Lifetime Test Procedure
Overview
Vertiq’s lifetime test seeks to determine the life expectancy of its modules under realistic operating conditions. During flight, drone propulsion systems spin at a wide range of RPMs. They will sustain operation at hover speed, run at full throttle when necessary, and rapidly change RPM to stabilize and maneuver effectively. Aerial vehicles are also subject to gyroscopic forces, which cause the propulsion system to experience combined axial and radial loads. Lastly, drones fly their mission and then land to stow away or recharge, causing the propulsion system to thermal cycle. The combination of the aforementioned factors eventually causes the mechanical and electrical deterioration of the propulsion system. Vertiq has therefore created a lifetime test that incorporates these characteristics of typical drone operation such that one hour of testing is equal to one hour of flight time.
Vertiq’s goal is to create the longest lasting motors and ESCs on the market. Data from lifetime testing allows us to iterate on designs to further improve the reliability and longevity of the product. These tests also allow us to state an expected lifetime with a high degree of confidence. Disclosing our lifetime testing method will ideally give our customers an equal amount of confidence in the published lifetime expectations.
Test Requirements
Hardware Selection
Module selection: Identify the desired module for testing.
Propeller selection: Regardless of the module size, customers use a wide range of propellers to achieve different goals. Some customers spin small propellers rapidly to achieve high top end thrust, compromising efficiency in hover. Others spin larger propellers slowly to increase hover efficiency but have a lower max thrust. For the purposes of lifetime testing, Vertiq tends to select smaller, reasonably sized propellers. We do this because the most common failure point on these modules are the bearings, whose failure correlates with the number of rotations. With a smaller propeller spinning faster, we will accumulate rotations more quickly and therefore reach a failure point faster, while having adequate load on the bearing. As such, our lifetime expectations should be conservative for those using larger propellers.
Hours
Minimum Test Hours: Based on market research, Vertiq identifies the minimum acceptable lifetime for its customers. If the tested modules exceed this threshold, we can release the product to the public. If the module does not exceed this threshold, we will conduct an immediate and thorough failure analysis followed by a redesign of the product.
Maximum Test Hours: Based on market research, Vertiq identifies a lifetime that matches or exceeds the high end of customer requests within reason. Due to resource constraints, we need a cut off time for these tests. Even if the modules are still performing well when they hit the maximum hour mark, Vertiq will publish the maximum hour value as the rated lifetime.
Published Lifetime: The published lifetime rating is the average hours achieved by the initial test units. Follow up lifetime tests may be conducted with more modules and a revised rated lifetime can be published.
Test Points
The lifetime test must hit the following test points:
Maximum Operating Point: the maximum speed and torque the motor-propeller combination can achieve while staying below its steady-state thermal limit.
Hover Operating Point: 60% of the Maximum Operating Point Speed.
Negative Maximum Operating Point: the maximum speed and torque the motor-propeller combination can achieve while staying below its steady-state thermal limit while spinning in the opposite direction. This operating point is used in the reversing portion of the lifetime test.
Tilting Period: The modules will be rotated during the lifetime test to apply gyroscopic forces on the modules similar to those they would experience in flight. The period of the tilting is an attempt to mimic real-world rotation rates. Rotation rates are positively correlated with propeller size, so larger propellers will have relatively longer periods. Vertiq determines the tilting period based on an analysis of available flight controller data for propulsion systems of a similar size to the test module and propeller.
Test Setup
Lifetime Test Jig
The lifetime test jig consists of multiple mounting plates on a rail. As such, multiple modules can be mounted and tested on the jig simultaneously. On either end of the rail, there are separate motors (“gyro motors”) used to rotate the rail. The power supply is set to the module’s max rated voltage. The test modules are paired with the selected propellers.
Data Collection Setup
Data is saved to text files on a local PC used to conduct the test. Remote access allows us to monitor tests over nights and weekends.
Test Script
The module(s) tested are commanded to run in two primary methods:
Sine wave of varying speed ±5-10% around desired operating points
Command a speed typical of a hover with the motor and propeller combination.
Command a speed typical of burst or maximum throttle.
Optional: Command 1 or more speeds between the two operating points above.
This is approximately 80% of the test.
Reversing:
Command hard speed reversals from a burst speed in one direction to the same speed in the opposite direction.
Command a low speed on other test motors to both reduce demand on the power supply and give those motors a cool-down period.
This is approximately 20% of the test.
