Just wondering has anybody has built a robotic arm using linear actuators like below? Something similar to the arms on excavators, but using the actuators instead of the hydraulic cylinders?
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well, this is an idea that i almost made a reality, but i was going to go with likkle teeny tiny actuators, it only wouldn’ve been 50 mms high 
dont know what i woulda done with it, but it would been cool 8)
It would be interesting to make an arm with an 8 foot reach just to see how easily it could be accomplished. The commercial actuators would be the expensive part, but maybe DIY ones could be made from threaded rod and cheap harbor freight drills.
yea, have fun strengthning that arm though, be hard to dude while keeping its light, evern with aluminum
At scales like that, it would probably be easiest, rather than trying to mimic an excavator’s arm with electrical actuators, to just build an interface that provides smooth control of an actual excavator arm, and provides feedback for the control electronics. Then it would just be a matter of replacing (or augmenting) the manually-controlled hydraulic valve stacks with a set of electrically-actuated stacks, and letting the computer treat the hydraulic rams as servos.
Giant servos.
That can kill you. 
LOL
Just as an experiment, I supported 14 oz of bananas on the end of a 36 in walmart yard stick which only weighs 3 oz. I used 2 in to hold the stick, so that is supporting 14 oz 34 in out with out significant issues on a 3 oz arm. My interest in this is using servos as controllers and positioning devices for the larger and stronger actuators and gear head motors. This would allow a computer and a servo controller to operate the arm much as they would control a purely servo operated arm. The commercial actuators are way too expensive for tinkering, so I need to think of a cheaper way to operate the arm.
Worm gear motors with a large sprocket would allow good speed and torque. One coud mount a pot to the joint to read the position and run the motor off an H-bridge. Then there’s just the controller to figure out.
In the setup I’m thinking about, a servo is placed at each arm joint to monitor the joint position and to control the geared motors via relays when the joint is not at the desired position. To make the servo the controller for the relays, the addition of a resistor and two diodes might be all that is needed.
When I was involved in Cal Poly’s Rose Float effort (hydraulics chair 1994), I worked a bit 
on computer-controlled hydraulic mechanisms. Our floats were animated using a computer driving a series of analog hydraulic valve stacks, to provide variable flow rates to the assorted cylinders and actuators. For the most part, these were just variable bi-directional valves which the computer could command to open from 0 to 100% in either direction.
Feedback was provided by placing what were essentially just bigger versions of feedback pots on every controlled axis, adding redundant sensors on the really important ones (when you’re controlling a steel structure the size of a house - and there are people riding inside - you want a bit of insurance). These feedback units were generally designed to work in a linear fashion, and I had a chance to dissect a few of them. For the most part, the transducers we used consisted of a large clockspring, which wound a small spool containing several turns of thin stainless steel aircraft cable. The axle of the spool was the shaft of a 10-turn potentiometer, so that as the wire was pulled out of the box against spring tension, it spun the pot’s shaft, and returned a value that varied in proportion to how much cable had been paid out. As tension was released, the spring recoiled, sucking the wire back in, and the pot’s value changed in the other direction.
If you want to turn a mechanism actuated by a hydraulic ram into a giant servo - that can kill you - you give the computer some way to control the flow of fluid to the ram (the analog stack), and you fasten one end of the transducer to the base of the ram or chassis, and the end of its steel cable to the end of its shaft, or a moving part of the mechanism. Adjust the bits of the program that control deadband, acceleration curves, and endpoints (endpoints are important in mechanisms that can tear their own welds apart), and you’ve got yourself a giant servo.
Lather, rinse, repeat as necessary. Give it a fresh tank of propane, cover it in flowers, and pretty soon you’ve got a float in an internationally televised parade.
Your mileage may vary.
I have built a robot arm for playing chess that uses a linear actuator for the shoulder and worm gearing for the elbow.
