Hot answers tagged

11

Back in the day, when I was learning, making this up as I went along, I used simple gradient following to solve the IK problem. In your model, you try rotating each joint each joint a tiny amount, see how much difference that makes to the end point position error. Having done that, you then rotate each joint by an amount proportional to the benefit it gives....


9

It's called compliance. Gravity compensation by itself is not enough to achieve this, as well it is not mandatory. For example, if reducers with high reduction ratios are used, robot arm will be very stiff to move around. One way to make robotic arm compliant is to have torque sensors that can measure the differences in expected load (i.e. weight of the arm)...


5

If you have no possibility to detect the obstacle apriori (e.g. with cameras, vicinity sensors...) If you already hit an obstacle and your position error increases, you can only detect the problem by the increasing position error or indirect by the increasing motor current. Most of the motion controllers, I have seen, have tons of configuration parameters, ...


5

The torque bandwidth is typically referring to the maximum frequency of motion at which the actuator can provide that torque. So your actuator can provide a peak torque of 100 Nm, as in it can hold up a weight of 100 N held at a torque arm of 1 m. If you want to swing that weight back and forth you could do it at up to 4 Hz, but no faster without damaging or ...


3

In 1981 Raibert and Craig wrote a paper Hybrid Position/Force Control of Manipulators which was published in the June 1981 issue of Journal of Dynamic Systems, Measurement, and Control. It was republished in Brady's book Robot Motion: Planning and Control. You can find many similar concepts today, some being called "position and torque control," and ...


3

Actuators Forces Do I get this right: you have a theoretical model of a rigid multibody system and would like to perform rigid body dynamics computations. You have implemented the model and now would like to compute how the model behaves when driven by an actuator. However what is an actuator for you? Is it simply a force acting at that joint? Is it a DC ...


3

This looks very much like a simplification of a traditional SCARA robot design. It is a nice simple design in which the weight bearing axes are all nicely horizontal, which means these axes behave similarly irrespective of the load weight. The only downside of this design is that the some positions can only be accessed from a left handed configuration, some ...


3

There a number of solutions to this problem that center around the Jacobian Matrix. This slideshow covers the Jacobian methods and also mentions a Cyclic Coordinate Descent method, which I am unfamiliar with. There are a plethora of resources on the subject - if you ask google for "inverse kinematics Jacobian". Also, check out Chapter 5.3 of the lecture ...


3

I'm not sure what the "standard techniques" are but here are a few a ideas. You could combine a standard jacobian based forward kinematics as your base and then apply a gradient search algorithm where you bias against things such as running out of slack on your wires or other constraints. Let me go into a bit more detail. Generally you would just give the ...


3

What you're asking for cannot be accomplished with a PID controller. As I understand your question, you want to be able to choose PID gains that would always produce a "good" trajectory, without tuning. You said it's alright if the motor output is unrealistic, i.e. the motors are "very strong" therefore can produce unlimited torque. However, you cannot ...


2

The cable has a certain amount of slack, and each joint it goes over takes up some of that slack as a function of the joint angle. The cable will break if the total amount of slack taken up by all joints exceeds the available slack. Firstly, it would help if you could thread the cable along the robot in such a way that that each joint has a calculable ...


2

It's called a slip ring. http://en.wikipedia.org/wiki/Slip_ring The Wikipedia page has several alternate names for it. Be careful when using these devices, a cheap or damaged one with poor brushes will destroy a high-speed digital signal (and even worse, it will get damaged after you build a working prototype.)


2

The closest I can find are this and this: "Swivel Joint".


2

Another name is pin and block - mcmaster carr is a go to place for many different fasteners, and mechanical doo-dads


2

If you haven't come across the Rigid Body Dynamics Library (RBDL) you might want to look at how they implement it, and/or contact the author Martin Felis.


2

Totally off topic and not really robotics related but that's an old gearless angle drive. Kind of like this... http://www.cal-vantools.com/p-5-90-degree-gearless-angle-drive38.aspx


2

The forward kinematics of the manipulator will correctly identify the larger displacements of the end effector for small rotations of the proximal joints, as opposed to the smaller displacements of the end effector for small rotations of the distal joints. When these motions are due to errors - all real mechanical systems have them - the established process ...


