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  1. #1
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    Default cnc tube bender retrofit

    Hello,
    I have recently purchased two Eaton-Leonard VB75 tube benders, a right hand and a left hand. These are small benders with a max tube size of 3/4". The controls on these machine are scrap so the machines need to be retrofitted. I would like to use some thing like Kmotion or Mach3 for a controller and use ClearPath step and direction servo motors to run the plane of bend motion and the distance between bend motion. I think I could control these directly from a step and direction controller. The bend axis is driven by a hydraulic cylinder and the cylinder motion is controlled by a Moog servo valve. My expericence with servo valves has not necessarily been all that great. On another tube bender that I have, their characteristic seem to constantly change as they run and I am constantly having to make minor changes to get it to work correctly. I think this is a problem with the valve/amplifier combination. I would like to avoid this type of problem and additionally I don't know how a servovalve could be driven from a step and direction output controller.

    I am seeing hydraulic press brakes that are driven by servo driven hydraulic pumps and got to thinking that this concept might work on my tube bender. I did some initial calculations on pump and motor size and it seems to be a reasonable concept.

    The concept that I have on the operation of the servo pump would be as follows:
    1. The controller would output signals to the servo amp tell it to move to say 90 deg. A closed center 4 way 3 position valve would shift to the forward direction. Concurrently the servo pump would accelerate up to the bending speed rate pumping oil into the cylinder, causing it to move toward the 90 deg mark. An encoder on the bend axis would feed position data back to the servo pump amplifier.
    2. As the bend axis approached the 90 deg position the servo pump would start decelerating and the bend axis would stop at the 90 deg mark. Concurrent with the axis reaching position, the 3 position valve would shift to the closed center position, locking the axis in place. I realize that the spring back on the tube would be wanting to move the bend axis away from the 90 degree position and since the hydraulic valve is not zero leakage, over time that would happen. Fortunately, that would not be problematic as the bend has already been made.
    3. When the machine was ready to take the bending axis back to zero position, the controller would send out positional signals to the servo pump amplifier, the 3 position valve would shift to the reverse position and the servo pump would accelerate up to the axis return speed and would return the bend axis toward the 0 deg home position. The encoder would be supplying positional data to the servo amplifier.
    4 Again as the bend axis approached the 0 position, the servo pump would begin decelerating and the bend axis would stop at the 0 deg position. Immediately the 3 position valve would shift to the closed center position. locking the axis at the home position, finishing the bending sequence.

    In the above scenario, the servo pump would always be pumping one direction. It would be controlling the axis acceleration, velocity, and deceleration into position, but the direction would be controlled with the 3 position 4 way hydraulic valve. Can this be done? Also, as the pump wears and the oil temperature changes, the amount of oil pumped per revolution will vary. That causes the pump to have to run faster as it wears and as the oil gets hot. The controller or amp would have to capable of sensing the axis not keeping up and adjusting the rpm of the motor to accommodate this variation. Can this be done?

    I have retrofitted several cnc machines in the past and have done several plc projects, but have never done any motion control, so I am breaking new ground. Any guidance on this project would be appreciated.

    Thanks

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  2. #2
    Registered Jim Dawson's Avatar
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    Default Re: cnc tube bender retrofit

    Quote Originally Posted by PROPAVER View Post
    Hello,
    I have recently purchased two Eaton-Leonard VB75 tube benders, a right hand and a left hand. These are small benders with a max tube size of 3/4". The controls on these machine are scrap so the machines need to be retrofitted. I would like to use some thing like Kmotion or Mach3 for a controller and use ClearPath step and direction servo motors to run the plane of bend motion and the distance between bend motion. I think I could control these directly from a step and direction controller. The bend axis is driven by a hydraulic cylinder and the cylinder motion is controlled by a Moog servo valve. My experience with servo valves has not necessarily been all that great. On another tube bender that I have, their characteristic seem to constantly change as they run and I am constantly having to make minor changes to get it to work correctly. I think this is a problem with the valve/amplifier combination.
    It could also be a problem with the controller, or position feedback system. A properly tuned system should compensate for normal variations in operating conditions. Maybe some adjustments to the PID loop is in order.

    Quote Originally Posted by PROPAVER View Post
    I would like to avoid this type of problem and additionally I don't know how a servovalve could be driven from a step and direction output controller.
    It is possible.....kinda It would require a step & direction to analog converter, they do exist, but tuning might be a nightmare.

    Quote Originally Posted by PROPAVER View Post
    I am seeing hydraulic press brakes that are driven by servo driven hydraulic pumps and got to thinking that this concept might work on my tube bender. I did some initial calculations on pump and motor size and it seems to be a reasonable concept.
    Seems reasonable to me. It would be possible to hang a servo motor onto a rather inexpensive gear pump. What size motor is it going to take to drive the pump?

