Practicum 1C

The parameters we seek to measure in this practicum include the ROV’s mass, buoyancy force, and thrust force.  Sensors called load cells, such as the one shown below in Fig. 1, convert such forces to voltage. We will be using the FC2231 10lb load cell connected to a MyDAQ to measure the forces we are interested in.  

There is one complication to using the load cells to measure the buoyancy response of a submerged ROV: load cells are not waterproof.  We will use a device called a force measurement stand (FMS) to translate the forces we’re interested in into forces applied to a load cell.

 Your data set should include a list of (voltage, mass) pairs, where the voltage was measured by the load cell for each different mass hung from the top hook of the FMS (see Fig. 1). This data will be used to create a calibration curve relating voltage to applied force.  There are a few points you need to consider when doing the calculations for this curve.

Open up your spreadsheet and type in (or load) your data set. Convert the calibration mass into force by multiplying by g.  This may be easiest to accomplish by adding an extra column to your spreadsheet to list the force value for each mass value.

Calculate the force each mass applies to the load cell using your torque balance equation from this week’s tutorial. It may be easiest to create another column in your spreadsheet.

Plot the four data points in Excel, e.g. using a Marked Scatter Chart. The plot should have Force on the y – axis and voltage on the x – axis.

The plot should be relatively linear. Under Excel’s Chart Design tab, go to Add Chart Element -> Trendline -> Linear to add a trendline to the plot. Right click the trendline and select Format Trendline. Scroll to the bottom and click the Display Equation on Chart option to yield your calibration equation. You now have an equation that converts your measured voltage to a force.  Screenshots of this process appear in Fig. 4.

At your FMS station, you should find a laptop running a VI named “P1C.vi”. If it is not running, click the run button. This VI reads measurements from the FMS load cell via a myDAQ, as well as opening up a PWM control. Fig. 5 shows the one front panel. The load cell measurements are plotted in real time (see chart with blue line). The mean value of the load cell data (using a moving window average) is displayed in a field just right of the chart. 

A slider is used to control the PWM signal being sent to the thruster at the bottom of the FMS. See the right pane in Fig. 5. This won’t be used until the next section.

Be sure the VI is running, and press gently but firmly on the top-most horizontal beam that rests on top of the load cell. You should see the voltage level displayed in the chart respond to your pressure. An example of a typical LabVIEW output when force is applied to the FMS appears in Fig. 6.

Place the 500 g weight on the top hook. You might need a step stool for this. If you can’t reach, ask a proctor or instructor for help. Wait for all motion in the system to dissipate and the voltage to reach a steady state value, i.e. the voltage tenths should not be changing much with time. Write down the mass being used, as well as the measurement displayed in the field labeled Filtered Voltage located on the front panel of the P1C .vi, (see Fig. 7). Repeat this for the remaining weight values (e.g. 500g, 1000g, 1500g, 2000g).  Finally, remove all of the weights and record the voltage with zero mass.

Construct a motor thrust curve using VI set up on the laptop. Adjust the duty cycle, which is the fraction of the motor’s thrust we are commanding, from 30-80% in 10% intervals. Record the duty cycles and the corresponding load cell voltages from the field labeled Filtered Voltage. 

You can try 100% as well. For some motors you may observe a phenomena called resonance which we will learn about in a few weeks.

Create a copy of the simulator from practicum 1B called VDSim.vi in your library and name it VDSim_PWM.vi. Modify the parameters in the new simulator to include the new experimentally determined values for mass and buoyancy.  

Make sure you use mass in kg, and that your buoyancy value accounts for the fact that the FMS was measuring both the gravitational force and buoyancy when the ROV was submerged. Assume that the drag coefficient is 10 kg/s.

Modify the new simulator to include the linear model of thrust as a function of PWM signal, i.e.: replace the throttle slider with a PWM slider and a multiplication block and addition block to make the slider match the linear fit to the thrust vs. duty cycle data. (See Fig. 10b for a hint). Consider if the simulator behavior matches what you observed in the tank. 

Discuss what happens with an instructor or proctor.