Nonlinear Weighing Scale With Tare Function

3D printed weighing scale with several attractive features such as

• More sensitive for light payloads (2g/div)
• Max 500g payload
• Tare function (subtraction of the weight of containers)
• No spring
• Easy to calibrate

Maximum part size : 126(W) x 179(D) x 192(H) mm

Supplies

• 11mm steel balls (36 balls)
• 1.6mm stainless wire for the shafts
• 2mm tapping screws (wood screws)
• Lubricant (silicone oil)

Step 1: Print

• Print the all parts with supplied posture.
• Print two copies of main weight holder parts.
• No support structures needed.

Step 2: Assembly

• Fill the weight holders with steel balls.
• Assemble the parts with screws.
• Bend the single end of the shaft to avoid it slipping out, then insert.
• Print the index (scale.pdf below). Please make sure that the magnification is correct. (it has 10mm reference scale).

Step 3: Calibration

1. Adjust balance weight (at bottom) to make the top and bottom links horizontal.
2. Stick the scale so that the 0g position is aligned to the hand (meter needle). 
3. Attach the main weight using tapping screws.
4. Using an object with known weight, adjust the vertical position of main weight.

Step 4: Learn

Roberval mechanism

• Parallelogram link makes the scale insensitive to the payload position.
• The mechanism can be regarded as a balance.
• Weight of Roberval mechanism is compensated by the balance weight on the line of the balance.

Nonlinear scale

• Main weight (W) is attached at the bottom of the pivot.
• Heavy weight changes the distance from the pivot to the main weight, and the balance becomes less sensitive.

Tare function

• Since the balance weight is on the line of the balance, the position of the weight does not affect to the sensitivity of the scale.
• Position of the main weight does not affect to the balance (0 position) of the scale also.