    Quote Originally Posted by PROPAVER View Post
    The concept that I have on the operation of the servo pump would be as follows:
    1. The controller would output signals to the servo amp tell it to move to say 90 deg. A closed center 4 way 3 position valve would shift to the forward direction. Concurrently the servo pump would accelerate up to the bending speed rate pumping oil into the cylinder, causing it to move toward the 90 deg mark. An encoder on the bend axis would feed position data back to the servo pump amplifier.
    2. As the bend axis approached the 90 deg position the servo pump would start decelerating and the bend axis would stop at the 90 deg mark. Concurrent with the axis reaching position, the 3 position valve would shift to the closed center position, locking the axis in place. I realize that the spring back on the tube would be wanting to move the bend axis away from the 90 degree position and since the hydraulic valve is not zero leakage, over time that would happen. Fortunately, that would not be problematic as the bend has already been made.
    3. When the machine was ready to take the bending axis back to zero position, the controller would send out positional signals to the servo pump amplifier, the 3 position valve would shift to the reverse position and the servo pump would accelerate up to the axis return speed and would return the bend axis toward the 0 deg home position. The encoder would be supplying positional data to the servo amplifier.
    4 Again as the bend axis approached the 0 position, the servo pump would begin decelerating and the bend axis would stop at the 0 deg position. Immediately the 3 position valve would shift to the closed center position. locking the axis at the home position, finishing the bending sequence.
    Your operating sequence sounds reasonable. To prevent the leak if needed could be done with pilot operated check valves.

    Quote Originally Posted by PROPAVER View Post
    In the above scenario, the servo pump would always be pumping one direction. It would be controlling the axis acceleration, velocity, and deceleration into position, but the direction would be controlled with the 3 position 4 way hydraulic valve. Can this be done?
    Yes, you would just feed back all of the operating conditions to the computer and let the computer figure out how to follow the motion profile. This may require a controller a bit more sophisticated than Mach3. The Kflop/Kanalog is user programmable so might be an option. I think you are going to need some special software. But it sounds like you are already a programmer, you just need to dive in a bit deeper. Should be a short learning curve, and a really fun project.

    Quote Originally Posted by PROPAVER View Post
    Also, as the pump wears and the oil temperature changes, the amount of oil pumped per revolution will vary. That causes the pump to have to run faster as it wears and as the oil gets hot. The controller or amp would have to capable of sensing the axis not keeping up and adjusting the rpm of the motor to accommodate this variation. Can this be done?
    Absolutely, that's why PID loops were invented.

    Last edited by Jim Dawson; 01-29-2017 at 10:42 AM.


  3. #3
    Community Moderator Al_The_Man's Avatar
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    Default Re: cnc tube bender retrofit

    You need a hydraulic servo valves and servo amps, using a PID loop to mimic a servo motor doing the same thing, 3 position valves won't cut it IMO.
    This is how pipe-bender manuf such as Pines do it.
    The feedback is done either with rotary encoders or linear (scales) type.
    I did a Pines once using a (now) legacy Galil Motion card.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


  4. #4
    Gold Member TomKerekes's Avatar
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    Default Re: cnc tube bender retrofit

    Hi Propaver,

    Sounds like it would be worth a try to me. How well it would work would depend on the dynamics of the system such as how fast the valves respond, how fast the pump motor responds to speed changes, friction, comprehensibility of the fluid, nonlinearities, etc...

    With KFLOP you can use the "No Output" mode to make use of KFLOP's trajectory planner, PID, Filters, Deadband, Feedforward, etc.. to create a Servo Output. See this diagram:



    You could then use a User C Program to read the desired Servo Output and apply it to your valves and pump speed control and handle any possible quirks (ie don't toggle the valve too frequently, if the desired output is below some threshold kill the pump and close the valve).

    Here is some example code of how to send the Servo Output to a KFLOP PWM in Sign and Magnitude fashion.

    Code:
    // put the servo Output to a specified PWM channel 
    // and direction bit in signed and magnitude mode
    
    void OutputSignMag(int ch, int pwm, int dir_bit)
    {
    	if (chan[ch].Enable)
    	{
    		if (chan[ch].Output >= 0)
    		{
    			if (chan[ch].Output > MAXPWM) // don't go to 100%
    				FPGA(IO_PWMS+pwm*2) = MAXPWM;  
    			else
    				FPGA(IO_PWMS+pwm*2) = chan[ch].Output;  
    			SetBit(dir_bit);
    		}
    		else
    		{
    			if (chan[ch].Output < -MAXPWM) // don't go to 100%
    				FPGA(IO_PWMS+pwm*2) = MAXPWM;  
    			else
    				FPGA(IO_PWMS+pwm*2) = -chan[ch].Output;  
    			ClearBit(dir_bit);
    		}
    	}
    	else
    	{
    		FPGA(IO_PWMS+pwm*2) = 0;  // whenever not enabled put 0% duty cycle
    	}
    }
    HTH
    Regards

    TK
    http://dynomotion.com


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