Hot answers tagged

89

It has more to do with the rocker bogie suspension than anything else. The system is designed to be used at slow speed of around 10 cm/s, so as to minimize dynamic shocks and consequential damage to the vehicle when surmounting sizable obstacles. In exchange for moving slowly, the rover is able to climb rocks that are double the wheel diameter (normal ...


37

This seems like a softball question but is surprisingly subtle. There are some excellent answers here, but I can add some basic rigor. The reason the rovers move so slow is essentially the need to be cautious with a multi-million-dollar piece of equipment. But there are some other design constraints worth mentioning. Energy is simply the worst bottleneck ...


25

I'm not such an expert in physics, but I can think of a few reasons: Power. The amount of power you need to do a task is inversely proportional to the time it takes to do that task. I think it is well known that doing something faster requires more power, otherwise you could do everything infinitely fast at no cost. Computation Speed. The statement about ...


21

At least in part quadrotors offer a nice balance between the complexity of the dynamics and power requirements. With traditional single rotor helicopters, control is a function of the orientation of the rotor which means you must change its orientation to change direction of the craft. This makes for very complex mechanical linkages comparatively speaking ...


20

The best option will depend on the type of terrain you expect to cover. You want the correct balance of several factors: ground pressure traction suspension steering Ground pressure from tracks is less than ground pressure than wheels, so they're more suited to soft surfaces. Larger tires can help, but there are limits -- they may not work in something ...


19

One reason is because of the communications delay between Earth and Mars. The round trip time for signals from Earth to Mars is several minutes, which means that you can't teleoperate the robot in realtime. That means that the robot needs some autonomous obstacle avoidance capability to help prevent it from getting stuck or otherwise in trouble. The ...


16

Well, you generally use wheels when: You want speed. Treads need a lot more torque to power, thus you generally use low-rpm/high torque motors. You want maneuverability: Treads are a big pain to turn with. Differential steering is very ineffective (the bot skids to steer, which may not work if you have really grippy treads) But, treads are better when: ...


16

You almost definitely want to have the battery rigidly mounted to the center of the airframe. Mounting a battery underneath each motor (though I have never seen this) will increase the multicopter's moments of inertia, which will make it more "stable" in that it will more sluggishly lose its balance and thus give a controller more time to react. This, ...


14

Assuming it is a rigid robot, then the only weight properties of interest is: the total mass the centre of mass In terms of tipping over, the robot is more stable if the angle required before tipping over is maximized. This is achieved by having a low centre of mass, and a centre of mass as far away from the edges of the support polygon as possible. ...


11

I think the surface of mars was pretty well known by the time Spirit / Op landed. The rocker-bogie system allows the robot to climb over obstacles up to twice the diameter of the wheels, while avoiding springs completely. Springs could cause the chassis to tilt on uneven terrain. Watch this video: https://www.youtube.com/watch?v=ON1zLBvYKRI, and imagine how ...


11

There are multiple reasons wheels may be prefered over treads. The main ones that apply to the mars rovers I would put as: Mass is a critical property, especially for space exploration missions. Treads are generally heavier than wheels. Treaded vehicles only support skid steering and are thus less precise to manouver. They also take more power when turning. ...


10

As Rocketmagnet mentioned, just because a motor is rated at 2.5 W doesn't mean it will be pulling 2.5 W all the time. Most robots have at most 1 or 2 servos that are running at full power at any one time; the rest have very low mechanical loads (and therefore pull much less electrical power) or are "off" and therefore pull practically zero electrical power. ...


9

You need 4 degrees of freedom to control yaw, pitch, roll and thrust. Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated. There is no restriction to only 4 props, hexa+ coptors are also very common. Generally you want an even number of props unless ...


9

Typically, a coordinate frame is placed at the robot center. The x-axis points forward, the y-axis points left, and the z-axis points up. Then, we measure angles with respect to the x-axis. So, a 90 degree angle would mean along the y-axis, as shown, So, "12" corresponds to 0 yaw, or straight forward. "9" corresponds to 90 degree yaw, or along the y-axis....


9

Let's look at how a quadrotor flies, then apply that to a trirotor. Let's assume that we want to remain in a stationary hover position. To do that, you need to balance all the forces: thrust from the propellers vs. gravity, and the torques of each motor. Each motor produces both thrust and torque according to the equations: $$ T = K_T\rho n^2 D^4 $$ $$ Q = ...


