Managing Electrical Noise and Static

Started in the 2023-2024 season, Tech Tips are segments released in the FIRST Tech Challenge Team E-mail Blast. Sometimes the Tech Tips are included in whole in the email blast, but sometimes there is more content than is reasonable in the email blast so partial content is included in the blast with the rest of the content here.

Signal Filtering with Ferrite Cores

For those about to use sensors, we salute you! This section focuses on signal noise and how to eliminate it with ferrite cores.

When deciding to use a sensor on a robot, we’re normally worried about how accurate the sensor’s detection is, how much the sensor costs, or how the sensor’s protocol will interface with the control system. It isn’t until the device is being mounted to the robot before we consider how outside electrical noise already present on the robot might significantly impact the performance of the sensor. This electrical noise almost exclusively comes from the electric motors and other sources of electric fields on a robot, such as power wires, power supplies, some sensors (especially ultrasonic sensors and cameras), radio frequency generators (like the Wi-Fi on the robot), and other places. This electrical noise can generate unwanted currents through electromagnetic induction in nearby wires, especially sensor wires, and these unwanted currents can wreak havoc (create “noise”) within the signals from your sensors. The amount of current induced in the wire depends on several factors including the strength of the magnetic field, the rate of change of the field, and the orientation of the wire.

Some buses and wiring are more sensitive to electrical noise than others. On a FIRST Tech Challenge robot, long signal-carrying wires (such as Servo wires or I2C sensor wires) are most susceptible to induced noise. So how can we eliminate this noise? The easiest way to remove noise is through the use of a Ferrite Core. Ferrite Cores, also known as Ferrite Beads, are made of a ceramic material called ferrite that has incredibly useful magnetic properties. When a Ferrite Core is clipped around a signal-carrying wire, the induced “noisy” alternating currents in the wire generate electrical fields in the ferrite that act to oppose those currents - this has the effect of canceling out or removing the high-frequency noise. It’s not typically required to “loop” the cable around the ferrite core, but doing so could increase the efficiency of the noise filtering in cases where excessive noise is being generated. You can find ferrite cores already installed in cables meant for high-noise environments or highly sensitive devices such as USB webcam cables and monitor cables. It’s best to place ferrite cores on the wire closest to the connector leading into the Control/Expansion Hub port.

The REV Resistive Grounding Strap

The REV Resistive Grounding Strap (RGS) is the only FTC-legal means of providing a grounding option for your robot frame or connected structural elements. Static electricity has two basic behaviors depending on whether it’s building up on a conductive or non-conductive surface; on non-conductive surfaces like polycarbonate or other plastics static electricity builds up in “pools”, on conductive surfaces like most metals static electricity spreads and distributes across the entire surface of the material. Aluminum extrusion used on robots typically has a clear non-conductive anodized layer used to prevent corrosion of the aluminum but the aluminum under the layer is conductive. When using the RGS, it’s important to connect the RGS to surfaces where you want to mitigate static buildup. If mounting the RGS to aluminum on your robot, it’s recommended to use a multimeter to test the continuity between the ring terminal on the RGS to different places on the robot to determine if the static buildup will be mitigated by the RGS. If testing for resistivity, remember that the REV Grounding Strap has a 470 Ohm resistor (with a ~5% tolerance) in-line in the strap - if not using an auto-range multimeter, be sure to select a range above 600 Ohms to ensure the resistivity is measured properly. It may be necessary to scrape the aluminum to create a conductive path between multiple segments of aluminum, just remember that a non-conductive oxide layer will eventually form on the exposed aluminum. Remember that if you’re probing aluminum extrusion to check for continuity or resistivity, those areas need to be scraped to expose bare metal in order to ensure electrical connectivity. “Jumper wires” screwed to aluminum elements can also be added to ensure conductivity between components.

Got any questions about electrical noise or static? Come start or join the conversation on the FTC Community Forums!