Commit c4004e55 authored by David Preiss's avatar David Preiss
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dyno progress

parent a6fcd5d2
# 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).
![alt_text](images/dyno/overviewTemp.png "image_tooltip")
### 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.
![alt_text](images/dyno/encoder.png "image_tooltip")
### 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.
![alt_text](images/dyno/torqueTransmit.png "image_tooltip")
Torque is measured with a [5kg load cell](https://www.phidgets.com/?tier=3&catid=9&pcid=7&prodid=224) and a [HX711 Load Cell Amp](https://www.sparkfun.com/products/13879) 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).
![alt_text](images/dyno/cad.png "image_tooltip")
### 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](https://bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/168xB_manual.pdf) 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](https://en.wikipedia.org/wiki/Flyback_diode), 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.
![alt_text](images/dyno/nonInverting.png "image_tooltip")
# Stall Condition Motor Test Rig
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