robotic air muscle actuator (biorobotics)

Biorobotics - Build Your Own Robotic Air Muscle Actuator

Anair muscle is a simple pneumatic device developed in the 1950's by J.L.McKibben. Like biological muscles, air muscles contract when activated.Robotists find it interesting that air muscles provide a reasonableworking copy of biological muscles. So much so that researchers can usea human skeleton with air muscles attached to the skeleton at primarybiological muscle locations to study biomechanics and low level neuralproperties of biological muscles. (See Internet sources)


Thisfeature is utilized by the research in many "Bio-Robotics" projectsinstitute by numerous researchers. In published papers air muscles arealso referred to as; McKibben Air Muscles, McKibben PneumaticArtificial Muscle, Rubbertuator and as I refer to them simply as airmuscle(s).

Applications

Air muscles haveapplications in robotics, biorobotics, biomechanics, artificial limbreplacement and industry. The principle reasons experimenters andhobbyists will like air muscles are ease of use (as compared tostandard pneumatic cylinders) and simple construction. Air muscles aresoft, lightweight and compliant, have a high power to weight ratio(400:1), can be twisted axially and used on unaligned mounting andprovide contractive force around bends, (see robot wars near the end ofthis article). Air muscles may also be used underwater.

How Air Muscles Work

TThere are two primary componentsto the air muscle are a soft stretchable inner rubber tube and abraided polyester mesh sleeve, see Figure 1. The rubber tube is calledan internal bladder and is positioned inside the braided mesh sleeve.


Figure 1Allthat is left to complete the air muscle picture is an air fitting onone end and two mechanical fittings (loops) on each end of the airmuscle that allow one to attach the air muscle to devices. The clampsin Figure 1 are made from 24-gauge wire tightly wrapped and twistedaround the ends of the air muscle.
When the internalbladder is pressurized it expands and pushes against the inside ofbraided mesh sleeve, forcing the diameter of the braided mesh toexpand. The physical characteristic of the mesh sleeve is that itcontracts in proportion to the degree its diameter is forced toincrease. This produces the contractive force of the air muscle.
Tooperate properly, it is important that the air muscle be in a stretchedor loaded position when it's inactive or in a resting state. If notthere will little if any contraction when activated. So the air musclemust be stretched in order for it to produce contraction when it isactivated, see Figure 2. Typically the air muscle can contract toapproximately 25 percent of its length.
The illustrationof the contracted air muscle in figure 2 is greatly exaggerated. Whenthe air muscle contracts, its diameter thickens equally along it'slength and contracts (shortens) as described. Air muscles typically donot develop a large bulge in the center when it is contracted, howeverfor purposes of illustration we will shown it this way.




Figure 2

Air Pressure

Airmuscles require a source of compressed gas (usually air). The airmuscle we will build operates at approximately 50 psi. Air pressure canbe generated by the easiest mean available to the experimenterincluding a small bicycle pump with an air pressure gauge. Aninexpensive automobile tire air pump that operates using 12 VDC. Othersources are a small air tank that can be filled up at a local gasstation that has an air pump for inflating automobile tires. If you usean air tank make sure it is equipped with an adjustable air pressureregulator, this will prevent pressurizing the air muscle with too muchair.

Making an Air Muscle

Air muscles are availablecommercially from Shadow Robotics in the U.K. in a variety of sizes.For experimental purposes its pretty easy to make your own air musclein whatever size you require.
The inner tube is made from soft silicone tubing, approximately ¼"OD and 1/8" ID, see suppliers index. Go to a local pet shop that sellsaquarium supplies. Purchase a small quantity of PVC clear tubing. (Samesize as the silicone tubing, but less flexible and tougher) Pick up afew aquarium air valves and couplings too.
Many electronic distributors sell the polyester braided mesh sleeve.The braided sleeve is used as a flexible conduit for electrical wiring.Purchase a small quantity (6 feet) of 3/8" diameter.
Finish the materials purchase with a few 3/8" long 10-24 screws anda small quantity of 24 gauge galvanized wire available at a localhardware store.
Cut a 4-inch length of silicon tubing. Insert the 10-24 screw in oneend of the tube. Insert an aquarium air coupling in the other end ofthe tube, see Figure 3.


