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Re: [SeattleRobotics] testing robot

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  • Kevin Ross
    I also agree that pneumatics is going to be the easiest way. You will need a fairly large diameter piston to generate 1000kg of force using off the shelf air
    Message 1 of 20 , May 16, 2013
      I also agree that pneumatics is going to be the easiest way. You will need a
      fairly large diameter piston to generate 1000kg of force using off the shelf
      air fittings. You are also likely to need a larger compressor than you are
      thinking. At 50,000 cycles for 1 second, you will need to cycle twice
      (compress/retract each take a cylinder load of air). Compressors have a
      series of different ratings you need to consider, most important is the
      expected duty cycle. They will get very hot if you exceed their duty cycle.
      I would expect this to take a larger (60 or 80 gallon) unit to keep the
      thing from running every couple of minutes.

      You could use the same large cylinder and have two regulators and two
      solenoids. One regulator might run at 10psi to generate 220lbs of force, the
      other would run at 100psi to generate 2200 lbs of force. That would make
      this mechanically much more simple. At 100 psi you are going to need a 5.75"
      or larger piston, so 6" it is OR you can use a multiple chamber unit like
      this
      http://www.bimba.com/Products-and-Cad/Actuators/Inch/Compact/Low-Profile/Flat-One-Multiple-Power/
      assuming you don't need full power to retract.



      -----Original Message-----
      From: Peter Balch
      Sent: Thursday, May 16, 2013 7:50 AM
      To: SeattleRobotics@yahoogroups.com
      Subject: [SeattleRobotics] testing robot

      I want to fatigue-test some load cells. For instance, apply 100kg for 1
      second 50,000 times and apply 1000kg 1000 times; total travel 10mm.

      In our local Ikea store, there's a demonstration device that uses pneumatics
      to open and close a cupboard door all day long. That's the sort of thing we
      need.
      http://www.youtube.com/watch?v=fPTyGGGaWq8
      (OK, I know that's a demo not a real test.)

      I can find lots of pneumatic stuff in catalogs, e.g.
      http://uk.rs-online.com/web/c/pneumatics-hydraulics-power-transmission/pneumatic-cylinders-actuators/pneumatic-compact-cylinders/

      But it's very hard to get started. What should we buy? What pressures do we
      use? How do we do the sequencing: via electric solenoid valves or pneumatic
      logic?

      What I need is a beginner's guide. Is there a website or a book? A DIY
      explanation?

      Peter





      ------------------------------------

      Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
    • Charlie H
      Peter, Have you thought about using a motorized automotive scissor jack? They have plenty of power and and the linear range is greater than you need. You would
      Message 2 of 20 , May 17, 2013
        Peter,

        Have you thought about using a motorized automotive scissor jack?  They have plenty of power and and the linear range is greater than you need.  You would need to drive the motor (typically 12 vdc) with some electronics, of course.  I've seen these online for < $100.00.

        Charlie


        On 5/16/2013 2:10 PM, Peter Balch wrote:
         

        > Wouldn't this be a good application for a cam mounted on a gear-motor with
        > a
        > spring to moderate the force? Then for the higher force, just substitute
        > a
        > heavier spring. This provides the advantage of not having high impact
        > loads
        > that could result from a pneumatically-actuated cylinder, plus you don't
        > need a control circuit, solenoids, valves, etc. to cycle the mechanism.

        Perhaps it does. That's one suggestion I've had from my wife's cousin who
        has done exactly that for small engineering firms.

        Once again, I'd probably use the crank or cam to lift the fence post rather
        than apply the force directly.

        When I start looking at gear-motors and cranks that can manage 100kg over
        10cm, I get into foreign territory. How many h.p. is that? How
        over-engineered must it be. You generally buy the motors and gearboxes
        separately but if they're from the same "series" then they bolt together.
        What gear ratio? Well the gear ratio depends on the motor speed which
        depends on the torque and the torque depends on the gear ratio. Hmmm. It's
        not obvious what to buy.

        Then I'd need to get the crank or cam made. What length do we need this
        week, or next month?And the engineering for the test rig becomes more
        complex.

        I can see a lot of advantages to pneumatics. It fits togther like Lego or
        Meccano.

