THE TRIBOELECTRIC EFFECT

What is the Triboelectric Effect?

The triboelectric effect is the static electric charge developed when different materials come into frictional contact. It is a surface effect and depends on the chemical composition of the surfaces of these substances.

Why do we care?

Our robots have been riddled with static problems for several years, peaking last season with the metal ramp in the RES-Q challenge. We believe our robot built up a static electric charge while running, and the sudden discharge on the metal frame caused our whole control system to crash, cratering our ability to contribute to our alliance’s score. We suffered more than most other teams, possibly because we used more unique materials and lots of sensors. Our static problems put us at a severe disadvantage in competitions, and there were some matches we were barely able to score.

What are we doing about it?

Last season, we tried using copper mesh as a Faraday Cage to shield our wiring and processors. This helped us through the latter half of RES-Q, but this year we aimed to fully understand and resolve our static issues. We looked to our experts, mentors and sponsors to help us achieve our goal. Rogers Corporation (the makers of Poron) kindly loaned us one of their electrometers this fall. We use this device to study the materials we use and to monitor our robot in operation to see the sources of static charge.

What have we learned?

We have learned that certain materials greatly increase our static charge. Tetrix omni-wheels and most commercial zip ties have both proved detrimental because they are made of nylon, which causes  large charges when it comes in frictional contact with the EVA foam (the playing field) and HDPE (the particles).

We also ran into strange problems and occurrences. Using the electrometer , we discovered that although most materials begin neutral, with their charge essentially at 0, molded silicone (our wheel rims and some pickup parts) seems to have an embedded charge, starting much more positive than other materials.

Measuring the charge of materials before and after rubbing them to see if a material has gained or lost charge helps us place materials on our triboelectric effect table. We have found that BUNA rubber, used in the rollers in omni wheels and as the tires on AndyMark wheels, conducts electricity. This can be determined and dealt with by measuring the effect BUNA has on other materials.

While it is very easy to see what materials develop which charges, it is much harder to measure the exact level of charge. The electrometer assumes that there is an infinite and flat surface in front of the detector, which we often do not have, and makes it hard to compare substances (tubing, zipties, and wheels). Nonetheless, the results are consistent: nylon develops a positive charge and latex a negative when rubbed, latex becomes positive and silicone negative when rubbed, and nylon grows positive against silicone. This enables us to rank our materials in a series:

The Triboelectric Series

Materials

+ Positive Ve +

Nylon 

PLA 

Polycarbonate 

PETG 

Cap Ball 

EVA** 

Delrin

Latex 

Neoprene 

HDPE 

HI 

Silicone Tubing, cast silicone

PVC 

– Negative Ve –

**We observed that PLA, Polycarbonate, PETG and EVA were very close in our series

What have we changed?

We are now a lot more careful about the materials we use and the way we use them. We dismantled our omni wheels and replaced the nylon frames with parts we printed using PLA. We also use silicone tubing instead of zipties for our harvester. We are also more careful with design. For example, we do not need the bristles of our harvester to touch the mat, so we make sure they do not, as surfaces that do not come in contact develop no charge.

Has this fixed all of our problems?

No! We still generate high charges, but so far we have not had the robot cut out. We are still learning exactly what actions generate the charges, and we are very interested in learning from other FIRST teams about their materials and their designs. We would also love to hear any feedback or suggestions!

What should other teams do?

We suggest that you use our triboelectric series to switch out materials for ones which have the properties you need but are closer in triboelectric potential to the key field components, because no one likes a dead robot!

We also suggest that you reduce or eliminate rubbing in your designs, as this process, especially with materials farther apart in our Triboelectric Series, can generate very high and damaging charges.

This year, we are also keeping our wiring and electronic components as far from the frame as possible, as the frame can hold a lot of the charge. When the frame discharges, it is best not to have your valuable parts suffer the same shock.

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