Figure 3 Cut a 7 inch length of 3/8" braided mesh sleeve. To prevent the ends ofthe sleeve from fraying and coming apart we singe the ends with a matchor candle flame, see Figure 4. The idea here is to just singe the endsof the polyester sleeve, its easy to go too far and melt too much ofthe sleeve. In that case cut another piece and start over.

Figure 4 Insertthe rubber tube inside the braided sleeve. Align one end of the sleevewith the bottom of the head on the 10-24 screw in the rubber tube. Wrapa piece of 24-gauge wire three or four times around the end, capturingthe sleeve, tubing and threaded portion of the 10-24 screw. Then twistthe ends of the wire together. Use a pair of pliers to make this astight as possible. Cut of any excess wire. See Figure 5.

Figure 5 Tofinish the other side, push down the sleeve until it is aligned withthe rubber tube on the air coupling. Wrap a piece of 24-gauge wirearound this end, tighten wire with pliers then cut off any excess wire.See Figure 6. At this point you may want to pressurize the air muscleto insure the two fittings do not leak. Since the air muscle is notloaded only use a pressure of 20 psi. If any air leaks, try tighteningthe 24-gauge wire.

Figure 6 Cuttwo 14-inch lengths of the galvanized wire. These we will use to makethe mechanical loops. Fold the wire in half to double. Form a 1-inchloop from the middle of the wire and twist the wire at the bottom ofthe loop, see figure 7. Next wire wrap the loop to the end of the airmuscle as shown in figure 8. Do the same to the other side. Pull on theloops to insure that they are secure.

Figure 7 Figure 8

Testing the Air Muscle

Thefirst test to perform is a simple static test. Wear eye protection whenpressurizing the air muscle. Attach one end of the air muscle to astationary object using the loop. At the other end hang about 5-6 lbs.of material to the air muscle using the other loop. This weight willload the air muscle (cause the air muscle to stretch). Pressurize theair muscle with approximately 50-psi. The air muscle should contractand easily lift the weight. While pressurized, listen for any airleakage. Repair any air leak, by tightening the 24-gauge wire.

First Mechanical Device

This first device illustrations the function and measures thecontract obtained with the air muscle, see figure 9. The air muscle ismounted to a piece of 1" x 2" lumber approximately 16" long. One end ofthe air muscle loop is looped over a wood screw secured into the wood.A thick rubber band is looped through the air muscle loop on the otherend. The rubber band is pulled until the air muscle is just fullyextended. Do not extend or pull the rubber band any further as thiswill just added additional resistance to the air muscle and notcontribute to its function. A woodscrew is secured into the wood atthis point and the rubber band is looped over the screw. When youpressurize the air muscle you can measure its contraction. Release theair pressure from the muscle and it should extend into its relaxedposition.




Figure 9

Second Mechanical Device

Alever is a simple mechanical device. A mechanical drawing is shown infigure 10 and photograph of a lever is shown in figure 11. Activatingthe air muscle causes the lever to rise. In the lever we are using anumber of rubber bands to load the air muscle.

Figure 10

Figure 11

Figure 12 illustrates how two air muscles may be configured to provide a counter force or load for each other.


Figure 12

Controlling the Air Muscle

Typically 3-way air valves are used to control the air muscle see figure 13.
Toactivate the air muscle, valve labeled #2 is opened. This causes theair muscle to contract. Once activated the #2 valve may be closedwithout impacting on the state of the air muscle. The air muscle staysin a contracted state. To relax the air muscle, the air pressure mustbe vented by opening valve labeled #1.

