Error detection using pneumatic logic

TL;DR

An air-powered error detector device that can detect and respond to failures in pneumatically actuated systems without using sensors, providing a simple and low-cost way to add fault detection to pneumatic actuation systems without using sensors.

Abstract

Pneumatic systems are common in manufacturing, healthcare, transportation, robotics, and many other fields. Failures in these systems can have very serious consequences, particularly if they go undetected. In this work, we present an air-powered error detector device that can detect and respond to failures in pneumatically actuated systems. The device contains 21 monolithic membrane valves that act like transistors in a pneumatic logic "circuit" that uses vacuum to represent TRUE and atmospheric pressure as FALSE. Three pneumatic exclusive-OR (XOR) gates are used to calculate the parity bit corresponding to the values of several control bits. If the calculated value of the parity bit differs from the expected value, then an error (like a leak or a blocked air line) has been detected and the device outputs a pneumatic error signal which can in turn be used to alert a user, shut down the system, or take some other action. As a proof-of-concept, we used our pneumatic error detector to monitor the operation of a medical device, an intermittent pneumatic compression (IPC) device commonly used to prevent the formation of life-threatening blood clots in the wearer's legs. Experiments confirm that when the IPC device was damaged, the pneumatic error detector immediately recognized the error (a leak) and alerted the wearer using sound. By providing a simple and low-cost way to add fault detection to pneumatic actuation systems without using sensors, our pneumatic error detector can promote safety and reliability across the wide range of pneumatic systems.

Figures (5)

• Figure 1: Using our pneumatic error detector to detect problems during the operation of a typical pneumatically actuated system. In this example, three pneumatic control lines (bits 1, 2, and 3) apply vacuum or atmospheric pressure to the system being controlled (a medical device, robot, etc.). A fourth pneumatic control line contains a pneumatic signal that represents the parity bit corresponding to the values of the three control bits at each step during the operation sequence, with vacuum = 1 (True) and atmospheric pressure = 0 (False). The pneumatic control and parity bit lines are connected to the pneumatic error detector, which uses an air-powered logic circuit consisting of 21 monolithic membrane valves to calculate the parity bit corresponding to the values of the control bits and compare it to the expected parity bit value. If the two values for the parity bit are different, this indicates that one of the pneumatic signals is incorrect due to e.g. a leak occurring in the medical device or soft robot, and the error detector responds by automatically outputting 1 (vacuum) on an error line. This pneumatic error signal can be used to alert the operator (using a whistle here), initiate a system shutdown, or take some other corrective action.
• Figure 2: Exploded (A) and cross-section (B) views of a single monolithic membrane valve. The pneumatic error detector (C) contains 21 valves; it calculates the parity bit corresponding to three pneumatic control bit signals and compares the result to the input expected parity bit. If the two values differ, then an error has been detected and the error bit output becomes 1 (vacuum); otherwise it outputs 0 (atmospheric pressure).
• Figure 3: Pressures inside the pneumatic error detector's channels (red for atmospheric pressure or 0, and green for vacuum or 1) during three example calculations. In examples A and B, the error detector confirms that the expected and calculated parity bits match, so no error is detected and the error output remains at atmospheric pressure ( 0). In the third example (C), the expected parity bit of 0 does not match the calculated parity bit of 1, so the error detector outputs a vacuum ( 1) indicating a problem has been detected.
• Figure 4: Pressure measured at each of the three control bit inputs (blue), one expected parity bit input (orange), and one error output (red) while applying all 16 possible combinations of 1's (vacuums) and 0's (pressures) to the control bit inputs and parity bit input. During the first eight combinations (times from 0 to 3.5 minutes), the expected parity bit is correct or consistent with the values of the three control bits, and the near-zero (atmospheric) pressures measured at the error output confirm that no error has occurred. However, during the last eight combinations (times from 3.5 minutes to 7 minutes), the expected parity bit is intentionally incorrect (the opposite of what it should be), and the vacuum pressures measured at the error output confirm that the device has successfully detected these errors. Results from additional experiments like this are provided in online Supplementary Materials.
• Figure 5: Frames from a video recording (available as online Supplementary Materials) of the pneumatic error detector monitoring the operation of a model soft-robotic medical device, an intermittent pneumatic compression or IPC device used to prevent blood clots in the wearer's legs. During normal operation (B $\rightarrow$ C $\rightarrow$ D $\rightarrow$ B $\rightarrow$ C $\rightarrow$ D...), the device control system contacts one bellows at a time and no errors are detected. However, when a bellows is punctured to create a leak (E), the pneumatic error detector recognizes the mismatch between the expected ( 1) and calculated ( 0) parity bit values and automatically alerts the wearer by blowing a whistle (F). Detailed explanations of each frame are in the main text.