        All through this project we've realised that we don't know what we're doing.
        And even at the testing stage, I think we can be pretty sure that we don't
        know what we want to test. OK, it's clear this month - but next month, and
        the month after ...

        What I'd like is to make a test rig bolted together out of Dexion with a
        piston (or two or three) we can move from place to place. That way, we can
        rearrange the parts next month to do a different job.

        People have said "why not get a local engineering firm to make a such and
        such". But that's what I'd like to avoid. We'd have to give them drawings
        then wait days for them to make it. All of which implies that we actually
        know what we're doing. Bolt the Dexion together ... oh, no, that part needs
        to be much longer ...Easy.

        Similarly "why don't you get a local university engineering department to do
        the fatigue testing?". Well, I'm pretty sure that half way into it, we'd
        realise we'd specified the wong test or we were collecting the wrong data.
        If we can keep it all in-house, we can recover more quickly from our
        incompetence.

        >> Wouldn't this be a good application for a cam mounted on a gear-motor
        > That's brilliant! Extra points for KISS.

        Except it isn't that much simpler. Not once you start looking into the
        strength and speed of the motor, getting the crank made, mounting it all,
        etc. With a piston, it's already a linear actuator and the "power" part of
        the motor is somewhere else.

        > I'll bet good money 50K cycles with high force peaks
        > will break something in your rig

        Yes, probably. But that applies equally well to a gear-motor system.

        I've tried to find the plastic-deformation force on a 50cm beam of Dexion
        and it doesn't seem to be given anywhere. But applying what I remember from
        high-school physics stress/strain implies that a Dexion box would be
        over-engineered by about 20-to-1.

        > Wouldn't a cam + motor + spring be dramatically cheaper, too?

        Somewhat. But heavy duty motors and gearboxes aren't cheap and then we'd
        have to pay an engineering firm to make whatever converts rotary to linear
        motion.

        As far as I can see, pneumatics is more like Lego than engineering. Lego is
        good when you don't know what you want.

        Peter


      • Peter Balch
        Charlie ... I think that it would be too slow for this particular project. They seem to take tens of seconds to extend. But I could imagine it being a useful
        Message 3 of 20 , May 17, 2013
          Charlie

          > Have you thought about using a motorized automotive scissor jack? They
          > have plenty of power and and the linear range is greater than you need.

          I think that it would be too slow for this particular project. They seem to
          take tens of seconds to extend. But I could imagine it being a useful device
          elsewhere.

          I wonder what their design-life is. Probably not 50,000 lifts. And probably
          not continuous operation.

          Ed

          > Sounds like a windshield wiper motor would be better suited.

          I wonder what the torque is - wiper blades are hard to move by hand. And
          they can opereate continuously for many hundreds of thousands of cycles;
          they have to, particularly in Seattle and here in Scotland.

          Peter
        • Peter Balch
          Kevin ... I was expecting to use a 10-to-1 lever so the piston has to lift 100kg over a distance of 10cm. ... Maybe. But the piston would be under load (100kg)
          Message 4 of 20 , May 17, 2013
            Kevin

            >You will need a
            > fairly large diameter piston to generate 1000kg of force using off the
            > shelf
            > air fittings.

            I was expecting to use a 10-to-1 lever so the piston has to lift 100kg over
            a distance of 10cm.

            > You are also likely to need a larger compressor than you are
            > thinking. At 50,000 cycles for 1 second, you will need to cycle twice
            > (compress/retract each take a cylinder load of air).

            Maybe. But the piston would be under load (100kg) so it could return
            passively.

            > Compressors have a
            > series of different ratings you need to consider, most important is the
            > expected duty cycle. They will get very hot if you exceed their duty
            > cycle.

            I hadn't considered that.

            At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
            is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
            cycle that's only 40 L/min. A small compressor you'd use to paint your fence
            is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

            Would that be enough?

            > I would expect this to take a larger (60 or 80 gallon) unit to keep the
            > thing from running every couple of minutes.

            That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

            > You could use the same large cylinder and have two regulators and two
            > solenoids. One regulator might run at 10psi to generate 220lbs of force,
            > the
            > other would run at 100psi to generate 2200 lbs of force.