8

It's always difficult speccing the power supply for a robot, and you've hit the exact problem we all face. Do you spec it to cope with the typical load, or the absolute maximum load when all motors are stalled at max current? There's no right answer to this, except that whatever happens it shouldn't damage anything. The good news is that the servos probably ...


8

There is no right answer. However, I think I can clarify what you've read so far. You need a buoyancy engine to supply the thrust, wings to direct that thrust, and some ability to change your pitch angle when you change the buoyancy. So, you'll need to balance the strength of your engine with the drag created by your wings. At the same time, your wings ...


8

Center, you want the moment of inertia to be as small as possible.


8

I think you've taken a good first step; you've divided the problem into a mobile platform (which has uncertainty of position and must navigate) and the arm (which has a fair certainty of position in real time, through encoders). I have looked at papers related to robots architecture [...] but I have yet to find information on how to have the low level ...


8

Addressing the shoulder joint, which is rather more complicated than elbows and knees... After this one, the other joints become far simpler to visualize or engineer. Here is how the ball and socket shoulder joint could work: Freedom of movement: Approximately 60 degrees end to end, all around. Less than for humans, but it can be tweaked to around 80 ...


8

You're trying to implement more PIDs than you have degrees of freedom. In a quadcopter, you have only 4: $(Z, \phi, \theta, \psi)$ i.e. (Altitude, Roll, Pitch, and Yaw). via (http://www.draganfly.com/uav-helicopter/draganflyer-x4/features/stability.php) Interestingly, from a PID perspective you definitely do have desired values for $\phi$ and $\theta$: ...


8

Yes, a state matrix with zero rows and/or columns makes sense and is viable. It typically signify pure integrators in the system. In the example you give, $$ \dot{v} = -\frac{b}{m} v +\frac{1}{m} u $$ where $v$ is the speed, $u$ is the externally applied force, and $bv$ is some viscous damping force. Now if the viscous damping coefficient is zero (no ...


7

You're making two mistakes that I can see, both related to the idea of "shrinking" the set of front or back wheels into a single wheel. Rather than thinking of Ackermann steering as (conceptually) a single wheel, imagine expanding the single front wheel of a tricycle into 2 wheels. At first, the tire gets wider, then splits into two tires, then they get ...


7

You have the right idea, just be sure to design for the servo to bear the moment force (aka torque) generated by the load at Y = 4 inches from the joint, not the 2.5 pounds of what you're trying to hold. $\tau = r*F*\sin(\theta)$ Where: r is the displacement (your 4 inch arm) F is the magnitude of the force (2.5 pounds + the gripper) Theta is the angle ...


7

If your goal is to experiment, then use the cheapest option possible -- it will give you the freedom to make more mistakes. Carbon fiber would be great if your design is in a fairly final state, but are you 100% assured that you correctly determined all the thicknesses, mounting holes, wiring guides, etc? Are you good enough working with carbon fiber to ...


7

As others have said in comments, a screw is probably your best bet. It's mechanically pretty simple to set up, and could be made to look fairly tidy, which is always nice in a home. But the main problem, as always is going to be doing this on the cheap. The cost of the parts soon adds up. Even if your motors are only $15, you still need to buy the lead ...


7

Your mechanism will depend on the type of dispenser you want. Like if its gravity feed or not. I will assume it is, so I make this sketch to give you some idea: The hole size will depend on how many candy you want to dispense at each stroke. (Of course its related to the candy size). The electro-mechanical actuator can be a crankshaft one, a solenoid, a ...


7

The principle lying underneath the sphero robot's design and locomotion is shifting of the centre of mass of the ball and making it unstable which makes the ball roll [1,3,4,5,6]. A controlled and calculated shifting of the centre of mass to the appropriate position can achieve desired trajectories of the ball. Apart from the above said principle, a few ...


6

Efficiency isn't the right thing to compare due to various advantages and disadvantages of each type of wheel. Comparing the efficiency of the different types of wheels is like comparing apples and oranges. However, comparing speed and force can give a good comparison of the different types of wheels. Here is a table that offers a quick and simple ...


6

I have some experience with using mecanum wheels, both indoors and outdoors (on grass, sand and dirt no less). Obvious advantage is holonomic movement. Disadvantages are weight (commercially available wheels are ridiculously heavy) and cost. For a given tread material traction will be along the lines of 65-70% that of a regular wheel due to the smaller ...


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