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Capacitive Force Sensor Circuit Project

Discussion in 'Sensors and Actuators' started by Evlov, Jan 5, 2018.

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  1. Evlov


    Jan 5, 2018

    I'm working on a project that requires the use of a capacitive force sensor like this one:

    I've broken the problem down into 3 parts, where Part II builds off of Part I, and Part III builds off of Part II.

    Part I

    If a force of 1.3 lbs or more is applied to the sensor, then a digital numeric display (7-segment display, for example) begins to count up from zero. (It's counting the time that has passed, so after one second it displays "1", after two seconds it displays "2", and so on.)

    The digital numeric display continues to count up indefinitely until the force applied to the sensor is 1.0 lbs or less. If the force applied to the sensor is 1.0 lbs or less, then the digital numeric display shows "0" or is blank (either is fine).

    The circuit is free to cycle between these two states, where State 1 is when the digital numeric display is counting up, and State 2 is when the digital numeric display is showing "0" or is blank.

    Part II

    Instead of instantly changing states upon meeting the associated force requirement, any change in state is delayed by 10 seconds.

    For example, if the circuit is currently in State 1 (the digital numeric display is counting up) and the force applied to the sensor drops to 1.0 lbs, then the circuit will change to State 2 (the digital numeric display shows "0" or is blank) 10 seconds later.

    Part III

    In order for the change in state to be successful, the force applied to the sensor must remain within the associated force range requirement over the course of the 10 seconds.

    For example, let's say the circuit is currently in State 1. The force applied to the sensor drops to 1.0 lbs and triggers the start of the 10-second delay. The force applied to the sensor varies between 0 lbs and 1.0 lbs for 5 seconds. At this time, a force of 1.1 lbs is momentarily applied to the sensor. Because the 1.1 lbs force is outside of the force range requirement, the change-of-state sequence is aborted. Immediately following the 1.1 lbs force spike, the force applied to the sensor drops back down to 1.0 lbs and the 10-second delay restarts (at zero).

    I'm fairly confident that this can be solved without using an Arduino, and instead with using only fundamental electrical components and simple ICs. However, all ideas are welcome :)

    Thanks in advance!

    Attached Files:

  2. kellys_eye


    Jun 25, 2010
    Sample and hold with peak detector. Both possible using op-amps.
  3. Evlov


    Jan 5, 2018
    Thanks for the quick response!

    I'm actually a novice when it comes to circuits... Can you possibly elaborate on your explanation?
  4. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Since you are an admitted novice when it comes to circuits, not even recognizing what @kellys_eye was referring to when he mentioned sample-and-hold and peak detector analog circuits, what makes you so confident that only "fundamental electrical components and simple ICs" are necessary to accomplish your objectives?

    The sensor (transducer) you linked to requires signal conditioning to convert its variable capacitance as a function of force into a useful electronic signal. Are you going to also purchase the signal conditioner that is offered on this web page? The signal conditioning device that is offered supplies both an analog 0 to 2.0 V output as well as a serial digital I2C interface to a separate microcontroller. On-board registers in the signal conditioner are useful in calibrating the transducer, but the I2C interface is required to access them.

    Clearly, the manufacturer intended the use of an Arduino to implement the I2C interface, as well as the USB interface protocol, with all the heavy lifting performed by a software application running on a personal computer (PC) at the other end of the USB connection. The software, for both the Arduino and the PC, is even provided as a free download. So, if your goal is to eliminate the Arduino and PC, you should also eliminate the signal conditioner board, which obviously contains more than just fundamental components and simple ICs.

    However, based on the conversation so far, I don't think you have a clue about how the capacitive force transducer works or how to interface it to anything that will provide the weight signals you need to control an up-counter.

    So, please answer this question: what will you connect the weight sensor to (electronically) that will provide a signal when the weight sensed is greater than 1.3 Lbf and another signal when the weight sensed in less than 1,0 Lbf? Also, what happens when the weight is between 1.0 and 1.3 Lbf? Does the counter continue to increment from where it stopped if the weight increases beyond 1.3 Lbf, or does it reset back to zero?

    And finally, is this a school or homework project?
    OBW0549 likes this.
  5. Evlov


    Jan 5, 2018
    Thank you for the thoughtful response, hevans1944! Everything you said is basically spot on!

    You are correct that I don't understand how to interface the capacitive force sensor to the circuit so that the weight signals can control the up-counter. This is part of the challenge. I could purchase the signal conditioner if it's absolutely required. However, the user manual for the sensor indicates that it can be connected directly to the circuit without the signal conditioner, as shown in the attached image. Alternatively, I MIGHT be able to use a load cell, but it's not ideal due to its bulk.

    Please find the answers to your questions below:

    What will you connect the weight sensor to (electronically) that will provide a signal when the weight sensed is greater than 1.3 Lbf and another signal when the weight sensed in less than 1,0 Lbf?

    I don't know. This is part of the challenge.

    Also, what happens when the weight is between 1.0 and 1.3 Lbf?

    Nothing will happen. The circuit will remain in the state that it is in.

    Does the counter continue to increment from where it stopped if the weight increases beyond 1.3 Lbf, or does it reset back to zero?

    The counter would reset back to zero.

    And finally, is this a school or homework project?

    This is a personal design project. It is NOT a school or homework project.

    Attached Files:

    hevans1944 likes this.
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Okay, you are perhaps a serious newbie who doesn't want to get involved with Arduino, for whatever personal reasons, but are willing design your own signal conditioning circuits for the capacitive force transducer.

    This is a huge mistake. You know absolutely nothing about how this transducer works, what is required to produce a voltage output that is function of force, or any of its maximum operating parameters... well, other than you shouldn't put the transducer into tension lest you break something inside of it, nor should you exceed three times its full-scale force lest you also break something inside of it.

