Nice to have a lot of vex parts around to use in FRC field element or robot element mock ups. This is a testing mockup of the touchpad which activates a light if any of the three posts holding the bottom plate are deflected at least 1/2 inch. The vex green rubber links provide the flexibility for any aluminum beam to be pushed without jamming in the holes of the top plate.
Here are some pictures of how a double reverse 4 bar using Vex 35 hole link arms. The main challenge is constructing a stable geometry. The lift has a middle link gear train to move the upper 4 bar in sync with the lower 4 bar.
Approx torque (inlb) required = 2*l_arm*(W_payload_lbs + W_lift_lbs + W_manipulator_lbs)*cos(angle)
height_delta = 2*l_arm*( sin(angle_finish) – sin(angle_start))
where l_arm is the distance between pivots of the arms in inches, W_payload_lbs is game piece weight, W_lift_lbs is the weight of the 3 arms used in the lift , W_manipulator_lbs is the weight of the gripper attached to the upper arm and angle is defined relative to the horizontal. The max torque occurs when the arms are horizontal or angle • 0 .
If you want to lift 4 1 lb Skyrise cubes with a 3lb lift weight , a 1 lb gripper and a l_arm of 16 in you would need about 256 in lbs of torque.
As a rule of thumb I use 6 inlb of torque per motor for sizing the number of motors. This assumes 2 inlbs of elastic support , 1 inlb of friction and 3 inlbs from active current (about .9 amps) hold per motor used. With these assumptions a 10:1 gearing and 4 393 motors might do the job. The lift would take about 5 seconds to go full travel. I’ll show a more exact torque derivation later.
Our GHCHS Algilata OpenROV project is not located near the ocean. The OpenROV community has found that salt water operation can be flakey compared to fresh water due to low resistance between the salt water and external wires that connect the battery tubes and the motors. This post deals with making simulated sea water for testing the OpenROV in the lab.
Sea water conducts electricity due to the dissolved salts that produce ions for transporting charge between conductors submerged into the water. Typical sea water has 35 parts per thousand by weight of salt in water. Since water at standard conditions weighs 1000 grams/liter then we can say that sea water has 35g of salt per liter.
I wanted to use just a cup measure to make a batch of sea water. So I weighed one cup some Himalayan salt and found that it weighed 8 oz. So we can estimate the weight of salt using this ratio…about 1 avoirdupois oz weight per 1 fluid oz . Of course this will vary with the granularity of the salt due to variations in packing density but it should be good enough for conductivity testing.
Given that there are 28.3 grams per avoirdupois oz and 33.8 fluid oz per liter the sea water concentration of 35 gm per liter converts to 1.24 avoirdupois oz per 33.8 fluid oz or
1 avoirdupois oz per 27.2 fluid oz.
Since 1 cup (8 fl oz) of Himalayan salt weighed 8 avoirdupois oz then I would need to mix this with 217.6 fluid oz of water or 27.2 cups of water (1.7 gallons)
So I now have a simple rule of thumb for adding granulated salt to water using a volume measure:
volume ratio salt:water 1 : 27.2
Other useful equivalents: 5.7 oz salt per gallon of water
1/4 cup salt to 6 3/4 cup water
1 tablespoon salt to 1.7 cups water
Measuring salinity using conductivity:
Scientist often use conductivity to estimate salinity. Standard units of conductivity are Siemans/meter (S/m). The electrical conductivity of 35 ppt salt water at a temperature of 15 °C is 42.9 mS/cm (ref). Thus 35 ppt equates to 42.9 mS/cm.
Conductivity measurements assume that there are two parallel electrical plates of area A in water at a distance L apart. If a voltage (V) is put across the plates and the current flow (I) measured then the conductivity k = I/V*L/A = L/(R*A) where R is the resistance V/I. Under ideal conditions the conductivity between the ROV wires and the water should be very low (high resistivity) but if a small area of copper is exposed then conduction can occur. eg some OpenROV forum members are finding resistance on the order of kiloohms rather than megohms.
Practically, if you put two probes from an ohm meter into water, the measured resistance will depend upon the area of the probe submerged and the distance between them. You can use this as a reference to test your simulated sea water at home.
I made a simple crude conductivity instrument out of a two prong to three prong electrical plug adapter. The plug prongs are separated by 1 cm and the exposed area between the prongs is almost exactly 1 sq sm. I covered the non-facing sides with tape or you could use paint or nail polish to insulate the surfaces from water. See photo.
In the field, dip the plug tester prongs into the water and measure the resistance between them. Note the temperature since conductivity varies a lot with temperature.
Theoretical plug conductivity prediction for sea water.
k = L/(R*A) = 1/R S/cm
= 1000/R mS/cm
Typically sea water resistance in ohms for this homemade instrument at 15 deg C would be
R_ohms = L/(A*k)= 1cm/(1cm ^2)/(42.9 mS/cm) = 1000/42.9 = 23.3 ohms
When creating your simulated sea water at home you would like to have similar conditions. You would add salt to your water tank/tub until the resistance level matched.
I did a quick conductivity test by dissolving 1 tablespoon of Himalayan sea salt in 1 3/4 cups of water. The measured resistance using my plug conductivity tester was around 3 kohms with a VOM meter and using the voltage / current method the resistance was 230 ohms. So it is reading much higher than the theory. The test was done at 70F (21C). Temperature changes the conductivity about 2% per degree. The measurement was 6 deg C higher so at most we would expect a 12% increase over the 15 C reference.
The absolute measurements can vary too much with conductivity so I would recommend just using the 35g/kg salt/water mixing method or just doing relative conductivity measurements..i.e. matching field conductivity to home conductivity under similar conditions.
GHCHS Team 599 Robodox attended the 2014 Inland Empire Regional FRC competition. We collected the Engineering Design award and also the competition finalist award and medals. It was a well run competition and we had a great time. Once again we had a top performing robot that got beat by a stronger no 1 alliance. We made an operational mistake in the finals that cost us the first match by breaking communication during setup and causing our Crio to need resetting after the match started and we sat out the Autonomous plus some seconds. Our alliance partners 294 and 4139 were having functional problems in the second so we were soundly trounced by the winning alliance led by 1678 citrus circuits who teamed up 399 and 4161. The 2nd final match (see video) was a thing of beauty with 1678 and 399 performing two truss catches and racking up a 229 to 72 score. Hopefully we can redeem ourselves at our next regional in Sacramento where we will once again tangle with 1678 citrus circuits from Davis. Also, thanks again to 294 for selecting us for the direct eliminations.
Winning the engineering design award means a lot to us since this year we focused on doing a 3D Solidworks design supported by solid prototyping. All fabrication was done based upon automated drawings made from the 3D model. This the best looking robot we have done in years and clearly the most durable. See this post for picture.
Lots more pictures on my facebook page.
The choo-choo catapult reset mechanism performed well so long as we kept the linkages in good order. The high forces caused holes in the linkages to elongate after a day worth of shooting. This was anticipated so we brought three spares and used them all. We will use steel linkages rather than aluminum at our next competition so they should last longer.
Robodox also ran the robot First Aid Station and the spare parts booth. We also had on display our underwater ROV which will be used by Algalita Research Foundation to do plastic pollution exploration in the Pacific Gyre this summer.