Precision test bench
for laboratory pipettes

The context

A manufacturer of laboratory pipettes needed to verify, very finely, the volume actually dispensed by its products. The reference method is gravimetric: the dispensed liquid is weighed, and the mass gives the volume.

The difficulty is not the balance, it is the gesture. A pipette is operated by thumb pressure on its piston, and no two human presses are identical: speed, force and dwell time all vary. A qualification campaign involves hundreds or thousands of dispenses. Until the gesture is reproducible, the measurement does not characterise the product — it characterises the operator.

The project was handed to us in full: neither the mechanics, nor the cabinet, nor the program existed. The mechanical and electrical design offices worked in parallel on the same data, from the design stage onward. The mechanical design remains confidential and is not shown here.

What we did

• Mechanical design of the bench: structure, kinematics, guidance and layout of the two driven axes, and of the weighing station.
• Electrical design and schematics in SEE Electrical Expert, run in parallel with the mechanical study.
• Two independent brushless axes: the vertical pipette axis brings the pipette into position, the vertical pusher axis operates the piston. Each is controllable in position and speed.
• Force control on the pusher: the motor is driven in closed loop on a force sensor, so the force applied to the piston follows the setpoint press after press. Motor torque and force are monitored continuously.
• PLC programming: motion profiles, dispensing sequences, incremental mode for setting up, travel limits and fault handling.
• Measurement acquisition: dialogue with the precision balance, automatic tare, conversion of mass to volume, and each weighing tied to its dispense.
• Recipe management: each test is described by a recipe of up to fifty programmable steps. The client composes its own sequences from the supervision screen, saves and recalls them — without writing a line of code and without involving us.
• Supervision under WinCC Runtime Advanced on PC: manual axis-by-axis control for setting up, automatic mode for campaigns, live mass and volume readout, real-time force curve and a dedicated alarm page.

The result

The bench reproduces the user's gesture with a controlled, constant force, and weighs every dispense. The measurement therefore characterises the product, not the operator.

It runs unattended for hours: endurance campaigns that were impossible to run by hand are started in the evening and analysed the next morning.

Above all, the bench is not locked to one protocol. The client writes its own recipes — up to fifty steps each — and evolves them as its products evolve. A new pipette model, a new qualification protocol: they do not need to call us back.

The cabinet and the supervision

The mechanics of the bench belong to the client and remain confidential: we do not show them. What we can show is what carries our workshop signature — the cabinet wiring and the interface we developed.