Why Does Speed Feedback (FG Signal) Matter in Your Precision Micro Pump Application?

You need consistent, repeatable results from your device, but performance drifts over time. Your micro pump seems to be running, but your system's output is becoming less accurate, causing failures.

This drift often happens because your micro pump's speed changes with load, temperature, or age. A speed feedback (FG) signal is the only way to know the micro pump's true speed and correct it in real time.

As a project manager here at BODENFLO, I often see OEMs design sophisticated systems around a simple "open-loop" micro pump. Initially, everything works perfectly. But months later, field reports come in about inconsistent performance. The root cause is almost always a change in the micro pump's motor speed that the system had no way of seeing or correcting. This article is about why that tiny third wire for the FG signal isn't a luxury feature—it's the foundation for building a truly precise, reliable, and intelligent device.

What Is an FG Signal in a Micro Pump and How Does It Work?

You've seen "FG Signal" on micro pump datasheets, but you aren't exactly sure what it is. You might think setting a PWM duty cycle is enough to control speed, but it's not the same thing.

An FG (Frequency Generator) signal is a digital pulse train from the micro pump's motor that reports its real-time rotational speed. Each pulse represents a fraction of a motor revolution.

Let's demystify this signal. It’s much simpler than it sounds and incredibly powerful once you understand its purpose.

How is it Generated?

Inside a brushless DC motor, Hall effect sensors are used to time the electrical switching that makes the motor spin. A side benefit of these sensors is that they can also be used to generate a feedback signal. As the motor's rotor turns, the sensors detect the passing magnets and output a series of square-wave digital pulses. This pulse train is the FG signal¹.

What Does it Represent?

The key is the pulse-per-revolution² principle. A micro pump's datasheet will specify how many pulses the FG signal outputs for each full rotation of the motor. For example, a micro pump might have a spec of "6 pulses per revolution." If your system's microcontroller counts 600 pulses per second, you can calculate the real-time motor speed:
(600 pulses/sec) / (6 pulses/rev) = 100 revolutions/sec = 6,000 RPM.

FG Control vs. Speed Control

It's crucial to distinguish between setting speed and knowing speed.

• PWM/Voltage Control: This is like pressing the gas pedal in a car. You are telling the micro pump how much power to use. It doesn't guarantee a specific speed.
• FG Signal: This is like looking at the car's speedometer. You are reading the actual speed the micro pump is achieving.

Why Is Open-Loop Speed Control Not Enough for Precision Applications?

You set your micro pump to a fixed 50% PWM duty cycle, expecting a constant flow rate. But over a long test, the flow rate slowly drops, even though your input command hasn't changed.

Open-loop control is not enough because it can't adapt. It assumes the relationship between your command (voltage/PWM) and the micro pump's speed is constant, but in the real world, it's not.

This is the most common "silent failure" I troubleshoot with clients. The system thinks the micro pump is doing its job because the command is correct, but the physical output tells a different story.

The Problem with Assumptions

An open-loop system³ operates on a simple assumption: X volts = Y RPM. This holds true only under perfect, unchanging conditions. In reality, several factors cause this relationship to break down:

Factor	How It Causes Speed Drift
Changing Load	As a filter clogs or tubing ages, system resistance increases. The motor must work harder, and with a fixed power input, its speed will drop.
Temperature	As the motor heats up during operation, the resistance of its windings changes, affecting its efficiency and speed for a given voltage.
Voltage Fluctuation	In battery-powered devices, the supply voltage drops as the battery drains. With a fixed PWM duty cycle, the effective power sent to the motor decreases, slowing it down.
Component Aging	Over thousands of hours, mechanical wear on bearings or demagnetization of magnets can slightly reduce motor efficiency, leading to a gradual speed reduction.

Without an FG signal, your control system is flying blind. It has no way of knowing that these factors are causing the micro pump speed⁴ to drift away from the target, leading to inaccurate and inconsistent performance.

How Does FG Speed Feedback Improve Flow and Pressure Stability?

You need to deliver a precise volume of liquid or maintain a stable gas pressure. Any fluctuation in micro pump speed directly ruins the accuracy of your device's primary function.

