Did anyone ever try to weigh products without releasing the product from the gripper?
Is it possible with the robot or by loadcells in the gripper?
Weighing products
- TRBe
- Thread is marked as Resolved.
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Possible, yes. But a great deal depends on your specific circumstances. Measuring the payload while the robot is in motion is also possible, but not at all easy, and would require some very careful modelling. Likewise, weighing the payload at some odd orientation other than vertical would also be difficult. Measuring a payload that does not have its center of gravity directly beneath the load cell would require extra complexity.
By way of an example, I have a robot that must apply a specific force against a work surface using several different tools. These tools mount to a plate which is supported by a triangular formation of three load cells. When the tool is colinear with the center of the triangle, measuring the applied force is relatively trivial -- simply sum the three load cells. On the other hand, some of the tools are offset and apply their force well outside of the triangle. In that case, I have to have an accurate measurement of the various lever arms between the point of contact and each load cell, and perform a trigonometric calculation to measure the actual force that is being applied at the point of contact.
Also, the entire tool had to be carefully designed so that all the forces were intercepted by the load cells. This meant that the tool mounting plate had to "float" on the load cell plate -- any structure pass through both plates would absorb some of the forces, and prevent the load cells from detecting all the forces in play.
A good tool for doing this kind of thing would likely be a Force-Torque sensor from ATI or Futek. These sensors are made to mount between the tool flange of a robot and the end effector. The sensor has multiple strain gauges and can report both forces and torques on the X, Y, and Z axes in realtime analog. Since the sensor has its own coordinate system, additional math in the robot or PLC will be required to translate the six analog values into the actual weight of the payload, taking into account distance, orientation, and gravity. The math is non-trivial, but should be something any decent mechanical engineer should be capable of.
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One of my customers did this a few years ago. The robot would weigh a plastic part that was just moulded in an injection moulding machine. If the weight was not within spec it would get dumped by the robot.
It was made by a company called Kissinger??..... Something along those lines anyway.
The problem they ran into was that it wasn't designed to be robot mounted. The cable coming from the sensor was not high flex and would always break. It was such a small gauge wire that it was happening all to frequently. They abandoned it and did something else.
RoboGuru
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Roboguru
This project is the same as you describe. We have to measure the weight within 5 grams accurate. So now we are thinking about dropping the whole gripper with the part on a scale.
I can't find anything from Kissinger. -
I have used a scale many times to weigh parts. I have always dropped the part on the scale then moved the robot away so as to not influence the scale reading. I have always used Mettler Toledo as the scale manufacturer. The systems work great. Analog signal into the controller.
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Unless the part weight is signigicant compared to the gripper, I don't see how you will be able to measure down to 5g accuracy by dropping them both on the scale. I believe you would be far better off dropping only the part on the scale, ideally using some kind of jig mounted to the scale to allow repeatable pick/placement. But the bigger the total weight placed on the scale, the harder it becomes to break out small deltas from the noise floor.
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I have the same doubts skyfire, the gripper wheighs a lot more then the part. I will here what the scale supplier has to say. The problem is there are small injection pieces that have to be cut off and they are only defined in the mould. So if i drop the part on the scale and release it, even with a jig or mould shape is will not be able to find the small pieces I have to cut off.
Oh yeah the small pieces have to be wheighed also with the part, so cutting them first will not work. Nice challenge. -
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