            Interesting idea. I was assuming that the cylinder lifted a lever which
            passively dropped to provide the force. The force would depend on what
            weight we hung on the lever.

            You're suggesting that the cylinder provides the force directly (or via a
            lever) and the force is determined by the air pressure.

            How accurately can you control the force that way? Doesn't friction and
            stiction affect the force? And how accurate is the air pressure?

            It would be nice if we could use the same rig for calibration as well as
            fatigue testing so a force accurate to a couple of percent would be good.

            Peter
          • ListServ
            Clippard has a pretty complete product line, and some good documentation on pneumatics. Look at their knowledge base and catalog for what is possible.
            Message 5 of 20 , May 17, 2013
              Clippard has a pretty complete product line, and some good documentation on pneumatics. Look at their knowledge base and catalog for what is possible.
              http://www.clippard.com/

              The symbols you see are likely pneumatic ansi symbols. A google search will describe them. If you can read a schematic, the pneumatic drawings should be understandable.



              On Thu, May 16, 2013 at 8:50 AM, Peter Balch peterbalch-at-btinternet.com |Listserv| <lf4fhwmmot@...> wrote:
               

              I want to fatigue-test some load cells. For instance, apply 100kg for 1
              second 50,000 times and apply 1000kg 1000 times; total travel 10mm.

              In our local Ikea store, there's a demonstration device that uses pneumatics
              to open and close a cupboard door all day long. That's the sort of thing we
              need.
              http://www.youtube.com/watch?v=fPTyGGGaWq8
              (OK, I know that's a demo not a real test.)

              I can find lots of pneumatic stuff in catalogs, e.g.
              http://uk.rs-online.com/web/c/pneumatics-hydraulics-power-transmission/pneumatic-cylinders-actuators/pneumatic-compact-cylinders/

              But it's very hard to get started. What should we buy? What pressures do we
              use? How do we do the sequencing: via electric solenoid valves or pneumatic
              logic?

              What I need is a beginner's guide. Is there a website or a book? A DIY
              explanation?

              Peter


            • Kevin Ross
              Compressor duty cycles are often times specified in 10 minute intervals or in 1 hour intervals. You need to check with the specific model you are looking to
              Message 6 of 20 , May 18, 2013
                Compressor duty cycles are often times specified in 10 minute intervals or
                in 1 hour intervals. You need to check with the specific model you are
                looking to use. A common value is 30% / 10 minutes, which means you can run
                it for about 3 minutes then it needs to cool down for the next 7 minutes.
                There are a lot of variables in this. You are also going to find compressors
                which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
                actually true in most cases and especially at the pressure you are going to
                need for your device. (We have 100% duty cycle compressors for use on FIRST
                robots, but if you attempt to run them for too long they heat up and fail.
                You have to let them cool down, meaning they are not really 100% duty cycle!
                It is marketing BS.)

                I see you have some of the math below. I don't really understand the details
                for your design so I am not able to verify your numbers at the moment, but
                you seem to get the idea. If you are generating 2200lbs of force with a 10cm
                stroke, my guess is that you are going to use much more air than what you
                have stated. I base this on a gut feeling from a guy who does a lot of
                pneumatics on FIRST robots and watches the compressors heat up very quickly.
                However, if you do the math, you get to tell me I am wrong and I won't feel
                bad at all.

                Pneumatics are a full of little variances. If you need more precision, you
                should consider hydraulic. Air acts like a spring, and is full of all sorts
                of surprises. It is not instant power, nor is it linear. It is, however,
                easy to work with and pretty reliable once you get it setup. The regulators
                do a good job at not allowing the pressure to exceed a certain amount, so
                they are usually pretty accurate at limiting the pressure.

                Friction will be an issue in any device you make. The fewer moving parts,
                the fewer sources of friction.

                -----Original Message-----
                From: Peter Balch
                Sent: Friday, May 17, 2013 11:43 AM
                To: SeattleRobotics@yahoogroups.com
                Subject: Re: [SeattleRobotics] testing robot


                > Compressors have a
                > series of different ratings you need to consider, most important is the
                > expected duty cycle. They will get very hot if you exceed their duty
                > cycle.

                I hadn't considered that.

                At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
                is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
                cycle that's only 40 L/min. A small compressor you'd use to paint your fence
                is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

                Would that be enough?