Tilting: as these tests occur, the rail on which the test modules are mounted will tilt ±15° continuously, at a rate appropriate for the propeller size and expected vehicle dynamics.
Alerts & Automatic Stoppages:
Since a Vertiq engineer is not continuously watching the lifetime test, we have set up alerts and automatic stoppages as a safety precaution.
Events such as the following will trigger an alert and/or stoppage:
Total power consumption goes above a set limit,
Motor speed is not close to the target for a long period of time,
Motor speed is zero
Minimum or maximum hour threshold met
Test Procedure
Pre-Lifetime Test Performance Sweep
Conduct initial performance tests before the lifetime test.
Go to the Performance Testing section of this document for more details.
This initial test data will provide a baseline for the module against which subsequent performance test data will be judged.
Lifetime Test
Attach the appropriate propeller to all motors under test. When testing more than one module at a time, use both counter-clockwise and clockwise propellers.
Mount one to four modules to the jig, depending on the module.
Set up the power supplies. Ensure the current and voltage settings are set appropriately with the current limit turned up to the maximum. Set the power supplies to an appropriate voltage to account for voltage drop on the line. Use a large capacitor in line.
Turn on the modules.
Turn on the gyro motor. Ensure that no propellers can strike the ground or other objects throughout the full range of tilt.
Run the Python test script once the module is setup on the jig
Conduct periodic inspections. More details can be found in the Inspection section of this document.
Note: if a module fails during lifetime testing, we will proceed to the Final Disassembly Procedure.
Post-Minimum Hour Performance Sweep
Once the module reaches its minimum hours of testing, conduct a performance test again and compare the results to the initial data.
If the module is still performing well, continue the lifetime test.
If the module is underperforming or has visibly concerning wear, we will conduct an immediate and thorough failure analysis followed by a redesign of the product.
Post-Maximum Hour Performance Sweep
Once the module reaches its maximum hours of testing, conduct a performance test again and compare the results to the initial and minimum hour data.
Whether the module has signs of wear or not at this point, we will conclude the test.
After the performance test, we will proceed to the Final Disassembly Procedure. Between the maximum hour performance sweep and the disassembly analysis, we will better understand how to improve the design to increase module lifetime in the future.
Performance Testing
Performance testing shall be performed in all test modules before and after the lifetime test to quantify any performance changes as a result of many hours of running. The following tests shall be performed, following the procedures outlined below.
Efficiency Testing
Mount the test module on the appropriate thrust testing stand using the proper mount.
Attach the selected propeller to the test module.
Run a voltage sweep test to measure the efficiency across the voltage range of the module.
Step Response Testing
Keep the motor and propeller combination on the thrust stand.
Run the step velocity test to measure the rise time of the module. The velocity steps that should be performed are:
10-50% throttle
30-70% throttle
50-90% throttle
10-90% throttle
Health Testing
Remove the motor and propeller from the thrust stand.
Remove the propeller from the motor.
Connect the module to the calibration jig.
Run the module “health check,” which measures the velocity ratios of forward and backward operation, among other basic parameter checks.
Inspection
Test modules should be inspected periodically. Our usual pace has been 16 hours/day on weekdays and 24 hours/day on weekends. Inspection should occur between these runs. There should be a maximum of 100 hours of runtime between each inspection. It is acceptable to log many hours throughout the weekend without physical inspection, if examination and graphing of the logs look okay.
The procedure for a scheduled inspection is as follows:
Visually inspect each test module. Look for signs of wear, particularly around the shaft.
Wiggle the rotor to check for play.
Attempt to pull the rotor off to check for loose shaft nuts and/or wear.
Document any irregularities.
Final Disassembly Procedure
After the conclusion of the test, whether at the end of the maximum hours or due to failure, all test modules must be disassembled and inspected for wear. The procedure for inspecting each module is as follows:
Visually inspect the test module. Look for signs of wear (similar to mid-test inspections)
Wiggle the rotor to check for play.
Attempt to pull the rotor off to check for loose shaft nuts and/or wear.
Remove the stator cover. Visually inspect the bottom of the ESC for any damage.
Desolder the PCB. Visually inspect the top of the ESC for any damage.
Inspect shaft nut and magnet. Note any visible wear/damage (e.g. magnet coming out of shaft, shaft nut loosening, etc.)
Attempt to remove shaft nut with socket wrench. Determine if glue is still intact. If the nut cannot be removed with tools, machine off the shaft nut.
Disassemble the rotor from the stator. Determine if glue is still intact.
