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tachometer

Discussion in 'General Electronics Discussion' started by sheminasalam, Jun 22, 2012.

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  1. sheminasalam

    sheminasalam

    3
    0
    Jun 22, 2012
    I would like to get help on a situation of mine.
    I want to design a circuit to measure the speed in rpm of a shaft and then in accordance to the measured value of the speed I would have to operate 3 motors.There is one more constrain to this,the 3 motors will have to be operated in the both directions(forward and backward) in accordance to the speed detected.
    Please help me.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Your question is VERY short on detail. You need to describe everything in MUCH more detail.

    Obvious questions that arise in my mind as I read the post are:
    What is the shaft connected to? How fast is it expected to turn? Do you have any ideas on how to detect its rotation? Can you attach a magnet to it and trigger a reed switch or hall effect sensor? Do you need to detect the direction of rotation or just the speed? How quickly will the speed change? Is it enough to detect a single rotation of the shaft or will you need multiple magnets on it (or a gearing system) to accurately follow acceleration and deceleration? What type of motors do you want to drive? Are they DC or stepper? What voltage? What power? Do you need to vary the motor speed or just turn them on and off? How do you detect the motor speed so you can ensure the motor is turning at the right speed? Do you want to make just one system or several or a production run? Do you have time or cost constraints? What experience do you have with mechanical stuff? What experience with electrical and electronics? Can you solder? Do you have a breadboard? Do you know about microcontrollers?

    The biggest fundamental question that arises for me is: WHAT ARE YOU TRYING TO DO? A THOROUGH description of what you want to do, including descriptions of the parts of the system you already have, and any ideas you have for what to buy for other parts of the system, would save a LOT of time.

    Without this basic background information, you may see lots of ideas suggested, which aren't appropriate because of restrictions and issues that you haven't described in your post. This wastes the time of many people. PLEASE, describe what you want to do THOROUGHLY and let the people here use their experience and gut feelings to make USEFUL suggestions.
     
  3. mechtronics

    mechtronics

    153
    2
    Aug 7, 2011
    maybe a rough diagram of what you want to do might be of help?
     
  4. sheminasalam

    sheminasalam

    3
    0
    Jun 22, 2012
    Actually this is part of my mechanical project that is based on dual clutch transmission.
    Being a mechanical student I will only be able to explain what I want to achieve and cant go anymore deep.I hope you can understand.
    We have a shaft coming from the engine and through a gear box.The electronics system that I requested was to detect the speed of this rotating shaft and at particular rpms operate actuators that will change the gear and also engage and disengage the clutch.
    Earlier we planned to use motors for this.But later we have devised a link mechanism that will make us replace the motors with actuators.We now plan to use 2 actuators to move the gear and one for engaging and disengaging the gear.
    As I have said this is a dual clutch system and thus we require a greater pull on that thus we will opt for a actuator that has more pull.But the 2 other will be simple.

    I hope you have got a simple basic idea about this.The maximum speed of the engine will be 1200 rpm.So I would require a system that could detect the speed up-to this value and and at a particular rpm do a process that is specified.
     
  5. sheminasalam

    sheminasalam

    3
    0
    Jun 22, 2012
    I hope you can understand,I know u will be totally confused.but ask questions and I will try to give the answers.
    I believe u can find me a solution.
     
  6. duke37

    duke37

    5,343
    753
    Jan 9, 2011
    I do not know whether I can help as this is not my expertise.
    Since you have a gearbox, you could use a Hall effect device to give a frequency output.
    This frequency can be turned into a voltage with a LM2917N.
    If you wish to engage a gear when two shafts match, then do the same for the second shaft.
    Compare the two voltages and when near enough, engage gear.

    All this could be done with a microcomputer.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Thanks for the explanation. It's still short on detail but it gives us somewhere to start.
    So you're talking about implementing an automatic gearbox.
    I've never done anything like this but I'll try to make some suggestions.
    What is the application? What is the output being used for?
    An automatic gearbox needs to know shaft RPM, but needs to know a lot more. Other factors are the "accelerator position" input, i.e. the user's "go faster"/"go slower" request (because if the user wants to go faster, you may need to drop down a gear to get more power), and the output torque (if the output torque increases, you may need to drop down a gear, and vice versa). Also, how is the fuel input to the engine being controlled?
    It's not enough to simply look at the output RPM and adjust the gear ratio accordingly, if you want good dynamic performance.
    In a practical modern vehicle application, everything is monitored and controlled by the engine computer, and it's a very non-trivial job to get a good combination of responsiveness and efficiency.
    Maybe I'm assuming too much complexity for this project. If we assume that you're just going to do something minimal that just monitors the shaft RPM and adjusts the gear ratio, it's fairly simple using a microcontroller, but you will need someone experienced with programming and interfacing to program the microcontroller. You'll need some way to drive the motors and/or actuators - try googling solenoid drivers, motor driver ICs, H-bridge driver ICs. There are plenty of components available. You'll also need a grunty power supply.
    It's not hard to get a microcontroller to remember the current gear, and produce timed signals to operate the clutches and gear change actuator. Monitoring the shaft RPM is not difficult either. You need a way to detect the shaft rotation. A single magnet on the shaft might be enough, with a reed switch or hall effect sensor (google those keywords).
    If you want to avoid using a microcontroller, it's possible to use discrete logic and timing to do simple actions, such as actuating the clutches, waiting a short time, changing the gear position, waiting a short time, and re-engaging the clutches, but I would suggest finding an electronics engineering person to help with the project since there will probably be some hands-on experimentation required.
    As duke37 suggested, you can use a frequency-to-voltage converter IC to convert a pulse stream from your hall sensor into a voltage, and voltage comparators to detect when gears need to be changed, and if necessary, to detect when speeds match closely enough that you can engage the next gear (that all depends on how the gearbox works and whether it has some sort of synchromesh arrangement). You'll also need some way to keep track of the current gear - ideally this should come from the gearbox itself, so it's not possible for the control circuit to get out-of-step with the mechanical stuff. Maybe just limit switches to detect "maximum gear" and "minimum gear" would be enough. How many gears does the gearbox have, anyway?
    You could do some googling on any unfamiliar keywords I've mentioned here, and decide a lot more details about your mechanical design - specifically, the requirements for the gearbox - how many gears, and how the actuators need to be controlled in terms of timing.
     
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