Moving horse legs

Welcome! This sounds like a fun project! There are a couple of ways you could get your desired effect. You could use a sensor to trigger a servo to move the leg when your leg moves. I did something similar when I made the Iron Man suit- I had a sensor in the boot that triggered a walking sound as soon as the boot was lifted from the floor. The sensor reads when your foot is lifted and sends a signal to a microcontroller (an Arduino works great) that then tells the leg servo to move.

The other method is to track your leg movement using something similar to what is known as a Waldo. A Waldo is a device that translates your body motions into a signal so something can mimic your movements. In this case what you do is have a potentiometer mounted at your hip (using something like a belt with a bracket) and then the potentiometer shaft is connected to a small piece of say aluminum tubing that is strapped to the side of your leg. A potentiometer is like a stereo volume knob- as your leg moves the knob rotates. An Arduino then reads this signal and tells a servo to follow your leg movement.

There are a couple of catches here. First there will always be a very slight delay from the time the sensor reads your leg movement to when it tells the servo to move. It's small but it shouldn't be too noticeable. Second (and this is a bigger problem) is that servos don't change their rate of acceleration- they only move at a fixed acceleration. You can change the rate of acceleration in some digital servos or sometimes in programming. In the case of having a sensor on your foot, the sensor only tells the servo when to begin moving through a programmed motion- it cannot change movements on the fly. So it's fine for say walking forward at a fixed rate but it will not directly mimic you leg movement.

When you read your leg position using a potentiometer the servo will follow that movement forward and back. It's like using a radio control transmitter- move the stick and the servo movement matches the stick movement. This is probably the way I'd go as it's much more versatile. The trick here is that the servo driven leg cannot exactly match your leg movement because you can swing your leg really fast or really slow and change direction very quickly- this range of movement (speed and acceleration) is typically outside of what a servo can do. Having said that I think with the right servo motors and practicing your movements to stay within a given speed range I think you would be able to pull off something pretty convincing.

The type of servo you use will depend greatly on the weight of the costume (specifically the legs) and the flexibility of the skin.

A less expensive way of making your costume work would be to have the rear legs directly connected to the costume front legs so they mimic the movement. Have you ever seen how some Bunraku puppets work? It would be very similar to that.

If any of this doesn't make sense or you need more help just let me know!

Yep I've used bend sensors a fair bit in the past and they tend to be pretty "noisy" and they need to bend a fair bit in order to work properly. They work OK in gloves but they're not too good for things like elbow and knee movement tracking- in those instances they work better as a sort of trigger sensor rather than a full range sensor. Potentiometers are much better for tracking small angular movements and rotations.

An accelerometer could also work but it may be more difficult to implement in actual use than a pot as accelerometers only work with respect to gravity- they only sense rate of change in movement. You would need to measure the +/- acceleration force of your forward/backward leg movement and then translate that into a specific range of motion. I've done some stuff before using a hacked Wii Nunchuck and tracked the motion from that to make a servo follow it and it certainly does work. There was a bit more latency than I would have liked but it did work. If you wanted to go that route you might be better off using an IMU (inertial measurement unit) as it combines an accelerometer and gyro. The gyro measures change of rotation- something an accelerometer cannot do. The higher end IMUs with on board processing combine the readings of the accelerometer and gyro and filter them in order to give you something useful to work with for motion tracking. That stuff gets complicated very quickly and most of it is honestly above my coding ability at the moment- LOL!

I think you're bang on the money with how to make the legs. For a wood leg I'd use a 1/8" plywood/foam core sandwich. You could use that thick blue or pink foam sheet they sell at Lowes or Home Depot. If you wanted to use aluminum you wouldn't need to weld it- you could bolt it all together using light tubing and Actobotics parts from Servocity.com and it would bolt right up to a geared servo motor.

Linear actuators are great for moving large loads but they tend to be pretty slow unless you go hydraulic/pneumatic. I'd use a good high torque geared servo drive like this one- www.servocity.com/html/spg7950a_standard_rotation.html

With a 7:1 gear ratio that servo is rated at 3402 oz-in and it takes .9 seconds to move 60 degrees (not exactly fast but not too bad- you won't be able to run across a stage though!) To figure out if that will work take your hip to floor measurement (horse leg measurement) in inches and divide that rating by the length to get your max torque rating. For instance, my hip to floor is roughly 38" so 3402/38 = 89.52 oz-in. That means that servo will max out trying to lift a fake horse leg that weighs 89 ounces (or roughly 5.5 pounds.) Ideally you never want to push a servo near its stall torque rating so I'd probably try to keep the leg weight down to around 3.5 pounds. It won't be easy but I think it's totally doable.

Happy to help! If you need drawings/schematics or help with coding let me know and I'll do my best.

Cardboard would also be a decent substitute for foam in the legs- just cut some 1" (25mm) wide strips and glue it on edge. You can make a pretty light/strong torsion box structure with that. I'd also make the leg bend at the knee. Use a radius rod/arm to pull the lower leg up as the entire leg rotates forward. That way it'll look a bit more realistic.

I also thought of using geared stepper motors or windshield wiper motors to move the legs. It wouldn't be as elegant as you would need to use an encoder to read the leg position and the power requirements would be greater but it may be a less expensive solution.

I could make you a kinematic model and maybe do some sketches. Would that help?

I'll get on it asap!

Wow, this is better than I'd even dreamed to hope for! I will try to look at the options with my boyfriend and see what he thinks we would be able to do.
Thank you sooo much for taking your time to look at this! And I'll look into the app, although I don't have any devices to run it on myself, but there's surely a way

Hi again,

I've been thinking hard, but I'm having a hard time deciding... Given my low skill level, I am leaning towards option 2, although I understand it's limitations.
Below link is a model of the character I'm aiming at - in the lower right corner of the pop-up window with the model¨, there's an option to choose animations, one of which is walking.
www.wowhead.com/npc=39930/mylune#modelviewer:10+0

To me, the hind legs actually don't seem to move that much at the lower joint, mostly at the hip joint. Could this movement be imitated with option 2?