Are the positions in the same frame?
Compare two object positions on a shared grid.
The question
A pick-and-place team reviews two object positions on an invented one-dimensional grid. The screen accepts an absolute error of at most 1 grid unit once both readings use the same frame.
| Robot-frame reference | Camera report | |
|---|---|---|
| Object A | 5 | 9 |
| Object B | 8 | 11 |
The camera report does not state its origin.
“What does this report tell you about the position error?”
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A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: Read the differences
What would you do here? Open the move you’d make.
Mark both objects as outside the position screen.
Tempting, but it misses something. The raw subtraction compares coordinates that may use different origins.
Report the raw differences and check whether the frames match.
The strong move. The reference and the camera can use different zero points for the same object.
See how a strong candidate says it
“The raw differences are +4 and +3 grid units. I need the camera’s frame before interpreting those numbers as position errors.”
Why it works: The reference and the camera can use different zero points for the same object.
Step 2: Ask for the frame
What would you do here? Open the move you’d make.
Check the frame and time information for both records.
The strong move. A comparison needs both coordinates to describe the same location at the same time.
Ask only for a higher-resolution camera reading.
Tempting, but it misses something. More precision does not put the readings in the same frame or time.
See how a strong candidate says it
“I’d ask for the camera origin, axis direction, scale and capture time relative to the reference.”
Why it works: A comparison needs both coordinates to describe the same location at the same time.
Step 3: Compare in one frame
The frame record places the camera origin at robot position −4. Both records use the same direction, scale and capture time.
What would you do here? Open the move you’d make.
Add 4 because the camera values look too high.
Tempting, but it misses something. The camera’s zero is at −4 on the robot grid. Moving a camera value into the robot frame adds −4.
Apply the reported offset before checking the absolute errors.
The strong move. The direction and scale agree, so the offset maps the camera coordinates to the reference frame for these records.
See how a strong candidate says it
“Subtracting 4 gives robot-frame positions 5 and 7. The signed errors are 0 and −1. Both meet the screen. B is at the boundary.”
Why it works: The direction and scale agree, so the offset maps the camera coordinates to the reference frame for these records.
Step 4: Give the limited result
What would you do here? Open the move you’d make.
Report the two results and ask what evidence would support wider use.
The strong move. The result supports the two comparisons in the packet. The perception owner can identify gaps in coverage.
Subtract 4 from every camera reading in later tests.
Tempting, but it misses something. This record supplies the frame relationship for these observations. Later tests may use different frames.
See how a strong candidate says it
“Both aligned records meet the screen. I’d keep the frame and time information in the report and ask the perception owner what other positions need a check.”
Why it works: The result supports the two comparisons in the packet. The perception owner can identify gaps in coverage.
The answer, in short
- Decision
- Compare the readings in the robot frame and report two passes.
- Because
- The recorded offset gives errors of 0 and −1 grid units.
- What would change it
- Further matched comparisons would show how often the error stays within the screen.
Common traps
- Checking raw values as if their zero points match.
- Using the result to cover untested positions.
Check your answer
- I read the raw differences.
- I checked frame and time.
- I applied the offset with the right sign.
- I kept the conclusion to the two records.
Last reviewed Nov 11, 2026