FG feedback enables a closed-loop system that directly stabilizes flow and pressure. By monitoring the real speed, your controller can instantly adjust power to hold the RPM steady, regardless of external changes.

The connection between micro pump speed and performance is direct and absolute. For a micro diaphragm pump⁵, flow rate is a direct function of the micro pump's cycles per minute, which is determined by motor RPM.

Real-Time Speed Correction

Imagine your application is a medical infusion device that must deliver exactly 100 mL/hour. This delivery rate corresponds to a micro pump motor speed of, for example, 3,250 RPM.

1. Set Point: Your controller commands the micro pump to run at 3,250 RPM.
2. Disturbance: The patient moves, slightly kinking the IV tube. This increases the backpressure (load) on the pump.
3. Speed Drop: Without feedback, the micro pump's speed might drop to 3,100 RPM, reducing the flow rate and under-dosing the patient.
4. FG Signal Reports: The FG signal immediately reports the speed drop to the controller.
5. Correction: The controller's closed-loop algorithm⁶ instantly increases the PWM duty cycle, providing more power to the motor until the FG signal confirms the speed is back at the 3,250 RPM set point.

This entire correction loop happens hundreds of times per second, ensuring the micro pump's speed—and therefore its flow rate—remains rock-solid even when the system's conditions are changing. This transforms an unstable system into a precise, reliable one.

What Happens When Load Conditions Change Without Speed Feedback?

Your gas analyzer works perfectly with a new filter, but after a week in the field, its readings start to drift. You might blame the sensor, but the real culprit could be the sampling micro pump.

Without speed feedback, changing loads cause a slow, silent degradation of micro pump performance. The micro pump appears to be running normally, but it's actually moving less fluid, leading to critical system-level errors.

Let's walk through a very common real-world scenario that I've helped clients solve many times. The application is a portable air quality monitor that samples a fixed volume of air per minute.

The Scenario: A Clogging Filter

• Day 1: The device is deployed with a new, clean particle filter. The system commands the sample micro pump to run at a speed that delivers 1.0 liters per minute (L/min). The load is low, and the pump easily achieves this.
• Day 7: The filter has started to accumulate dust and particulates. The resistance to airflow (load) has increased. The pump motor, receiving the same fixed voltage as before, now has to work harder and its speed naturally drops. Without an FG signal, the system doesn't know this. The actual flow rate might now be only 0.9 L/min.
• Day 30: The filter is significantly clogged. The pump's speed has dropped even further. It's now only sampling 0.7 L/min, but the system's software is still calculating pollution levels based on the original assumption of 1.0 L/min.

The result? The device reports pollution levels that are 30% lower than reality. This is not a pump failure; it is a system design failure caused by the lack of speed feedback.

Which Precision Micro Pump Applications Truly Require FG Feedback?

You're designing a new device and trying to decide if the added complexity of an FG signal is worth it. You need to know if your specific application falls into the "must-have" category.

FG feedback is not optional in any application where repeatability, traceability, or adaptive control are functional requirements. This includes most analytical instruments, medical devices, and advanced industrial automation.

Instead of thinking in terms of industries, it's more helpful to think in terms of functional needs. If your device's success depends on any of the following, you need FG feedback.

Application Type	Why FG is Critical	Example
Analytical Instruments	Repeatability: Ensures the same volume of sample or reagent is used in every single test, guaranteeing consistent and comparable results.	Gas Chromatograph, Water Quality Analyzer
Medical Devices7	Accuracy & Safety: Delivers a precise, documented dosage of medicine or maintains a stable negative pressure for therapy. Essential for patient safety and treatment efficacy.	Infusion Pumps, Negative Pressure Wound Therapy
Gas Sampling Systems8	Traceability: Provides a verifiable record that the micro pump was running at the correct speed to sample the required volume, crucial for environmental compliance.	Stack Emission Monitor, Personal Exposure Monitor
Industrial Automation	Adaptive Control: Allows a system to maintain stable performance while handling materials with varying viscosity or as components like nozzles wear down over time.	Automated Dispensing, Pick-and-Place with Suction

In these applications, "close enough" is not good enough. The ability to know and control the exact micro pump speed is a fundamental requirement.

