RPM Measurement in an Electric Impeller Drive

As the proud owner of a Unitest2, I’ve made it a habit to measure the power and RPM of an electric model’s motor before its maiden flight. Now the finished EIT (Electric Impeller Trainer; www.keiro.ch) with a built-in Wemotec Minifan (without a propeller, of course!) is sitting in front of me—what now?

Despite the brushless RPM sensor announced by SM-Modellbau, I decided to challenge our electronics apprentice, Timm Baumann, with this problem. The circuit he came up with enables contactless RPM measurement and is easy to replicate.

The UniTest21 features a speed measurement function using a built-in light sensor. Normally, the speed can be easily measured using a flashlight that shines into the sensor through a rotating propeller. With an impeller drive, however, this is more difficult because it is hard to access.

However, the motor’s mechanical speed can also be determined if the electrical frequency and the number of poles of the motor are known. The electronic circuit described below drives a super-bright light-emitting diode (LED) in such a way that the electrical frequency is emitted as modulated light. This light can then be detected (as with the propeller) by the UniTest2’s light sensor.

The speed controller generates a three-phase alternating current in the motor using clocked voltage signals. Pulse-width modulation (PWM) (at least during partial-load operation) is used to generate a quasi-sinusoidal AC voltage on each phase of the motor (U1, oscilloscope channel 1). The high-frequency components of the PWM must now be filtered out. In the circuit, this is achieved using the RC filter (R1, C1). The LED then emits light only at the frequency relevant for speed measurement (U2, oscilloscope channel 2).

As can be seen on the oscilloscope, the voltage across the LED has a large DC component. This causes the LED not to “flash,” but rather to glow more brightly and dimly. The UniTest2 still detects the rotational speed, however, since it responds only to changes in light intensity.

To display the rotational speed, select the “Drive Measurement” function. To ensure that the correct mechanical rotational speed is displayed, you must also enter the number of poles of the motor (switch from the number of blades to the number of poles). This information can be found (if you’re lucky) in the manufacturer’s motor data sheet.

Now hold the UniTest2’s light sensor in front of the lit LED. The maximum reading distance for an error-free display is about 10 cm. Tests confirmed the expected speed range up to 27,000 rpm.

Subsequent field testing outdoors showed that the speed measurement works even in sunlight.

• Motor*
• MEGA ACN 16/15/3 (Brushless in-runner, 6-pole)
• Speed Controller*
• PULSO DL33A+
• Battery (LiPo)*
• Hyperion Lite Storm CX, 3S1P, 4250 mAh
• Speed Sensor*
• UniTest2
• Oscilloscope*
• Tektronix TDS 1001B

Note: The circuit has only been tested with the drive components listed above. With other speed controllers and at significantly higher speeds, it is likely that component values will need to be adjusted. If the circuit does not function, connecting an oscilloscope between the battery’s negative terminal and the motor phase will likely provide clarity. If the oscilloscope displays something different from the measurement shown above (signal shape, DC offset), changes to the circuit will likely need to be made.

The most important thing is to use a super-bright LED, since only a small amount of current flows through the selected resistors and a long viewing distance is desired. Incidentally, the test circuit also worked with red, green, yellow, and white LEDs.

¹ SM-Modellbau, Dipl. Ing. (FH) Stephan Merz, D-82407 Wielenbach; www.sm-modellbau.de

Figure 2: Receiver with drive (green) and speed measurement circuit (blue).

(if you’re lucky), this can be found in the manufacturer’s motor data sheet.

Now, hold the UniTest2’s light sensor in front of the lit LED. The maximum reading distance for an error-free display is about 10 cm. The tests confirmed the expected speed range up to 27,000 rpm.

Subsequent field testing outdoors showed that the speed measurement works even in sunlight.

CH2 (top): Signal across the capacitor (U2)
CH1 (bottom): Unfiltered speed control signal (U1)

Signal waveforms on the oscilloscope.

LED

• red, 5 mm, clear; brightness 3000 mcd at $I_F = 20$ mA; beam angle 30°
• Resistors**
• Metal-film $\frac{1}{2}$ W 3.3 k $\Omega$
• Capacitor**
• Polyester 100 nF/63 V
• Heat-shrink tubing**
• $\varnothing$ 6.4 mm polyolefin

TQ strands

• 0.25 mm^² black (0 V) and yellow (motor phase)

The replica can also be built without a circuit board. The compact circuit shown in Figures 2 and 4 is now soldered directly to the speed controller’s plug-in terminals (any motor phase and battery negative) using 0.25 mm^² stranded wire. It is therefore not necessary to open up the speed controller—which would void the warranty! Finally, the entire assembly is wrapped in heat-shrink tubing. But be careful: Do not allow a short circuit to occur between the wires of the components!

By the way: The circuit should actually also work with an outrunner motor with a propeller drive. But in this case, there’s a simpler solution: The LED permanently installed in the model emits a steady light through the propeller (LED + 3.3 kΩ in series, powered directly from the battery).

In any case, holding a flashlight dangerously close to the spinning propeller is now a thing of the past!

Print it out and head to the hobby room
www.modellflugsport.ch

^Two people interested in building a replica can receive the parts by sending a pre-stamped and addressed C6 envelope plus CHF 2.00 in stamps to Telma AG, Timm Baumann, Gewerbeweg 10, CH-3662 Seftigen (offer valid through August 2010).