacitance, the improvement in isolation is usually worth the effort.

Also, to improve the performance, use a higher value of RCOUNT, along with a lower discrete sensor capacitance. Often, using a smaller fixed inductor is helpful, as the inductor will have it's own parasitic capacitance which adds onto the total sensor capacitance. Also, increase the sensor IDRIVE to get the sensor amplitude to 1.8Vp.

Attached is a diagram showing the sensor design recommended for most robust operation - this construction uses a balanced sensor design, and includes some ferrites for high frequency protection.

Regards,

ChrisO

Hi Chris,

I was trying to do active shielding using buf634 it was suggested by TI member in answer to a question:

The pin out of buffer is as follows from data sheet from TI:

So FDC channel 0 output was connected to VIN (pin 3) and also directly to main sensor.

The Vo is connected to shield around the sensor which covers the sensor except one direction which is exposed to target object.

The BW is connected to V- and both are connected to GND on FDC2214 EVM

V+ is connected to VCC on FDC2214 EVM

overall connections are like this:

But the problem is that after connection of buffer, by putting the oscilloscope probe I could not see any oscillation as if it loaded the sensor and FDC2214 altogether.

If disconnect the buffer, I can see some oscillation on IN0A using probe of oscilloscope.

I really do not understand why it has such bad effect? The fact that V- and BW are connected to GND of FDC2214 EVM and V+ is connected to VCC on FDC2214 EVM is correct? As Buf it is not on FDC2214 I connect it to buffer via some wires soldered to EVM and to bread board. Could length wires cause a probelm.

I have changed the buf from SMD to DIP package still the same problem. and it can not sense anything.

I am desperate for some help.

Thanks

Amin

Hi Chris,

Thanks for giving advice regarding the lowering the frequency.

Just to confirm, by lowering sensor frequency, you meant change the fixed capacitance or/and inductance of LC tank connected between IN0A and IN0B so that resonance frequency changes? 

Or you meant to change the external clock frequency using the GUI which I changed to 0.5MHz from 40MHz as shown below:

Also if you meant changing clock frequency, shall I change the frequency of input Deglitch to 1MHz?

By changing the clock, a slight pattern in change capacitance appeared, but it is still vert far from the scenario where buf634 was not in place?

I also have bought one FDC1004 EVM and did an experiment with this one by connecting the sensor board I was using for FDC2214 EVM which is of size 15cm x 2.3cm. By connecting this sensor and shielding which is much bigger than built-in sensor and shield on FDC1004 EVM, I noticed a significant decrease in rate/frequency of  capacitance measurement in comparison to case where only embedded sensor plate/shields on FDC1004 is used. Is FDC1004 using a method of measuring decay time of RC to measure capacitance?

because if that is being used by FDC1004 for measuring capacitance then by using a bigger sensor to increase sensitivity then we have bigger C which would cause loner RC decay time and therefore measurement time would increase significantly or capacitance measurement frequency would drop significantly?

Many thanks for your help Chris,

Amin

Hi Chris,

I also have a question regarding the shape of sensor.  I managed to get some gain in sensitivity and detection range by putting a cuboid shape shield around the main rectangular sensor where the main rectangular sensor would fit inside this shield. The shield was like a housing where an insulator around main sensor make sensor and shield disconnected inside housing. However, both shield and main sensor were connected to pin IN0A of FDC2214. Even both were connected to same pin directly and there were no buffer used, I managed to get considerable gain in sensitivity, and this gain make thinking to improve over shielding by using a buffer which might not be suitable till now.

As the shield is like a housing around main sensor, it caused the electric field from main sensor to be target on one direction towards the target and no other direction.  Now, I was wondering if I could change the shape of main sensor so that electric field would focus on one point or region. I wonder a sensor with concave shape (part of ellipsoid or sphere) would focus the electric around the focal length point which then would detect the object in distance around focal length with higher sensitivity?

I also have FDC2214PROXSEN-EVM () which has the sensor plate like a ring around the board so if I use a rectangular/elliptic ring  with the same width of rectangle instead of a rectangular shape sensor, would it make more sensitive? or because the area of ring with same diameter is less than rectangle, the sensitivity would be less?

Thanks

Amin