Preloaded Hydraulic Muscle

Universal , scalable hydraulic muscle module.

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A new way to build an actuator for use in robots , exoskeletons and prosthetics . Built using easy to get components and easily scalable . The muscle module will use silicone bladders encased in aramid fiber sleeves , a small gear pump and a dedicated controller and will be able to be used as easy as a typical RC servo . By using a simple trick , the whole hydraulic control system is limited only to the bidirectional gear pump , no fluid tanks or valves are needed.

he basic idea behind the Preloaded Hydraulic Muscle is to use two inflatable bladders and keep them partially inflated in the idle state ( t. A gear pump transfers the fluid from one bladder to the second one moving the actuator.

Each of the bladders is inserted into a sleeve made from the aramid fiber , which transforms the swelling of the bladder into linear motion of the actuator.  Since the amount of hydraulic fluid in the both bladders is constant , there is no possibility to rupture the bladder with excessive pressure as long as the muscle system was filled correctly initially.

The module consists of  four parts :
    - two actuators ( one for bending , one for straightening motion )
    - miniature gear pump with a BLDC motor
    - BLDC motor controller
    - logic board with microcontroller , responsible for interfacing with control circuits , servo mode operation , PID , monitoring and communication .

The two videos below show the operation of the prototype , still without the logic board.

In the next posts I will present the whole design of this actuator


BLDC motor driver GERBER files for PCB production

x-gtar-compressed - 33.37 kB - 05/27/2018 at 16:37



Schematic of the BLDC motor driver

Adobe Portable Document Format - 20.97 kB - 05/27/2018 at 16:37



technical drawing of the motor to pump adapter

Adobe Portable Document Format - 59.60 kB - 05/27/2018 at 16:36



technical drawing of the actuator mounting element

Adobe Portable Document Format - 24.81 kB - 05/27/2018 at 16:36



technical drawing of the actuator top block

Adobe Portable Document Format - 60.23 kB - 05/27/2018 at 16:35


View all 7 files

  • Test with asymmetric load

    consciousflesh08/27/2018 at 18:39 0 comments

    One more test . This time with a 15kg asymmetric load.

  • BLDC motor controller and filling the system

    consciousflesh05/27/2018 at 19:54 0 comments

    The hydraulic system of the muscle module is powered with a 540 type , brushless motor. This type of the motor provides full control of torque and speed but require a dedicated controller.

    The controller was build  using MC33035 Brushless DC Motor Controller IC. The outputs of the chip are used to drive the high current MOSFET's  with the help of the MIC4427 driver IC's. The current control loop is realized with the feedback resistor etched directly on the PCB.  The complete schematic of the controller , as well as the GERBER files of the PCB can be found in the files section.

    Filling the system , and bleeding the air of it is done with the tank installed above the muscles and pump and two service connectors for the schrader valves ( the type used for air conditioning repair ) .

    The filling procedure:

         - connect the service valves to the schrader valves

         - open  the valves and let the gravity fill the system with fluid

         - start the pump and let it bleed the air.

         - close the outlet valve and let the pump fill the muscle to its maximal capacity

         - close the inlet valve and turn off the pump .

         - disconnect the service valves and secure the schrader valves with caps.

    This way both muscles are filled and have a positive pressure in the idle state.

  • The gear pump

    consciousflesh05/27/2018 at 18:55 0 comments

    The second , most important part of the muscle module is the bidirectional , miniature gear pump . After a lengthy search I found a pump which is almost perfect for this project - a miniature gear pump made by MAGOM HRC company , and intended to be used in RC construction machinery models.

    The pump needs to be modified a bit :
       - first remove all the hoses from the pump and unscrew the hose connectors
       - remove the oil tank
       - dismantle the pump
       - remove the screw holding the pressure regulator , and dismantle it
       - drill the pressure regulator hole with 4mm drill , and tap it with M5 tap
       - thread the inlet hole in the pump cover with the M5 tap.
       - plug the hose connector holes and oil tank mounting hole with headless screws
       - install the 4mm push-in connector in the newly threaded M5 holes
       - reassemble the pump

    The modified pump should look like this :

    The last step is to install the BLDC motor. If the motor does not fit the pump it may be required to build an adapter and modify the original mount ( as it was it my case  ) .

  • Building the actuator

    consciousflesh05/27/2018 at 17:41 0 comments

    The main part of the actuator is a silicone bladder made from 8 mm silicone hose. The hose is then encased in the aramid braided sleeve ( aramid cable sleeve )  and screwed into hose connectors. The length of the hose should be chosen to match the application , remembering that the shortening of the actuator is around 30 %.

    Here is the cross section of the single "muscle" 

    The bladders are installed between two aluminum blocks acting as the mounting elements and the conduit for the hydraulic fluid . The technical drawings of the blocks can be found in the files section of the project.  If one needs more force exerted by the actuator the blocks can be extended and equipped with more "muscle bladders" connected in parallel .

    As you can see , there are two types of mounting blocks . One is designed to be fitted with a 4mm push-in connector , the other should be fitted with a Schrader Valve . The 4mm connectors would be connected to the pump , while the Schrader Valve is needed for filling the system and bleeding the air out of it.

    When all the parts are ready , all the connectors should be installed into the blocks first and sealed along the thread using either a dedicated sealant or a bit of epoxy . Then the hoses with the sleeves should be screwed into the connectors.  The finished actuator should look like this : 

View all 4 project logs

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Starhawk wrote 08/30/2018 at 20:01 point

I know I'm the only one commenting here -- but -- have you considered (a) using something a little less rigid than silicone tubing, and (b) using the muscles in opposing pairs, the way nature does it?

  Are you sure? yes | no

Starhawk wrote 04/02/2019 at 22:43 point

Speaking of this. There is a kind of tubing used in the medical field called "Corr-A-Flex" (that's the brand). It's the blue accordion airflow tube stuff, and it's made of EVA. It occurs to me that using it with some liquid (glycerol and mineral oil come to mind, if yhey won't react with it) and a small but torquey gear pump would do wonders with this project.

I have some Corr-A-Flex on hand -- I may do some experimenting with it -- let me know if you DON'T want me to do that...

  Are you sure? yes | no

Starhawk wrote 08/21/2018 at 22:04 point

Sent your project up the tip line. You should submit it there as well -- this deserves attention!

  Are you sure? yes | no

Starhawk wrote 08/20/2018 at 21:17 point

This project is cooler than liquid nitrogen, and entirely too clever by half. I love it already :)

  Are you sure? yes | no

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