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• Unit 2 - Food For Thought (Chapters 4 & 5)

## Unit 2 - Food For Thought (Chapters 4 & 5)

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By Darren Hurren July 9, 2019 - 9:06pm

Arms, Legs, Wheels, Tracks, and What Really Drives Them (Chapter 4)

Food For Thought

Q1: How would you measure a motor's torque? How about it's speed?

A1: I'm not exactly sure how to measure a motors torque. The easiest way would be to take a calibrated torque wrench, that hopefully has a higher maximum torque then what the motor can output and attach that somehow to the shaft  I'm thinking you would use an ammeter and record the current draw when the motor is free spinning with no load, and then record the current where there is maximum load (where the motor is stalling because it cannot produce enough force to move the load. This can't be good for the motor though...) and then I'm assuming that you could correlate this with the motors datasheet to get a rough estimate of the torque value. In Matarić's "the Robotics Primer" Section 4.3.1, she writes:

When a DC motor is provided with nice constant voltage in the right range, it draws current in the amount proportional to the work it is doing.

Matarić, M. J. (2007) The Robotics Primer. Cambridge, Massachussetts: The MIT Press

Since torque is directly proportional to power and rotational velocity, one way would be to know the power when the rotational velocity is zero (motor is stalled), in order to calculate what the torque would be. Alternatively if there is no load, then the rational velocity is the highest it can be for that specific motor but the output power is zero, so the torque would also be zero.

Q2: How many Degrees of freedom (DOF) are in the human hand

A2: This is a complex question. I'll try to break it down for myself just like it was done for the human arm in the book; Each finger knuckle (at base of hand) has two DOF * 4 (up/down, side-to-side), the secondary / middle knuckle (or joint) on each finger has one DOF * 4 (up/down), the third / end knuckle (or joint) has only one DOF *4 as well (up / down... and this is barely even true it seems... on my hand atleast). The thumb's main joint has 2 DOF I would say (Up/Down, Side-to-side) and the second joint (close to the end of the thumb) has only one DOF (up/down).

So, it may sound crazy, but I think that the human hand has a whopping 19 DOF!! (4+4+4+4+2+1)

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Move It! (Chapter 5)

Food For Thought

How does an automated web path planner, such as Google maps, find the optimal path?

Since it would be dangerous for any driver of a vehicle to simply take the shortest trajectory to their destination (i.e. driving straight through fields, lakes, houses and any other thing in their path), the driver typically chooses the fastest, safest, most efficient, most law-abiding route to their destination (although this is not necessarily the shortest route when compared to a straight trajectory path). Google maps and other similar applications take the predetermined routes into consideration to get you to your destination long with those four specifications I mentionedd earlier. According to Matarić in "The Robotics Primer" this is called the optimal trajectory. The way they (Google and other navigation assisting companies) determine those routes I don't exactly know but I'm assuming they use data from maps, gps signals and likely user provided feedback on road closures & construction etc. In addition, since so many people use the google maps application, they can gather data from all users in order to calculate & adjust the expected travel time & delays for everyone in order to ake the trip even more efficient.