                > I would expect this to take a larger (60 or 80 gallon) unit to keep the
                > thing from running every couple of minutes.

                That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

                > You could use the same large cylinder and have two regulators and two
                > solenoids. One regulator might run at 10psi to generate 220lbs of force,
                > the
                > other would run at 100psi to generate 2200 lbs of force.

                Interesting idea. I was assuming that the cylinder lifted a lever which
                passively dropped to provide the force. The force would depend on what
                weight we hung on the lever.

                You're suggesting that the cylinder provides the force directly (or via a
                lever) and the force is determined by the air pressure.

                How accurately can you control the force that way? Doesn't friction and
                stiction affect the force? And how accurate is the air pressure?

                It would be nice if we could use the same rig for calibration as well as
                fatigue testing so a force accurate to a couple of percent would be good.

                Peter



                ------------------------------------

                Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
              • Kevin Ross
                Compressor duty cycles are often times specified in 10 minute intervals or in 1 hour intervals. You need to check with the specific model you are looking to
                Message 7 of 20 , May 18, 2013
                  Compressor duty cycles are often times specified in 10 minute intervals or
                  in 1 hour intervals. You need to check with the specific model you are
                  looking to use. A common value is 30% / 10 minutes, which means you can run
                  it for about 3 minutes then it needs to cool down for the next 7 minutes.
                  There are a lot of variables in this. You are also going to find compressors
                  which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
                  actually true in most cases and especially at the pressure you are going to
                  need for your device. (We have 100% duty cycle compressors for use on FIRST
                  robots, but if you attempt to run them for too long they heat up and fail.
                  You have to let them cool down, meaning they are not really 100% duty cycle!
                  It is marketing BS.)

                  I see you have some of the math below. I don't really understand the details
                  for your design so I am not able to verify your numbers at the moment, but
                  you seem to get the idea. If you are generating 2200lbs of force with a 10cm
                  stroke, my guess is that you are going to use much more air than what you
                  have stated. I base this on a gut feeling from a guy who does a lot of
                  pneumatics on FIRST robots and watches the compressors heat up very quickly.
                  However, if you do the math, you get to tell me I am wrong and I won't feel
                  bad at all.

                  Pneumatics are a full of little variances. If you need more precision, you
                  should consider hydraulic. Air acts like a spring, and is full of all sorts
                  of surprises. It is not instant power, nor is it linear. It is, however,
                  easy to work with and pretty reliable once you get it setup. The regulators
                  do a good job at not allowing the pressure to exceed a certain amount, so
                  they are usually pretty accurate at limiting the pressure.

                  Friction will be an issue in any device you make. The fewer moving parts,
                  the fewer sources of friction.

                  -----Original Message-----
                  From: Peter Balch
                  Sent: Friday, May 17, 2013 11:43 AM
                  To: SeattleRobotics@yahoogroups.com
                  Subject: Re: [SeattleRobotics] testing robot


                  > Compressors have a
                  > series of different ratings you need to consider, most important is the
                  > expected duty cycle. They will get very hot if you exceed their duty
                  > cycle.

                  I hadn't considered that.

                  At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
                  is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
                  cycle that's only 40 L/min. A small compressor you'd use to paint your fence
                  is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

                  Would that be enough?

                  > I would expect this to take a larger (60 or 80 gallon) unit to keep the
                  > thing from running every couple of minutes.

                  That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

                  > You could use the same large cylinder and have two regulators and two
                  > solenoids. One regulator might run at 10psi to generate 220lbs of force,
                  > the
                  > other would run at 100psi to generate 2200 lbs of force.

                  Interesting idea. I was assuming that the cylinder lifted a lever which
                  passively dropped to provide the force. The force would depend on what
                  weight we hung on the lever.

                  You're suggesting that the cylinder provides the force directly (or via a
                  lever) and the force is determined by the air pressure.

                  How accurately can you control the force that way? Doesn't friction and
                  stiction affect the force? And how accurate is the air pressure?

                  It would be nice if we could use the same rig for calibration as well as
                  fatigue testing so a force accurate to a couple of percent would be good.

