# Tag Info

## Hot answers tagged joint

Accepted

### Why aren't more bipedal (or even quadruped) robots made with joints/articulations similar with those found on atmospheric deep dive suits?

Why aren't they, you ask? Typically, it would be a worse design. Atmospheric deep dive suits, and, for that matter, space suits, are designed to maintain constant internal volume. Otherwise the ...
• 1,291

### What is the meaning of joint space and configuration space and task space?

All "spaces" are used to define the current state of the robot, but with differences. The state of the robot is a point (also known as a configuration or pose) in a space. Please note that ...
• 6,682

### 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 ...
• 619

### 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, ...
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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 ...
• 15.9k

### 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.
• 4,396

### 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 ...
• 671

### 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 ...
• 1,074
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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: <...
• 143

### 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 ...
• 6,682
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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 ...
• 1,859
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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 ...
• 588

### 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 ...
• 6,682

### 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 ...
• 1,712
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### Can I change the axis of rotation of the joints from urdf file?

Yes, you can modify it from its URDF file. Look for the joint tag: ...
1 vote
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### Adding joint plugins cause ODE internal error

The error was caused by the collision engine ode . I changed it to the bullet engine, now everything works fine.
• 11
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### joints not showing up in rviz

As already mentioned, it seems that your are missing the tf2 tree, which rviz needs to render your robot. You'll get this by adding robot_state_publisher to your launch file. A tutorial of the ...
• 1,944
1 vote

### How to implement self-locking motor in Gazebo/SDF?

Self-locking is a phenomenon that to my knowledge only occurs in worm drives. Other gear units such as planetary gears can have a very high friction, and hence be seemingly non-backdrivable, but if ...
• 2,139
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### Why aren't more bipedal (or even quadruped) robots made with joints/articulations similar with those found on atmospheric deep dive suits?

The Kinova Jaco arm has some joints on a 45 degree angle. This arm is specifically designed to aid people with disabilities. I believe they used these joints to eliminate pinch points wile still ...
• 5,865
1 vote
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### DH Parameters of a 2 DOF RP Robot

Using the DH convention in Robot Modeling and Control by Spong, the DH table is  T_2^0 = \begin{bmatrix} c_{\theta_1} & 0 & -s_{\theta_1} & 0 \\ s_{\theta_1} & 0 & c_{\theta_1} ...
• 2,452
1 vote

### Estimating distance between joint positions

I'm guessing you're wanting to change the weighting to modify some joint optimization code? But if you want to change the weighting then you need to decide how those weights change. The best advice I ...
• 15.9k
1 vote

### Why are serial manipulators almost always made of single dof joints?

Conceptually multiple-dof join is a series of 1-dof joints in sequence. Such decomposition allows you to model your control system and do stuff like inverse- and forward kinematics. For example - if ...
• 111
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### How to calculate displacement of the end effector by the yaw, pitch, and roll in the spherical joint?

Write the rotation matrix from the first coordinate system to the coordinate system moved by the first motor. Four of the elements should include a cosine or sine of the first motor angle. Then ...
• 4,396
1 vote

### what are shoulder ball and socket joints called? (i am trying to design and build a hydraulic assistance mech suit)

A 2 degree of freedom shoulder joint can be called a gimbal or pan-tilt. It can be modeled with a universal joint (aka U-joint). Usually, the third degree of freedom to twist the arm can be added ...
• 5,865
1 vote
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### My robot keeps getting saved as one cylinder leaving out the bottom cube

I suppose that what you're seeing when you add your robot model to the world is something like this: That is, only the top part of the robot is visible. The problem is not that the bottom part isn't ...
• 1,363
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### What is the purpose of an empty link without any child elements?

In this particular case, it's probably used as a projection of the robot's root link on the floor. This is often useful for motion planning or SLAM, where the robot's pose is read (or set) as the ...
• 1,363
1 vote

### IMU based joint angle measurement

"roll value of the second IMU also increases whenever the first axis rotates" — yep, that's how it always is. For the second joint, obtain the difference in the roll angles from both the IMUs. You're ...
• 337
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### Making a motor controlled joint passive

Depending on the motor type, you may be able to freewheel the motor. With a brushed DC motor this could be achieved with most commercial H-bridges, controlled with fast current decay. Flyback diodes ...
1 vote

### How to determine the direction of X-Axis in Denavit Hartenberg process? [Sketch inside]

The DH method is ambigous. There are multiple solutions which are correct and leat to the same TCP pose. Try to make a minumum amount of changes in the coordinate systems with the transition. If you ...
• 6,682
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. ...

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