Figure 13

Electronic Control System

Athree-way stopcock air valve may be purchased, see figure 14. Thestopcock air valve allows you to control the air muscle manually. Athree-way valve may be simulated or constructed using two one-way airvalves. Small plastic aquarium valves purchased at a local pet shophave work quite well. However they are not rated for work at 50 psi andthat's a good reason to wear eye protection, just in case they popapart from the air pressure.

Figure 14

Usingmanual valves are fine for testing air muscles, to build a robot orindustrial device we must provide a way for electronic control.Fortunately this is not difficult. There are a number of solenoid airvalves available. I prefer the Isonic valves from Mead Fluid Dynamics.These are 3-way air valves that are activated using 5 Volts see Figure14.
The 3-way Isonic values automatically vent the airmuscle (through the back) when it is deactivated. To see how thisparticular valve works look back to figure 13, in the Isonic valve, thevalve labeled #1 is normally open, this is the vent, through the backof the valve. The valve #2 is normally closed, this is labeled "In" onfigure 14. The air stream to the air muscle is labeled "Out" in figure14. When the electronic valve is activated valve # 2 opens allowing airpressure to the air muscle and valve #1 closes. When deactivated eachvalve changes state; valve #1 opens, venting the air muscles airpressure and valve #2 closes separating the air supply from the airmuscle.
The front of the Isonic air valve has two quickconnects-disconnects for air tubing. This particular air valve connectshandles semi-ridge 4mm tubing, which is a good size for running air tothe air muscle. To use the quick connect simply push the air tubinginto the hole, it will lock in. To disconnect, hold the valve flange tothe valve body with your fingers and pull the tubing out.
MulitpleIsonic air valves may also be mounted onto a common manifold that makemaking multiple air muscle control that much easier.
Figure 15 is a simple manual control schematic for a 5-volt control circuit for the Isonic valve. Figure 15

Going Further

Using pressurized air is aninconvenience, but one that may be overcome. For instance at CaseWestern Reserve University a team of faculty and students are buildinga cricket micro-robot that utilizes air muscles for walking andjumping. The micro-robot will walk and jump just like its biologicalcounterpart. What makes this project so interesting is that themicro-robot is no larger than 5 cm (2 inches) in any dimension. Topower the air muscles the team made a micro-pump that supplies 35 psifor the muscles.

Robot Wars

Many ofthe robots used in the Robot Wars series use pneumatic devices andflippers to over turn competitive robots. It is quite conceivable tosubstitute air muscles for pneumatic cylinders in these robots andthereby improve the strength to weight ration. In addition alignmentfor these flexible pneumatic devices are very forgiving. Therebyallowing the builder an easier construction since the robot may bebuilt with far more open tolerances.
Internet WEB Sites to visit for more information on air muscles are:
Cricket Micro-robot
http://biorobots.cwru.edu/projects/c_mrobot/c_mrobot.htm
Construction of McKibben Artificial Muscles:
http://brl.ee.washington.edu/Research_Past/Biologically_Based/Device_01_McKibben/Mckibben_Costruction.html
Powered Prosthetics Project:
http://brl.ee.washington.edu/Research_Past/Biologically_Based/Project_02_Prosthetics/Powered_Prosthetics.html
Pneumatic Robot Home Page:
http://www.ks.uiuc.edu/Research/Neural/robot.html
Anthroform Arm Project:
http://brl.ee.washington.edu/Research_Past/Biologically_Based/Project_01_Arm/Anthroform_Arm.html

Suppliers Index:

Soft Silicon tubing PN# 75-300-016

Barnant Company
Dept 77-3397
Chicago, IL 60678-3397
847-381-7050 Telephone

Local Suppliers:

polyester braided mesh, air tubing, air valves, air couplings, 24 Gauge galvanized wire

Manual Electric Control:

5VDC Mead Isonic three way solenoid air valve
10K ¼ watt resistor
220 ohm ¼ watt resistor
Push Button Switch (Normally Open)

Commercial air muscles available from:

Images Company U.S. 718-966.3694
Shadow Robot Group U.K. 44 207700 2487