need help on a control system

Discussion in 'The Projects Forum' started by jobertgalea, Nov 27, 2008.

  1. jobertgalea

    Thread Starter Member

    Nov 27, 2008
    Hi all,

    I am building a simple control system and wish some help on it.

    I have a motor attached to a vise which closes the jaws on a load cell to measure the force. The load cell gives feedback of a maximum of 30mV at full load. The motor is a 12VDC 60W 50rpm motor.

    I have three questions:
    1. do you think it is best to use an H bridge with transistors or mosfets to reverse the motor or just a simple H bridge with switches due to high currents?
    2. And in the differential opamp for the control system what determines the value of the resistors apart from the amount of gain required? i.e. does the current effect which value to choose for a particular opamp?
    3. And is there any simple way how to calculate the transfer function of the motor as I do not have and cannot find any data sheet on my type of motor
    I would be very grateful if anyone could help me cause even tutors did not help me much.

    thank you
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    A simple switch can reverse the motor.

    You will have to give a better description of what you're trying to accomplish before it's possible to say what you may or may not need. This applies to the motor, load cell, and sensing.
  3. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007

    There are two ways to approach the control system design: analytical and experimental. If you know the transfer function of the motor and transducer, then the task is fairly simple. Designing the PI or PID compensator based upon these, and the common control system parameters.

    However, it seems that you do not have enough information. You can derive the motor parameters if you have the appropriate test setup.

    If you want to do this experimentally, you can use a generic PID. I would amplify the signal of the transducer to a reasonable value (1-5V?). Then, it would go into the PID with the setpoint signal. The output would be a voltage that would be responsible for the PWM. Clearly, it must be a bipolar type signal for forward and reverse control. I have done the 2.5V center, 0V lower, 5V higher type of scheme in the past with a microcontroller. There are circuits for this out there. Then, you would employ some generic tuning algorithms such as the ZIEGLER - NICHOLS method.

    Use mosfets, they're fairly easy to work with and are more efficient in saturation..

  4. jobertgalea

    Thread Starter Member

    Nov 27, 2008

    thank you very much. I will try and approach the control system experimentally.

    But does anyone know what are the most important things to take into account when choosing the type of opamp? And also obviously the associated resistors?

    thank you
  5. mik3

    Senior Member

    Feb 4, 2008
    Actually, you cant plug any resistors values you want in the circuit. They have to be chosen as the op amp can draw the necessary input bias currents. The maximum values of the resistors depend on each op amp.
  6. leftyretro

    Active Member

    Nov 25, 2008
    Well first you should better describe the control objectives you require.

    It sounds like it could be much simpler then one might think at first if the purpose of the control is to start closing the vise and stop when the load cell reaches a predetermined force?

    If so all that would be required is a comparator function that stops the motor when the force is => then the desired force when closing. If that is the case the motor would only require a 3 position switch (off/open/auto) and possibly a limit switch to turn off the motor just prior to full open on the vise.

    Again a better description of your control objective would help define the need control functions required.
  7. jobertgalea

    Thread Starter Member

    Nov 27, 2008
    yes you're right sorry.

    My objective is as you said to set a predetermined value of force by a potentiometer and the motor closes the vise until that value is reached according to the feedback given by the load cell.

    To control the opening and closure of the vise I will be using two pedals for forward and reverse of the motor. this enables the user to be hands free for working. For this I will be using a simple H bridge with normal switches, although any suggestions would be more than welcome.

    The only problem I have is how to choose the type of Opamp for the comparator and if it is important to insert a buffer before
  8. leftyretro

    Active Member

    Nov 25, 2008
    Well basically you need to work in stages of circuit functions.

    First you need to have signal conditioning and amplification stage for the load cell, such that the 0-30mv load cell output signal is scaled to say a 0-10vdc measurement signal. Then you need to have a set-point generator that takes an potentiometer setting and outputs a 0-10 vdc signal. Next function would be the comparator function that has two inputs, load cell and set-point and outputs a logical on or off depending if load cell measurement is below or above the set-point value.

    I fail to see the need for an H bridge drive for this kind of application. A simple on/off relay or power FET switch can control the power to the motor on or off (but wired in series with the forward foot pedal) under control of the comparator function. A simple reversing switch (the reverse foot pedal) can then be used to open the vise manually by reversing the polarity to the motor. You still should have some kind of full open limit switch in series with the open foot switch in case the operator falls a sleep with the open pedal on and the motor burns out when the vise reaches full open ;-)

    So for your two questions:

    A buffer is required for the load cell, it could be an instrumentation type op amp with it's gain set to have an output range of 0-10vdc in my example.

    You can use virtually any standard op-amp or a comparator type op-amp, the IC used is not as important as the circuit design using the IC. Many op amp data sheets show typical application examples.

    PS: If I was tasked with designing such a system I would use a small microprocessor board like my Arduino. The load cell would still be scaled up to say a 0-5vdc. All switches would read as digital inputs to the board and a small serial display could show the actual measurement and the set-point desired. Would be a very low cost but effective solution. The programming would be very simple. Might still hardwire limit and safety switch contacts directly to the motor power.
    Good luck
    Last edited: Nov 28, 2008
  9. jobertgalea

    Thread Starter Member

    Nov 27, 2008
    thanks a lot for the information.

    It seems pretty interesting your suggestion and I like it also. They suggested me a PLC to control the system but as you said, with the microprocessor you can produce a display of the measurement, so its better with the microprocessor.

    Can you give so more information please on the microprocessor or you can buy it ready made? As first of all I do not have a lot of electronics knowledge cause in reality I am in the mechanical study section. And also do you think its relatively simple to work on a microprocessor?