# Tag Info

### What is the technology used for no-resistance robot arms?

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 ...
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### What to do when position control with trajectories is interrupted?

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 ...
• 1,647
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### What is torque bandwidth in actuated joints and how does it affect the control systems?

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 ...
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### Can we simulate a actuator with a very strong torque with a PID controller

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, ...

### What to do when position control with trajectories is interrupted?

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 ...
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### Adding an Actuator or Force to a (Featherstone) Articulated Rigid Body Model

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 ...

### Velocity description in Screw Motion Theory

The body velocity $V_{b}$ is the velocity of the frame with respect to the world, as seen from the frame's perspective. Its rotational component $\omega_{b}$ contains the rotation rates around the ...
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### Adding an Actuator or Force to a (Featherstone) Articulated Rigid Body Model

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.
Accepted

### What type of mechanism is this?

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
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### Denavit–Hartenberg parameters of a robot with spherical wrist

A spherical joint can be represented by three consecutive rotary joints with intersecting rotation axes. This statement is true, your restrictions, I'm afraid are not. The position of spherical ...
• 226

### How can the inverse kinematics problem be solved?

There are two broad approaches: analytic solutions, given an end-effector pose, directly compute the joint coordinates. In general the solution is not unique, so you can compute a set of possible ...
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### Determine what the rotation axis is given a rotation matrix

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}...
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### Stewart platform as robotic wrist joint

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 ...
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### Stewart platform as robotic wrist joint

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 ...
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### Joint Space Singularities

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, ...
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### Lateral load on a servo motor

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 ...
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### Reference request: Path accuracy algorithm in the joint angle space

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 ...
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### how to find maximum force of a robot joint

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 ...
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### Gazebo joint->SetForce() call applies forces to the whole model. How to circumvent that?

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 ...
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### State of the art trajectory for fine precision robots

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 ...
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### How to calculate effort/torque values of a joint

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 ...
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### Spherical (Ball) Joint in Robot Model not Working in Gazebo

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: <...
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### What is the rectangular-shaped joint in this diagram?

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 ...
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### What are the differences between wheel and manipulator joint encoders?

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 ...
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### How can I attach an arm to a double-flat drive stepper motor?

You can use a D or Double D shaped shaft collar/coupler. Or obtain a Double D shaped punch, depending on the design of the piece you are attaching to the shaft.
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### How does this end effector work?

The actuator has two raceways cut into it. One is a screw thread, and engages with a rotary screw nut that is turned by a motor. Turning the nut moves the actuator linearly. The other is a spline ...
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### How do I decide the minimum number of link lengths and joints my manipulator must have from the configuration space?

If you should consider not just points, but poses (position and orientations). If you can write these as a table, where the columns are the coordinates ...
• 6,322
1 vote
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### Adding a floating joint to a sphere in urdf

The joint type you've added to the URDF is fixed rather than the desired floating. ...
1 vote
Accepted

### Parallel axis theorem to determine the moment of inertia

If the distance between the axes is zero, then there is no change in the inertia for that specific axis. You can verify your calculations with a CAD software. Most of them (Solid Works, Inventor, ...
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1 vote

### Can we simulate a actuator with a very strong torque with a PID controller

Industrial robot controllers (and most "real" robot arm controllers) will have a feedforward term to remove the effect of gravity. Often called gravity compensation. You noticed how necessary this is ...
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