                  Peter



                  ------------------------------------

                  Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
                • Kevin Ross
                  Compressor duty cycles are often times specified in 10 minute intervals or in 1 hour intervals. You need to check with the specific model you are looking to
                  Message 8 of 20 , May 18, 2013
                    Compressor duty cycles are often times specified in 10 minute intervals or
                    in 1 hour intervals. You need to check with the specific model you are
                    looking to use. A common value is 30% / 10 minutes, which means you can run
                    it for about 3 minutes then it needs to cool down for the next 7 minutes.
                    There are a lot of variables in this. You are also going to find compressors
                    which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
                    actually true in most cases and especially at the pressure you are going to
                    need for your device. (We have 100% duty cycle compressors for use on FIRST
                    robots, but if you attempt to run them for too long they heat up and fail.
                    You have to let them cool down, meaning they are not really 100% duty cycle!
                    It is marketing BS.)

                    I see you have some of the math below. I don't really understand the details
                    for your design so I am not able to verify your numbers at the moment, but
                    you seem to get the idea. If you are generating 2200lbs of force with a 10cm
                    stroke, my guess is that you are going to use much more air than what you
                    have stated. I base this on a gut feeling from a guy who does a lot of
                    pneumatics on FIRST robots and watches the compressors heat up very quickly.
                    However, if you do the math, you get to tell me I am wrong and I won't feel
                    bad at all.

                    Pneumatics are a full of little variances. If you need more precision, you
                    should consider hydraulic. Air acts like a spring, and is full of all sorts
                    of surprises. It is not instant power, nor is it linear. It is, however,
                    easy to work with and pretty reliable once you get it setup. The regulators
                    do a good job at not allowing the pressure to exceed a certain amount, so
                    they are usually pretty accurate at limiting the pressure.

                    Friction will be an issue in any device you make. The fewer moving parts,
                    the fewer sources of friction.

                    -----Original Message-----
                    From: Peter Balch
                    Sent: Friday, May 17, 2013 11:43 AM
                    To: SeattleRobotics@yahoogroups.com
                    Subject: Re: [SeattleRobotics] testing robot


                    > Compressors have a
                    > series of different ratings you need to consider, most important is the
                    > expected duty cycle. They will get very hot if you exceed their duty
                    > cycle.

                    I hadn't considered that.

                    At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
                    is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
                    cycle that's only 40 L/min. A small compressor you'd use to paint your fence
                    is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

                    Would that be enough?

                    > I would expect this to take a larger (60 or 80 gallon) unit to keep the
                    > thing from running every couple of minutes.

                    That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

                    > You could use the same large cylinder and have two regulators and two
                    > solenoids. One regulator might run at 10psi to generate 220lbs of force,
                    > the
                    > other would run at 100psi to generate 2200 lbs of force.

                    Interesting idea. I was assuming that the cylinder lifted a lever which
                    passively dropped to provide the force. The force would depend on what
                    weight we hung on the lever.

                    You're suggesting that the cylinder provides the force directly (or via a
                    lever) and the force is determined by the air pressure.

                    How accurately can you control the force that way? Doesn't friction and
                    stiction affect the force? And how accurate is the air pressure?

                    It would be nice if we could use the same rig for calibration as well as
                    fatigue testing so a force accurate to a couple of percent would be good.

                    Peter



                    ------------------------------------

                    Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links
                  • Peter Balch
                    Thanks Kevin. I think I m beginning to understand what we need to buy but the valves are still a mystery. If I want active retraction of the cylinder (i.e. a
                    Message 9 of 20 , May 19, 2013
                      Thanks Kevin.

                      I think I'm beginning to understand what we need to buy but the valves are
                      still a mystery. If I want active retraction of the cylinder (i.e. a
                      double-acting cylinder) then maybe I'll need a 5-port 2-Way, 3-way or 4-way
                      solenoid valve. Or maybe a 5-port air-spring valve operated by two 3-port
                      2-way solenoid valves. Or maybe just use two 3-port 2-way solenoid valves.

                      When I look at the catalogues, there's an enormous range of valves to choose
                      from with no clear way of making a decision.

                      Then I look at the photos of the valves and realise the connectors for the
                      hoses need to be the right type. Are there different ranges? Which is bect
                      for what we want to do? Presumably, it depends on pressure and airflow. And
                      so on and so on.