How Do FG Signals Enable Closed-Loop Control and System Diagnostics?

You want to build a "smarter" device that can not only perform its function but also monitor its own health. You need a way for the core components, like the micro pump, to report back their status.

FG signals are the key. They provide the raw data needed for a controller to implement true closed-loop speed regulation and enable powerful diagnostic features like blockage detection and preventative maintenance alerts.

Moving from open-loop to closed-loop is like upgrading from a basic tool to an intelligent instrument. The FG signal is the sensor that makes this upgrade possible.

Enabling True Closed-Loop Control

As we've discussed, the primary role of the FG signal is to enable a closed-loop control system (often a PID controller) in your device's firmware. This system constantly works to minimize the "error" between your desired speed (Set Point) and the actual speed reported by the FG signal. This is what provides the rock-solid stability that precision applications demand.

Unlocking Advanced Diagnostics

The real magic happens when you start analyzing the relationship between your command and the FG signal's response.

• Blockage Detection: Suppose you command the micro pump to run at 4,000 RPM, which normally requires a 60% PWM signal. If the controller suddenly has to push the PWM to 95% to maintain that same 4,000 RPM, it's a clear indication of a blockage. The system can then trigger an alarm.
• Leak Detection: In a vacuum application, if the micro pump suddenly requires much less power than usual to achieve the target speed, it could indicate a system leak.
• Predictive Maintenance⁹: By logging the required PWM to maintain a target speed over months of operation, you can track the micro pump's health. A gradual increase in required power can signal that the motor is aging and may need to be serviced, preventing unexpected field failures.

How Should You Evaluate and Integrate FG Signals Correctly in OEM Micro Pump Design?

You're convinced you need an FG signal. Now you need to know what questions to ask and what to look for to ensure a smooth and successful integration into your design.

Proper integration requires confirming specifications with your supplier and interpreting the data correctly. Getting this right early on prevents costly hardware and firmware revisions later in the development cycle.

As an engineer on the manufacturer side, I always appreciate when a client asks these questions upfront. It shows they are thinking about system-level performance and sets the project up for success.

What to Confirm with Your Micro Pump Supplier

1. Pulses Per Revolution (PPR)¹⁰: This is the most critical spec. Is it 2, 4, 6, or something else? Your firmware calculation depends entirely on this number.
2. Signal Type (Pull-up/Open-Collector): Most FG signals are "open-collector," meaning you need to add an external pull-up resistor on your circuit board to get a readable high/low voltage signal. Confirm the required resistor value and pull-up voltage.
3. Maximum Voltage and Current¹¹: Ensure your pull-up voltage and the resulting current do not exceed the absolute maximum ratings for the FG output pin. This prevents damage to the micro pump's internal electronics.

Best Practices for Integration

• Use Interrupts, Not Polling: For the most accurate speed reading, configure your microcontroller to use a hardware interrupt to count the rising or falling edges of the FG pulses. This is more reliable than "polling."
• Implement a Moving Average: To get a stable RPM reading, calculate the speed based on the number of pulses counted over a fixed time window (e.g., 100 milliseconds) or use a moving average filter in your firmware.
• Balance Cost and Benefit: The added cost of a micro pump with an FG signal is minimal. When you weigh this against the immense system-level benefits—stability, reliability, and diagnostics—the decision is clear for any performance-critical application.

Conclusion

Speed feedback via FG signals is not a luxury feature for micro pumps—it is a fundamental enabler of precision, stability, and long-term reliability in performance-critical systems.
For applications where flow accuracy, pressure consistency, traceability, or system diagnostics matter, open-loop control is simply not sufficient. FG speed feedback transforms a micro pump from a passive component into an actively controlled, measurable, and diagnosable subsystem.

At BODENFLO, we work closely with OEMs to design and supply micro pumps with properly implemented FG feedback, optimized for closed-loop control, long-term operation, and real-world load variation. Whether you are developing medical devices, gas sampling instruments, or precision automation equipment, our engineering team can support you from signal definition and electrical integration to pump selection and system validation.

📩 Contact BODENFLO to discuss your FG-enabled micro pump requirements:
Email: jean@bodenpump.com

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