2

Like Andy said, a Harmonic drive seems to be the most likely answer. But your final question is very vague. The other option is that it is a direct-drive system (with a clutch to protect the motor), and they are using a very high-torque motor. They certainly seem to have the diameter, as long as you aren't trying to lift something too heavy. As for your ...


2

From the Wikipedia article on conversion between the rotation matrix and Euler axis/angle: Given a rotation matrix: $$ \ R = \begin{bmatrix} r_{11} & r_{12} & r_{13}\\ r_{21} & r_{22} & r_{23}\\ r_{31} & r_{32} & r_{33} \end{bmatrix} \\ $$ You can get the rotation angle: $$ \theta = \cos^{-1} \left[ 0.5(r_{11} +r_{22} + r_{33} -...


2

The "best" robot wrist in terms of human analog is probably the omni-wrist by Mark Rosheim. It has a large range of motion, and does not have singularities or gimbal lock that plague other more conventional wrists. However, it is fairly complicated mechanically, and thoroughly patented i believe. The book Robot Evolution: The Development of Anthrobotics ...


2

Bear in mind a Stewart platform has six degrees of freedom - yes it does reproduce the main motions of the human wrist, but it adds extra ones such as the ability to change its overall length. There is (I would guess) additional complexity in building one - finding suitable compact actuators and linkages, additional work to control its path, perhaps ...


2

Assuming it is a robot with a serial structure: You will always be able to move every joint of the robot in any pose. However the singularitites still exist, and you can end up in a singular pose, but if you do not have any cartesian space contrains (and do not even calculate inverse kinematics/inv. jacobi), then from a motion point of view these are ...


2

The second design will put more stress on the servos over time, so there are indeed real structural reasons for the design. However, it also looks like the second design is more of a prototype compared to the first. And it's possible that the second design is trying to save weight, sacrificing some servo life in the interest of that goal. Reducing total ...


2

As I understand, you do not actually need the force, you just want that your simulation to behave somewhat realistically. Instead of complicating everything with dynamics, I suggest you remain at kinematic models (will be much much faster, considering you will evaluate your models probably millions of times, if I undertand you goals corretly, in the ...


2

As you mentioned in your update, One of the possibilities is to simply add mass to the chassis This works because, at some point, you need a reaction force. An applied force (or torque) at any point in a structure will be transmitted through the structure back to a "fixed" object. The fixed object is generally the Earth, and connection to the Earth is ...


2

For very high-precision applications such as finishing, milling by CNC machines, jerk-bounded trajectories (that is, trajectories comprising polynomials of degree 3 of higher) are often used. If you search on Google Scholar using the term "jerk bounded", you can find loads of methods to plan such trajectories. High-order polynomial trajectories (or splines) ...


2

The wrench characterizes the forces and torques acting on the respective linkages. Part of this is motor torque, the other forces and torques are loading the mechanical structure (and motors down the chain). If you have followed the Denavit Hartenberg convention in defining the coordinate systems and you have a rotational joint the joint torque is the z ...


2

Answer: The issue is that <origin>, <mass>, and <inertia> tags must be nested inside a <inertial> element for each of the links, instead of being nested in the <link> tag. correct: <link name="${leg_name}_psi_link"> <visual><geometry><sphere radius=".01"/></geometry></visual> <...


2

That diagram appears to be taken from the paper Design and development of a Mechanism of Robotic Arm for Lifting (Part 5), by M.H.Ismail et al - which is available to download as a pdf file from ResearchGate. That paper also includes the following free-body diagram of the arm: It is, perhaps, easier to see from this diagram that the 'rectangular-shaped' ...


2

Encoders used in wheels and manipulator joints are essentially the same. It is the post-processing pipeline that changes their usage and significance. These encoders basically sense the rotational motion. All optical encoders or magnetic encoders or hall effect encoders record the amount of rotation that occurred in a fixed time. Generally, a magnetic or ...


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