                      I can see what I need in theory and I can see where to buy Stuff. But
                      there's still a big gap between those two.

                      Peter
                    • mark@kenworthymachine.com
                      Rotary screw type compressors are designed to run continuously. I don t know of any type designed for continuous service. Connected by DROID on Verizon
                      Message 10 of 20 , May 19, 2013
                        Rotary screw type compressors are designed to run continuously.  I don't know of any type designed for continuous service.

                        Connected by DROID on Verizon Wireless


                        -----Original message-----
                        From: Kevin Ross <kevinro@...>
                        To:
                        SeattleRobotics@yahoogroups.com
                        Sent:
                        Sun, May 19, 2013 05:25:46 GMT+00:00
                        Subject:
                        Re: [SeattleRobotics] testing robot

                         

                        Compressor duty cycles are often times specified in 10 minute intervals or
                        in 1 hour intervals. You need to check with the specific model you are
                        looking to use. A common value is 30% / 10 minutes, which means you can run
                        it for about 3 minutes then it needs to cool down for the next 7 minutes.
                        There are a lot of variables in this. You are also going to find compressors
                        which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
                        actually true in most cases and especially at the pressure you are going to
                        need for your device. (We have 100% duty cycle compressors for use on FIRST
                        robots, but if you attempt to run them for too long they heat up and fail.
                        You have to let them cool down, meaning they are not really 100% duty cycle!
                        It is marketing BS.)

                        I see you have some of the math below. I don't really understand the details
                        for your design so I am not able to verify your numbers at the moment, but
                        you seem to get the idea. If you are generating 2200lbs of force with a 10cm
                        stroke, my guess is that you are going to use much more air than what you
                        have stated. I base this on a gut feeling from a guy who does a lot of
                        pneumatics on FIRST robots and watches the compressors heat up very quickly.
                        However, if you do the math, you get to tell me I am wrong and I won't feel
                        bad at all.

                        Pneumatics are a full of little variances. If you need more precision, you
                        should consider hydraulic. Air acts like a spring, and is full of all sorts
                        of surprises. It is not instant power, nor is it linear. It is, however,
                        easy to work with and pretty reliable once you get it setup. The regulators
                        do a good job at not allowing the pressure to exceed a certain amount, so
                        they are usually pretty accurate at limiting the pressure.

                        Friction will be an issue in any device you make. The fewer moving parts,
                        the fewer sources of friction.

                        -----Original Message-----
                        From: Peter Balch
                        Sent: Friday, May 17, 2013 11:43 AM
                        To: SeattleRobotics@yahoogroups.com
                        Subject: Re: [SeattleRobotics] testing robot

                        > Compressors have a
                        > series of different ratings you need to consider, most important is the
                        > expected duty cycle. They will get very hot if you exceed their duty
                        > cycle.

                        I hadn't considered that.

                        At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
                        is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
                        cycle that's only 40 L/min. A small compressor you'd use to paint your fence
                        is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

                        Would that be enough?

                        > I would expect this to take a larger (60 or 80 gallon) unit to keep the
                        > thing from running every couple of minutes.

                        That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

                        > You could use the same large cylinder and have two regulators and two
                        > solenoids. One regulator might run at 10psi to generate 220lbs of force,
                        > the
                        > other would run at 100psi to generate 2200 lbs of force.

                        Interesting idea. I was assuming that the cylinder lifted a lever which
                        passively dropped to provide the force. The force would depend on what
                        weight we hung on the lever.

                        You're suggesting that the cylinder provides the force directly (or via a
                        lever) and the force is determined by the air pressure.

                        How accurately can you control the force that way? Doesn't friction and
                        stiction affect the force? And how accurate is the air pressure?

                        It would be nice if we could use the same rig for calibration as well as
                        fatigue testing so a force accurate to a couple of percent would be good.

                        Peter

                        ------------------------------------

                        Visit the SRS Website at http://www.seattlerobotics.orgYahoo! Groups Links

                      • Mark Kenworthy
                        That should be “any other type that is rated for continuous service”. Our rotary screw compressor has two operational modes. One where it just runs
                        Message 11 of 20 , May 19, 2013

                          That should be “any other type that is rated for continuous service”.  Our rotary screw compressor has two operational modes.  One where it just runs continuous and bleeds off excess air, and another mode where it cycles off when reaching the preset pressure level.

