INV 11 – Robot Arm Wrestling – Leaf 2,3

Attrib: Aralani

Leaf 2 – Sally Meets Ferdinand

“Hello, I saw you watching me wrestle with the robot over there, just then” , Ferdinand said.

“Hi, yes, I saw it”, she said looking away for a bit.

“Wasn’t smart enough,… or quick enough”, said Ferdinand

“mmm, … perhaps I will need to to reprogram it.. thoughtfully “, said Sally

“OK,  lets get a drink, Let me get you one, lets talk over the next steps..”, said Ferdinand

“Well, thats kind of you, but, ok meet me here again Monday night, it’s getting late, I got to go.. sorry..  Whats your name ?” asked Sally

“I’m Freddo, ” said Ferdinand

“Thats got to be not for real..”, said Sally

“Nickname, my mum gave me back home in Oz, apparently Dad had something for St Ferdinand, patron saint of engineering etc… so mum just called me Freddo” said Ferdinand.

“Anyway – my name is Sally, Sally Jo, but it’s a bit Southern, so I just say Sally”

“Here’s to robot arm wrestling Sal”, said Freddo, raising his nearly empty glass of beer..

“Monday then”, said Sally.

Leaf 3 – Sally and Ferdinand… in the local pub..

So Ferdinand  found a reasonably nice quiet part of that same bar, on Monday evening.  He only had one more week in that town, but this bar had been a useful place to come in the evenings, while he was around.

Ferdinand had his laptop open and was working on a problem relating to something totally mundane, and just had to be done,,  a discusion topic for one of his internal meetings.., something about the new digital standards in control, not too heavy.

Sally walked in and looked around for a bit, she finally found Ferdinand, the only guy in a busy pub with his laptop open, and a beer of some sort, looked like a shiner, but anyway she walked over to meet him.

“Hello Mr Ferdinand”, “Hi Ms Sally, Thanks for coming” , they greeted each other.

Sally took a seat…

… “so how were you able to beat my robot arm ?” she asked.

“you must’ve been counting on the force being with you,  the logic in your robot arm seemed to be, I will always use a restoring force proportional to the angle the arm is from its correct position.  This was your ARM rule.  Then you had another rule which was your timer rule.  This was, from the time the arm selection is made, and the arm is grasped, the correct arm position is vertical.  Then sometime later the robot would give a warning of 30 seconds, which I think I correctly presumed meant that the correct position or setpoint would be adjusted to 85 degrees or so, which would then cause the ARM controller to move the arm to the winning position.”  said Ferdinand

“It really wasn’t 30 seconds”, said Sally, “it was only 15 seconds, and then it was programmed to shift the setpoint over at 5 degrees per second. Not too fast, not too slow, but just enough time for the punter to realise he was going to lose, and kind of get to laugh about it as it happened. I found that if the robot arm won too quickly, people just woudn’t play again.  This way they would play over and over and lose but then keep coming back, sure they could win eventually.”

Ferdinand, took some beer, and carefully looked through the glass, it was an ordinary glass, but he was thinking about stuff..

“You see, the weakness of the design of proportional control, is that it is ok as a basic regulator, with enough gain and feedback, but too much gain and it goes unstable, and not enough gain, you get too much error.   Since I was strong, and I got it all the way over to 45 degrees, your program simply doubled the gain and therefore doubled the force per degree error.  This actually worked in my favor later on.”, said Ferdinand.

“Yes you are right, It was a way of kind of compensating for the strength of the player holding the arm.” said Sally.

“Yes, you are figuring on a simple change to cope with each new situation the controller is facing. Controllers are generally tuned to the situation, which in most cases is fixed, not changing, but now with this arm, each player is a different situation. You will have to improve on the controller and the strategy perhaps, keeping in mind, as you have, safe operation, and player appeal.” Said Ferdinand.

“You see, by your increasing the gain, I was able to exploit the weakness by means of providing abnormal expected behaviour.  You set this thing up so that it expects to control against a counter force at all times.  Not a problem, since the idea is to wrestle the robot arm.  But by actually pulling the robot arm in the direction of its target prematurely, it allowed me to use its force in addition to my force, and the momentum of the arm, to overcome it and win.”

The jukebox was playing a song Ferdinand had selected some time back in the queue.  It was the Bluesbreakers’  with Eric Clapton, “Have you heard”….. Ferdinand loved the brilliant mix of Hammond Organ, Guitar, but mostly the wonderful saxophone, right at the start of the piece… an honest blowing style full of blues and promise.  He liked the way it sounded like New Orleans, and a kind of slow march start with just like… anyway… he kind of hoped Sally would like it as well.

“… and ok,  I get the idea,..” said sally.  “So you had trouble pushing further  than the first 45 degrees, I mean you were stuck there for a bit.  It is when it stops that I get the robot to increase the gain.” she added.

Ferdinand explained. “Classically, you could have avoided the gain change to reduce the error by including reset control.  It’s a way of integrating the error, adding more force, without changing the gain, which basically compensates for the error, so that 45 degrees would have been reduced back to vertical by means of reset action.  Each time step, take a proportion of the error at that time, and then just add it to the output of the ARM.”  There are a couple of ways to do it. But this means your proportional band does not get too strange by increasing the gain too much and making it too reactive.”

[10 minutes passed…]

“Well thanks Freddo, let me make some adjustments” Sally said.

“OK we will see how you do then” Ferdinand said.  “Listen, I will be leaving at the end of the week, but you can keep in touch by email if you like.” Ferdinand gave his email address, on the back of a beer coaster.

“Thanks, .. Ok Bye” said Sally


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