Dynamometer

Below is a high level overview of the dynamometer showing each major subsystem (aside from the power measurement IC which is being shipped from digikey). Each one is discussed in further detail below (please pardon my use of a breadboard at this stage :D).

RPM Measurement

RPM is measured with a 32ppr optical quadrature encoder mounted to the back of the absorber. Only one channel of the encoder is used, but rising and falling edges are both used. Measuring RPM is then done with a single interrupt triggering a counter and a timer.

Torque Measurement

From the pictures below you can get an idea of how torque from the test motor travels through the frame and is ultimately constrained by the load cell. From early testing this system is sensitive down to the g*mm.

Torque is measured with a 5kg load cell and a HX711 Load Cell Amp on a breakout board with serial output using its own protocol (it doesn't actually use a peripheral). Torque was calibrated using 1kg and 0.5kg weight at the 75mm moment arm shown below. With the configuration below, the setup is capable of measuring a peak torque of 54kgcm, which can be increased to 208kgcm with a 20kg load cell (or ~20Nm).

Absorber and Back-Torque Generation

A brushed DC motor with a protected variable power supply is being used as the absorber, with the primary benefit of low RPM torque generation. Alternative absorbers considered were an eddy current brake (or an industrial induction motor with variable DC applied to one of the windings), or a bicycle disc brake.

Here's a link to the BK-1687B power supply which can accept 0-5V voltage and current control (also via USB and the DRO). The motor is a Midwest Motion Products brushed DC motor rated to 12A and 24V and 2000 rpm MMP D33 655E 24V. The power supply is protected via a diode in series with the absorber motor, which effectively keeps the absorber in open circuit until the power supply overcomes the back EMF of the absorber, and then (at least from my understanding) the DC motor starts generating torque proportional to the forward current. Inside the variable power supply is a rectifier and buck converter capable of closed loop voltage and current control. It's dangerous to drive large inductive loads (like motors) with switching power supplies because of flyback, and so there are two diodes oriented as shown below, where D1 keeps the absorber's windings (while being driven) in an effectively open circuit, and D2 is insurance in case that diode fails.

Because the power supply can only be controlled to 24V with our 3v3 analog outputs, a non-inverting op amp (OPA2337) was used to achieve the full 0-5v range. Because most of the micros that I use operate at 3v3 I am planning to permanently adhesive the breadboard that I used to do this to the back of the power supply (also convenient because the power supply has both the 0 and 5V rail voltages required.

Stall Condition Motor Test Rig

Goals

• Standardize peak torque data relative to power (servos are typically giving peak torques when subject to many amps of current)
• Add real time temperature data to get an idea of sustainability of above ^
• Better understand tradeoffs between DC motors (hobby servos) - Steppers - BLDCs (both with and without reduction)
• Inform design/selection for future actuator use cases
• Complement a real dynamometer to acheive higher fidelity at stall condition

Link to motor testing data

This rig allows for peak torque measurements at stall for a variety of actuators, and precise measurements of torque ripple (in motors with less/no reduction). It also allows for measurement of torque required for backdrivability, particularly in motors with significant amounts of reduction, as well as angular jitter from the motor's feedback loop.

Measurements include torque and rotational position and a 5kg (interchangeable) load cell with an HX711 amplifier and an AMS 5047D rotary encoder measuring 14 bits of shaft rotation. The stepper shown below is just a shell for mounting the shaft, and has had its rotor removed and replaced with a dummy stainless shaft (to eliminate cogging). The EDM'd 1-piece shaft coupler shown in the photo below added a ton of torsional elasticity to the measurement and was later replaced with a spider'd shaft coupling.