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Sherlock Ohms

Running the Robot to Ruin

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GeorgeG
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Platinum
Re: Original by-line mislead me
GeorgeG   7/10/2013 10:45:54 AM
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"I figure if the manufacturer didn't want it to fall apart from running fast, they should of limited the maximum speed that I can set!". First, speed isn't directly the issue: it's the acceleration and jerk required to get up to speed and down again; obviously, you can do a long motion at a high speed with relatively low acceleration but a short motion requires higher acceleration. Also, acceleration is the result of applying a force to a mass as well as other opposing forces. It is mostly the force required to produce a desired acceleration that resolves into stress in the various mechanical parts producing wear out; this is a complex function of payload, tool-point vectors, drag and other factors that are very application specific. No one would be happy if manufacturers set performance limits at the 'safe' value for the most demanding application conceivable. No one would by a car if the speed was restricted to a safe value for parallel parking. Also, schools of practice such as TPM and OEE recognize that wear always happens; the economic solution is to balance costs of maintenance and downtime against productivity. Factors like utilization play into this: even the same robot doing the same job will have significantly different service requirements if producing 1000 parts per week versus 100,000 parts per week. What you do with a robot is an economic as well as technical decision. Also, any multiaxis robot is in some ways its own payload. While the mass of all subsequent links must be  accelerated by an axis, the reaction force of the relative motion of all subsequent axes must also be overcome as the reaction force of each axis supporting it must be overcome; consequently, the amount of force acting and the amount of resulting stress is very much a result of tool trajectories which are ever so application specific.  Keep in mind that for any revolute link, even constant velocity implies centripetal acceleration. Additionally, there is thermal stress on the drive system to consider: short bursts of high effort may be possible without elevating the temperature of motors and bearings so the peak dynamic limits are also a function of duty cycle. Applying robots in manufacturing is not, as you wish, child's play. On the other hand, maintenance guys are right to restrict the feeds and speeds to only what is necessary to meet production capacity with the least effort and the highest yield possible which translates into least operating costs for the equipment.  Never forget that the purpose of an industrial robot is to maximize profit.      

       



GuidoBee
User Rank
Iron
Re: Haste makes waste
GuidoBee   7/10/2013 10:53:11 AM
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As someone not in the robotics business / industry, your explanation of the limits of speed and accuracy were most informative.  Thanks for a good explanation.

GTOlover
User Rank
Platinum
Re: Original by-line mislead me
GTOlover   7/10/2013 10:57:15 AM
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I understand all that and agree. But in injection molding the time the robot goes into and out of the tool has to be minimized. My comment was in that venue in that this is a critical time delay for the cycle time for the next molded part. After the robot has the part and is clear of the molding machine, I always optimize the robot to match the press cycle (slow it down). What got me was that technicians would slow down the overall speed parameter and the entire robot maximum speed would be inhibited, thus impacting cycle time. instead i would argue to maximize the overall speed and then slow down individual moves non-critical to press cycle time. Always with the intent that the end of arm tooling was robustly designed and within the payload capacity specified by the robot manufacturer.

And in no way did I imply that this was "child's play".

I have to admit that some EOTs (end of tooling) over-loaded the robot and the speed and accelerations had to be adjusted downward. But this was usually a top candidate to redesign EOT or split the EOT requirements.

Charles Murray
User Rank
Blogger
Re: Original by-line mislead me
Charles Murray   7/10/2013 7:12:00 PM
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That makes sense, HarryB. Jack rabbit starts mean higher forces (for autos or any kinds of machines), and more deterioration.

William K.
User Rank
Platinum
The running robots.
William K.   7/11/2013 10:03:37 PM
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We did learn about the precise stop versus the direction changing intermediate  move and we learned about acceleration and decelerations during moves. So there are ways to speed up cycles without a lot of stress. None of it is trivial, although the process is straightforward. But some times it is possible to reduce the precision and speed things up even more, but only when accuracy is less critical. After all, not every operation demands high precision. But that should only be done by those who understand the product very well.

