Rise time is generally defined as the time taken to go from 10% to 90% of a commanded value. In propulsion systems, the most relevant measurement is the rise time of thrust. In our tests we measure the rise time of the thrust from 1 N to 4 N because most drones using 2306-sized motors will have a hovering thrust in this range. For every motor-propeller combination, there is a specific command signal that corresponds to a desired thrust. The command values change for each motor-propeller combination because propellers have varying inertias and drag coefficients. The types of command signals include PWM, DShot, and serial, among others. It is important to note that, while certain command signals are faster than others, rise time is independent of the type of command signal.
Before measuring rise time, we find the command values that match with 1 N and 4 N of thrust for each motor-propeller combination. Then, we spin the motor such that it is generating 1 N of thrust, send the command signal to generate 4 N of thrust, and measure the amount of time it takes to get from 1.3 N (10% of the change in command signal) to 3.7 N (90% of the change in command signal) of thrust.
The tests use a 1500 mAh 5s battery, the battery is fully charged before each response is carried out. Thrust is measured using the ATI nano 17 along with an NI DAQ. During the test, we take 8,000 measurements per second. For this first test, we used the same IQ Speed Module (same motor and ESC), so the only variable in the system is the propeller. We used serial communication in this test. We also measured the rise times in open loop and closed loop to demonstrate one of the many benefits of having a closed loop system:
When an IQ motor is in closed loop, its rise time is about three times faster compared to open loop for these propellers. Faster response, or smaller rise time, is better because it allows the motor to follow command setpoints more closely. For the user this means the drone will respond faster and the gains can be more aggressive without overshoot.
As for the propellers, reversible props are meant to produce equal thrust in both directions. This is great for 3D pilots because they need adequate thrust to fly upside down. For any other application, reversible props are bad because they tend to produce significantly less thrust in the desired direction. In other words, props that are meant to spin only clockwise should outperform reversible props of the same size.
Comparison to KISS
How does this reaction time compare to the competition? The KISS ESC is used by both enterprise drone companies and hobbyists, so we are starting with them. For the KISS test, we used the same motor and propeller, but swapped out the IQ ESC for the KISS 32A ESC. We used DShot with the KISS ESC. Here are the results:
The KISS ESC has us beat in open loop mode, so we have some room for improvement on that front. Now that we have figured out the hardware, we can turn our focus to optimizing the firmware. Fortunately, we perform best in closed loop (velocity mode). KISS is not capable of running closed loop, so if we are comparing bests, the IQ ESC is about 3 times faster than the KISS ESC.
Configuring your Module
With this data in mind, we suggest that all pilots use closed-loop, or “Velocity Mode,” to get peak performance. When you receive your IQ Module, you’ll need to connect to the IQ GUI to initialize it. Once you connect to the GUI, you’ll have the ability to change a wide range of settings, including motor direction, 2D vs. 3D, closed loop vs. open loop, and much more.
The one downside of Velocity Mode is that you need to know the characteristics of the propeller you’re using, particularly the motor’s max velocity with a particular propeller. This max velocity is the speed at which your motor begins to overheat, and this value varies from propeller to propeller.
Let’s take a motor-propeller combination that has a max velocity of 25,000 RPM as an exmaple. If you set your max velocity higher than 25,000 RPM, you’ll risk overheating your motor. If you set your max velocity below 25,000 RPM, your quad will feel underpowered.
Fortunately, we’ve managed to characterize a few propellers, including the APC props that were sold as part of the Standard and Reversible Kits. On the “General” page of the GUI, you’ll have the ability to load pre-made settings for your application. If your propeller appears on that list, then select it and press “Set.” You’ll notice the settings will update for your propeller. If your prop doesn’t appear on the list and you do not know its max velocity, we suggest running PWM or voltage mode.