Society of Robots - Robot Forum
Mechanics and Construction => Mechanics and Construction => Topic started by: ErikY on January 20, 2013, 07:16:52 AM
-
Would most people consider servos to be the best overall option for actuators for a Biped?
I have been reading a lot on this, and while very expensive, it seems like you can get the best combo of speed, power and control using servos.
Others I have been looking into are:
Linear Actuators
DC Motors with a built in feedback system, either potentiometer, encoder, other.
Pneumatic actuators
I would be curious to hear if anyone feels I am missing any other options, and what anyones opinions are on the subject.
One thought I have is for different types of joints, and different types of applications, perhaps different actuators may be best suited for the job.
-
DC Motors with a built in feedback system, either potentiometer, encoder, other.
What do you think a servo is? ;-)
There are only so many ways of developing force that can be well controlled. Rotary motors, linear motors/solenoids, pneumatics, heat contraction wire, hydraulics, and direct electromagnetics are the main ones I can think of. Combustion engines are not great, because there's no good/immediate/robust way to directly control the amount of torque/force per-joint. (Slip couplings wear out, throttle response is sluggish, etc.) I guess you could use them for driving the compression in a pneumatic or hydraulic system...
Given those constraints, DC motor based servos are the best trade-off. Even so, there's a big difference between a $5 Chinese analog hobby servo, and a $500 robot servo like a Dynamixel EX-106. The robot specific servos (Dynamixel, Herculex, some specific HiTecs) are much better, having idlers on the back, digital feedback systems, etc.
-
DC Motors with a built in feedback system, either potentiometer, encoder, other.
What do you think a servo is? ;-)
There are only so many ways of developing force that can be well controlled. Rotary motors, linear motors/solenoids, pneumatics, heat contraction wire, hydraulics, and direct electromagnetics are the main ones I can think of. Combustion engines are not great, because there's no good/immediate/robust way to directly control the amount of torque/force per-joint. (Slip couplings wear out, throttle response is sluggish, etc.) I guess you could use them for driving the compression in a pneumatic or hydraulic system...
Given those constraints, DC motor based servos are the best trade-off. Even so, there's a big difference between a $5 Chinese analog hobby servo, and a $500 robot servo like a Dynamixel EX-106. The robot specific servos (Dynamixel, Herculex, some specific HiTecs) are much better, having idlers on the back, digital feedback systems, etc.
I understand about the servos being DC motors :)
I probably should have put more verbiage into that one. I have seen some people do some really cool things with DC motors, making some very powerful joints at a fraction of the cost as a powerful servo. I was really factoring cost into the equation as well, as the high end servos as you mention are extremely expensive, ecspecially if you want to get any significant DOF in your Biped.
I also was throwing stepper motors into the DC motors category, which was probably a bad idea since they do behave quite differently.
-
I have various dynamixel servos in the passed and have had very good experiences with them just remember that they use half-duplexed communication instead of the traditional PWM signals. This way you can get feedback from them.
~Idan
-
I have various dynamixel servos in the passed and have had very good experiences with them just remember that they use half-duplexed communication instead of the traditional PWM signals. This way you can get feedback from them.
~Idan
I have some AX-12A Dynamixels.
The functionality is very cool, no doubt.
They are very expensive however, and the cost of using them adds up quickly, but that really goes for any quality actuator I suppose.
-
Unless you're building a human-sized biped (>4 ft high, or so), you're going to use servos.
Are you planning a humanoid biped or just the legs?
-
Unless you're building a human-sized biped (>4 ft high, or so), you're going to use servos.
Are you planning a humanoid biped or just the legs?
Eventually I would like to build a humanoid, right now I am still in the learning phase.
Out of curiosity, if you were to go over 4 foot, what would you use in that case?
-
I don't know what I'd do, personally. I based my statement on the fact that that's about the height at which I start seeing non-servo walkers (e.g. PETMAN).
-
Gertlex, Thanks for the reply.
PETMAN is very interesting, I did not even know BD was doing a humanoid like that, let alone PETMAN and Atlas, I had seen Big Dog many times, but not that.
It looks to me like they are using hydraulics as actuators, does anyone know more about the type of hydraulics they are using?
I have been looking into air muscle, and I just cannot find any realistic options for air pressure, switching valves to actuate different muscles, etc.