GlennA
User Rank
Gold
Re: Original by-line mislead me
GlennA   7/11/2013 10:34:12 PM
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Old Curmudgeon;  You have struck a sore point.  The 'technician' that changed the termination type and was patting himself on the back was one of two self-proclaimed 'robotic welding experts'.  Neither was competent, let alone proficient at robot programming or welding applications.  They were Managers, not union,  Several times I had to fix their mistakes - which they did not even realize were mistakes.  A robot does not just do what it is programmed to do - it does EXACTLY what it is programmed to do.  A common mistake is programming a joint move followed by a linear move.  When stepping backward the robot moves in joint, and does not re-trace the linear path.  I have seen many programmers baffled by this fact.

OLD_CURMUDGEON
User Rank
Platinum
Re: Original by-line mislead me
OLD_CURMUDGEON   7/12/2013 7:58:23 AM
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Glenna:  Not witnessing this event, I have no direct knowledge of all the parameters that went into the situation causing the failure.  However, that being said, I have been employed by two very successful private electronics businesses in my long career, which were non-affiliated for many years, and then became unionized.  And, in each case, these companies suffered drastically to the point of death.  While the initial explanations didn't point to the unionizations as the culprits, history did show that it WAS the fundamental reason for their demises, sorry to say.  The shame of it all is that IF it weren't so, and all else being equal, I could have envisioned myself getting that proverbial "gold watch" after 50 years of dedicated service.

Furthermore, I have designed enough electromechanical devices in my career to understand the importance of component specifications, and why writing "cautious" software to control these devices is UTMOST important!  As others have blogged so learnedly, acceleration, deceleration, impact & momentive forces MUST be held in high priority in all designs.

Thank you. 

dougspair
User Rank
Gold
Robits tossing tools
dougspair   7/12/2013 7:10:13 PM
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All through the 1970's I was an Elec-tech at Burgmaster, a company that made turret drills, with NC programming. We tried designing and building a vertical spindle mill, with tool changer for a 3 axis CNC, as the CNC was just getting started in Machine Programming. The enginerrs did a nice job on the mechanics of the tool changer,but didn't allow ramping up/down for the accelleration at end of cycle, many tools got tossed from the gripper and sent flying.

I'd have thought they'd incorporate ramping in the toll changer arm, as all the X-Y-Z axis had .400" ramping from max speed to final position commanded. 

oldtimer8080
User Rank
Gold
robots driven to destruction
oldtimer8080   7/12/2013 8:20:53 PM
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after my work in supercomputers, I used my digital to analog experience in Robotics, both in AGVs and tracking devices for parolees.

 One of the hardest problems to solve is how do you control motion in a 5000# AVG without making it too slow or so fast it damages it's hardware.

 Knowing how PWM controlled motors act to ramp-up and ramp down is critical and THEORY VS REALITY is very important.

 As in supercomputer design, you must test and get the repeatable motions in places to get the parameters for proper program code to program the PWM motor controllers....the real problem is that AGVs MUST have a HARDWARE EMERGENCY STOP SYSTEM that is not dependent on other progrmming.

 To make the AGV work, a default servomechanical wheel brake was put on the driven wheel(s). The bad news: a stop from full speed created a flat spot on the driven wheels and we could never get a full stop in the 5' requirement for AGVs.

( the boss gave us that requipement )

What we finally did was to set up a guided " racetrack " in the parking lot to test maximum speed before guidance loss ( around 7.5MPH ) due to feedback overcorrection and maximum rate of travel when the safety overrides were triggered.

With those real world parameters set, we could make the AGV safe and controllable for the next step: proper motion of the servo arm to handle explosive airbag triggers...

One wrong step....BOOM!!

bobjengr
User Rank
Platinum
ROBOT TO RUIN
bobjengr   7/13/2013 12:04:51 PM
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Very informative post Glenn.   Two years ago I experienced a fairly similar situation with one client.  The robot (Janome --SCARA ) dispensed an adhesive onto a metal substrate; i.e. control panel.  After application of the adhesive a decorative trim piece was placed onto the panel.  This will be hard to believe but, the entire work cell was a cost center in which the employees received a base salary and paid a bonus based upon daily production.  (By now you  know where I'm going. )   One interprising individual felt the need to "speed things up" a bit so he altered all speeds for each path the robot was programmed to take.  We were negligent in not blocking access to the program but qute frankly we did not realize anyone in the cell could (or would ) make adjustments.  Boy were we incorrect about this one.  We fried two servo motors -- down for over a week waiting on replacements and  cost our client well over $2K for replacement parts. 

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