                           

                          They are fairly expensive, as compressors go, so you might want to try to find a business that has one and work out a deal to do your testing there.

                           

                          Mark

                           

                          From: SeattleRobotics@yahoogroups.com [mailto:SeattleRobotics@yahoogroups.com] On Behalf Of mark@...
                          Sent: Sunday, May 19, 2013 8:13 AM
                          To: SeattleRobotics@yahoogroups.com
                          Subject: Re: [SeattleRobotics] testing robot

                           

                           

                          Rotary screw type compressors are designed to run continuously.  I don't know of any type designed for continuous service.

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                          -----Original message-----

                          From: Kevin Ross <kevinro@...>
                          To:
                          SeattleRobotics@yahoogroups.com
                          Sent:
                          Sun, May 19, 2013 05:25:46 GMT+00:00
                          Subject:
                          Re: [SeattleRobotics] testing robot

                           

                          Compressor duty cycles are often times specified in 10 minute intervals or
                          in 1 hour intervals. You need to check with the specific model you are
                          looking to use. A common value is 30% / 10 minutes, which means you can run
                          it for about 3 minutes then it needs to cool down for the next 7 minutes.
                          There are a lot of variables in this. You are also going to find compressors
                          which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
                          actually true in most cases and especially at the pressure you are going to
                          need for your device. (We have 100% duty cycle compressors for use on FIRST
                          robots, but if you attempt to run them for too long they heat up and fail.
                          You have to let them cool down, meaning they are not really 100% duty cycle!
                          It is marketing BS.)

                          I see you have some of the math below. I don't really understand the details
                          for your design so I am not able to verify your numbers at the moment, but
                          you seem to get the idea. If you are generating 2200lbs of force with a 10cm
                          stroke, my guess is that you are going to use much more air than what you
                          have stated. I base this on a gut feeling from a guy who does a lot of
                          pneumatics on FIRST robots and watches the compressors heat up very quickly.
                          However, if you do the math, you get to tell me I am wrong and I won't feel
                          bad at all.

                          Pneumatics are a full of little variances. If you need more precision, you
                          should consider hydraulic. Air acts like a spring, and is full of all sorts
                          of surprises. It is not instant power, nor is it linear. It is, however,
                          easy to work with and pretty reliable once you get it setup. The regulators
                          do a good job at not allowing the pressure to exceed a certain amount, so
                          they are usually pretty accurate at limiting the pressure.

                          Friction will be an issue in any device you make. The fewer moving parts,
                          the fewer sources of friction.

                          -----Original Message-----
                          From: Peter Balch
                          Sent: Friday, May 17, 2013 11:43 AM
                          To: SeattleRobotics@yahoogroups.com
                          Subject: Re: [SeattleRobotics] testing robot

                          > Compressors have a
                          > series of different ratings you need to consider, most important is the
                          > expected duty cycle. They will get very hot if you exceed their duty
                          > cycle.

                          I hadn't considered that.

                          At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
                          is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
                          cycle that's only 40 L/min. A small compressor you'd use to paint your fence
                          is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.

                          Would that be enough?

                          > I would expect this to take a larger (60 or 80 gallon) unit to keep the
                          > thing from running every couple of minutes.

                          That's 200 to 200 L. Seems quite large if I'm using 4 L/min.

                          > You could use the same large cylinder and have two regulators and two
                          > solenoids. One regulator might run at 10psi to generate 220lbs of force,
                          > the
                          > other would run at 100psi to generate 2200 lbs of force.

                          Interesting idea. I was assuming that the cylinder lifted a lever which
                          passively dropped to provide the force. The force would depend on what
                          weight we hung on the lever.

                          You're suggesting that the cylinder provides the force directly (or via a
                          lever) and the force is determined by the air pressure.

                          How accurately can you control the force that way? Doesn't friction and
                          stiction affect the force? And how accurate is the air pressure?

                          It would be nice if we could use the same rig for calibration as well as
                          fatigue testing so a force accurate to a couple of percent would be good.

                          Peter

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