    When a designed, tested, and available signal conditioning board is available for just a few dollars, why do you want to re-invent that particular wheel? Just purchase this instead and use the analog output. We can probably help you with the rest of it, but bear in mind we also expect you to exercise due diligence, do your homework research, and at least propose some sort of initial design using "only fundamental components and simple ICs."

    IF you are still in the wheel re-invention frame of mind, the inventors and manufacturers offer OEM (Original Equipment Manufacturer) assistance in integrating their sensor. Visit this webpage for more information.
    OBW0549 likes this.
  7. Evlov


    Jan 5, 2018
    Thank you for your patience. I apologize for my lack of expertise. Please keep in mind that if I knew what I was doing, then I wouldn't be on this forum asking for help ;)

    I'd like to keep the cost low and the size small. However, if I have to start with a solution that's neither, that's ok. As I mentioned in my original post, all ideas are welcome.

    I also welcome partial answers to my question. If you're getting stuck on the capacitive force sensor, then feel free to replace it with a simple load cell instead. Perhaps this one:

    I'd still be thrilled to see a circuit design solution that solves my problem using a load cell. I can always go back and figure out how to replace the load cell with the capacitive force sensor later.

    I'd also be happy to see a proposed solution to Part I only, or just through Part II. Every bit helps!
  8. Alec_t


    Jul 7, 2015
    It often helps to find a solution if we know the end purpose of the system. Exactly what will the force sensor and display be used for?
  9. OBW0549


    Jul 5, 2016
    As an analog design engineer who has done this many times over the years, I can tell you that designing the signal conditioning circuitry for capacitive sensors is NOT a trivial task, even for an experienced designer.

    The design task becomes especially difficult with sensors like these, where the capacitance is very small (75 pF or 230 pF), the capacitance changes less than 3% from no force to full force load, and the capacitance change is highly non-linear to boot. While you might be able to come up with a circuit that "kinda-sorta" does something detectable in response to force using this sensor, making it give an output voltage that is repeatable, stable, insensitive to interference, temperature fluctuations and the effects of stray capacitance, and which can be reliably and consistently interpreted in terms of applied force, is quite out of the question for a novice (as you describe yourself). It is definitely a "don't try this at home" thing.

    I strongly suggest following the advice @hevans1944 gave you in post #6:

    Do that, and you'll almost certainly save yourself an enormous amount of frustration and confusion, and quite possibly avoid failure as well.
    hevans1944 likes this.
  10. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    My patience has just about run out. Your lack of expertise is correctable with education, training and experience. So far, I have seen no indication that you are willing to put forth the effort necessary to gain any expertise. Instead you ask for ideas, suggestions, and answers to your questions. Perhaps if you are in a "monkey see, monkey do" mode of rote learning, then a more suitable forum would be this Instructables website.

    Most of us who post on a regular basis here on Electronics Point are "hands on" experimentalists, some of us with more than average abilities imagining solutions to problems and going forth to implement a particular solution. If we "get in trouble" with an implementation, having tried our best to figure out what went wrong, we post as much information as possible about (1) what we are trying to DO, (2) how we are trying to do it, (3) and when and where we think things went wrong. If we are thorough and clear in posting of our difficulty, we usually receive prompt responses from other EP members, and a back-and-forth conversation often ensues as we narrow the focus down to a particular "fix" for the problem. I see no evidence that your efforts to date are anywhere near or similar to the preceding description.

    EP gets a lot of "drive by" posters who join, post a question, and are never heard from again, probably because they didn't receive instant gratification. We are not about instant gratificaiton. Nor is it our purpose to train you.

    We are here to help, but you are expected to do the actual work. There once was a highly knowledgeable and respected member who did do the actual work,,, designing circuits, complete with bill-of-materials and often with a printed-circuit-board (PCB) artwork layout... all backed up with extensive explanations of how his circuits worked to solve whatever problem the original poster (OP) presented. @KrisBlueNZ (deceased) was truly amazing and one-of-a-kind.

    I, and others here who knew Kris, miss his presence on these forums. I doubt that anyone will ever step up to try to fill his shoes. Nor do I think that would necessarily be a good thing to do if it short-circuits our goals to discuss, advise and debate electronics related topics.

    There are many here who can "solve" your three goals, although perhaps not within the constraints you have presented: small, low cost, no advanced integrated circuit circuit technology. You copied a graphic from the user manual that indicated four approaches to using the capacitive force transducer you have chosen. The first three of these use the signal conditioner board mentioned earlier. Only the last one indicates a direct connection to "User Hardware" with the explanation "User reads capacitance using their own circuitry." The first choice uses the signal conditioner, but uses only its analog output. This is what I and @OBW0549 have suggested you try if you insist on using your own circuitry to implement the three parts of your original post.

    However, since you did ask for suggestions, I would suggest you take advantage of the huge software resources available free to the Arduino community as well as to the Windows and Linux communities and implement a USB interface to a PC. Software on the PC (or tablet or whatever) can display your count-up clock and perform the logical discrimination for which you are asking. Your entire project can probably be completed in one weekend or less. After you get to that point, you can go to work eliminating all the "extra and superfluous" hardware and software leaving you with just a 7-segment display, a counter, and other "fundamental electrical components and simple ICs." Maybe somewhere along this journey you will even discover how to use those things to perform signal conditioning of the capacitance force transducer.

    If you want to try using a load cell, that's fine. There are plenty of circuits available on the Internet for you to choose from. Be sure to make Google your friend to find those.
    Alec_t and OBW0549 like this.
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