PYTHON · NUMPY

What the number is per

A vendor publishes capacity and runtime but not duty cycle. Who decides the efficiency number?

Separate a property of a battery from a property of a machine, and identify the unit of work a claim is denominated in.

01Challenge

A vendor sheet gives you 2.3 kWh and 'up to 4 hours'. A second gives 0.842 kWh and '4 hours'. Decide which machine is more efficient, and defend the answer.

Predict firstA vendor publishes capacity and runtime but not duty cycle. Who decides the efficiency number?

02Model

Almost every energy figure published about a robot is a property of something other than the robot. Watt-hours of battery capacity is a property of the pack: it tells you what was installed, not what the machine does with it. Hours of runtime is a property of a duty cycle somebody chose, and the person who chose it also chose how much of that time was spent standing still. Neither answers the only question a buyer has, which is how much work comes out per joule in. The fix is a denominator. Energy per task, per metre, per unit moved, per kilogram lifted through a metre: any of them makes two machines comparable, and none of them appears on a spec sheet. Cost of transport is the version physics already agreed on for locomotion, and it is dimensionless, which is why a 30 kg robot and a 70 kg human can appear in the same column.

03Guided practice

Compute average power from capacity and runtime, then divide by the work actually done. The trap is that runtime alone hides the duty cycle, so start by asking what the machine was doing for those hours.

04Feedback

PASS WHEN You produced two comparable numbers from sheets that were not comparable, and you had to invent the duty cycle to do it. That invention is the whole problem: the vendor left the most decisive input to the reader.

If your run fails, check:

avg_power_W is capacity in watt-hours divided by hours. Watch the kWh to Wh conversion.
metres_walked uses only the fraction of the time spent walking, not the whole runtime.
If both machines come out identical, check that you used each one's own mass in the CoT step.

05Retrieve & space

Watt-hours is a property of the pack; hours of runtime is a property of a duty cycle somebody chose.
A comparable figure needs a denominator: per task, per metre, per unit moved.
Cost of transport is dimensionless, which is what lets machines of different mass share a column.

06Mastery & project

In one sentence, say what 'four hours of runtime' tells you about a machine, and what it does not.

Take two published spec sheets for any powered machine and compute one comparable figure from them. List every input you had to assume, and mark which of those the vendor could have published and did not.

GO FURTHER · THE LIVE PLAYGROUND

The instrument ↗ · Set a gait and a duty cycle and watch the derived figures move. Same machine, different claim.

← Reading an Energy Claim The grade, not the number →

PYTHON · NUMPY

What the number is per

Two machines, two spec cards, and a duty cycle the sheets do not state. Find the figure that makes them comparable.

You read

the arrays and values already in scope

You change

the code you write in each cell

Fixed

the dataset and the checks that grade you