Integrating the New FENYX® Pump from Fluid Metering with Your PLC System

This white paper aims to shorten the integration time for the FENYX® variable dispense pump, which uses valveless rotating piston pump technology—a design that’s less commonly known—and ensure it operates at its best. A great pump is only as good as its control system.

How Rotating Piston Pumps Work

Piston pumps are known for their accuracy due to the fixed volume dispensed per stroke. For comparison, peristaltic are “less” accurate because they rely on flexible tubing, which introduces tolerance variability.

Standard piston pumps, such as OEM syringe pumps, and micro piston pumps require valves—this adds mechanical complexity and reduces component lifespan. They also operate in two stages: aspiration and dispense. In contrast, the rotating piston pump aspirates and dispenses continuously, simplifying the system and increasing throughput.

The Fluid Metering design improves on traditional piston pumps by combining rotation with reciprocation. The piston rotates and moves up and down, drawing fluid into a defined cavity and pushing it out—over and over. This eliminates the need for auxiliary valves, maintaining piston-level accuracy while simplifying control.

The valveless pumping function is accomplished by the synchronous rotation and reciprocation of the ceramic piston in the precisely mated ceramic cylinder liner. One complete piston revolution is required for each suction/discharge cycle as shown. The piston always bottoms for maximum fluid and bubble clearing.

1. Suction stroke

The piston rotates and reciprocates. As the piston is pulled back and the piston flat opens to the inlet port, suction is created and the fluid fills the pump chamber. As the piston reaches the highest point in the reciprocation cycle, the pump chamber is now at its maximum volume capacity.

2. Crossover point

Continuing the rotation, the inlet port is then sealed and crossover occurs. As the inlet port is sealed and the pump chamber is full, the outlet port opens up. Only one port is open at any time and at no time are both ports interconnected.

3. Discharge stroke

Continuing the rotation and reciprocation, the piston is forced down and the piston flat opens to the outlet port. Discharge is created and fluid is pumped out. The piston bottoms for maximum fluid and bubble clearing.

4. Crossover point

Continuing the rotation, the outlet port is then sealed and crossover occurs. As the outlet port is sealed and the pump chamber is empty, the inlet port opens to start another suction stroke. Only one port is open at any time and at no time are both ports interconnected.

Key features and benefits

FENYX® pumps can run for 80 million+ cycles (depending on application) without needing recalibration—significantly reducing consumables and calibration overhead.

Understanding Pump Accuracy: Coefficient of Variation (Cv)

Rotating piston pumps provide exceptional accuracy, particularly at low volumes. This is measured using the Coefficient of Variation (Cv), a common metric in precision dosing:

Cv(%) = (σ/ μ) x 10

Where:

• σ\sigma = Standard deviation of dispensed volume
• μ\mu = Mean dispensed volume

Lower Cv = Higher accuracy and consistency

Interpretation:

• Cv < 1% → Excellent (e.g., lab dosing, medical devices)
• Cv 1–3% → Good (e.g., industrial dosing)
• Cv > 3% → Lower precision (e.g., fluid transfer)

Adjustment of Dispense Volume

Dispensed volume is controlled in two ways:

1. Rotations of the piston (rotational stepper motor)
2. Stroke length (linear actuator)

The stroke length is controlled by the angle of the pump head relative to the drive shaft:

• 0° angle = no reciprocation, flow is zero
• Increasing angle = piston begins reciprocating, flow starts
• Greater angle = more volume per stroke

This adjustment is infinitely variable between 0–100%. The design ensures the piston always bottoms out, eliminating air bubbles—especially critical at low dispense volumes.

Fenyx: Automating Dispense Control

Previously, stroke adjustments had to be done manually or were fixed. The Fenyx pump automates this via a stepper motor with encoder, allowing stroke length to be adjusted programmatically.

• Do not adjust stroke length while the pump is running. Always stop the pump first.

For accurate dispensing, the rotational stepper motor must complete a full revolution. The two stepper motors involved are:

• Rotational motor: rotates the piston (causing both spin and up/down motion)
• Linear actuator: adjusts stroke length (angle of pump head)

The linear actuator resolution must not be exceeded. Overdriving can damage the pump. The encoder at the end of the actuator provides positional feedback. The pump is shipped pre-calibrated, and Fluid Metering will provide the default settings.

Understanding Parked Position

The “parked position” is when the piston is nearly touching the top of the pump chamber. This minimises residual volume when pump is not in operation.

Each dispense cycle should start and stop at defined motor positions. A flag sensor provides rotational feedback. Standard sensors are available, but custom options can be provided to match specific PLC platforms.

• Home: Starting position for dispense
• Park: End position where piston is almost touching the chamber bottom

Priming and Line Filling

FENYX® makes priming simple, even for small-volume applications:

• Set stroke to maximum to quickly fill lines
• Once filled, reduce stroke to normal dispense volume

Line-fill detection methods include:

• Flow meter
• Bubble detector
• Vision sensor (detecting fluid at nozzle)
• Time/rev-based estimation
• Weight/gravimetric check

The optimal method depends on your system. Flow meters are often overkill, as Fenyx does not need recalibration. “Blind dispensing” (based on timing or revolutions) may be sufficient.

Dry Running and Flow Reversal

The pump head can operate dry for short periods only—i.e., with no liquid inside.

FENYX® pumps are bidirectional:

• Reverse flow by reversing motor direction
• Alternatively, move the head past the zero angle in the opposite direction using the linear actuator

PLC Compatibility and Control

FENYX® can be controlled by Fluid Metering’s stepper controller or any PLC-compatible stepper driver. Most PLC integrators prefer their own brand of driver. Controller selection is outside the scope of this document.

This guide assumes the correct pump head, piston size, and wetted materials have already been selected for your application.

Flow Curve Considerations

Changes in suction and discharge pressure will affect flow performance. Consult the flow curve to anticipate these changes in your system design.

Recommended: Evaluation Kit

We recommend using our evaluation kit, which includes:

• A Fluid Metering controller
• Free setup software

This enables initial testing and integration before transitioning to your preferred PLC controller.

Linear Actuator Travel Range

• Total travel: ~5000 steps
• From home (0 point):
  • +4500 steps (forward)
  • –500 steps (backward)

Volume per step varies by piston size:

Quick Start Procedure

1. Connect tubing to inlet and outlet (1/4”-28 flat bottom fittings). Inlet is marked with a nub.
2. Home both motors (rotational and linear).
3. Increase linear position by 4000 steps, run rotary motor at 100–150 RPM to fill lines.
4. Reduce linear by 2500 steps (to position 1500), increase rotary to 300–400 RPM.
5. Once primed, home both motors again.
6. Begin dispensing/metering operations.

Summary

If you know the piston size, you can calculate dispense volume per step based on actuator position.

• The linear motor’s 0 point = 0 volume

• Each step adds volume per the table above.

The linear stepper motor includes an encoder, and the rotational motor uses a flag sensor to determine position. Once you know both motor positions, you can calculate the dispense volume. Dispense rate is controlled by rotational speed and stroke length (angle).