Gimbal lock is a phenomenon that occurs when the three gimbals used to stabilize an object in space, such as a spacecraft or aircraft, align in a way that results in the loss of one degree of freedom of movement.
This can occur when two of the gimbals line up on the same plane, causing the third to lose its ability to control motion in one direction. Gimbal lock can be a serious issue for spacecraft, as it can result in the loss of control over the vehicle’s attitude and orientation in space.
Gimbal lock was a concern during the Apollo missions, as the spacecraft used a system of gimbals to stabilize the vehicle during its journey to the Moon and during its return to Earth. However, NASA engineers developed techniques to mitigate the risk of gimbal lock, including using a different set of gimbals for different phases of the mission and designing the spacecraft’s software to detect and correct for gimbal lock if it occurred.
In this article, I will answer what a gimbal lock is and why the instantaneous flip of a gimbal by 180 degrees prevents a gimbal lock. If you are interested in this gimbal lock phenomenon and this question, please read on.
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What Does Gimbal Lock Mean?
The name “Gimbal lock” is the loss of one degree of freedom in a three-dimensional, three-gimbal mechanism. This happens when the axes of two of the three gimbals are driven into a parallel configuration, in other words, “locking” the system into rotation in a degenerate two-dimensional space.
The term lock is misleading because no gimbal is restrained. All three gimbals can rotate freely about their respective axes of suspension. Nevertheless, because of two of the gimbals’ axes’ parallel orientations, there is no gimbal available to accommodate rotation about one axis.
While MIT’s Draper Lab had the contract for the guidance system in the Apollo spacecraft, they were not involved in Saturn’s guidance equipment. Bendix produced the ST-124 for the Instrument Unit’s primary contractor, IBM.
What Exactly is a”Gimbal Lock”?
We need to break this one down just a little bit before getting into the gimbal lock question. At the heart of the Apollo guidance computer was an inertial measurement unit or IMU.
What is the IMU
The IMU is a spherical housing about the size of a soccer ball that contains three nested gimbals. They’re set at right angles from one another. Supporting a central platform mounted on the innermost gimbal.
The outer gimbal is mounted on the navigation base, and this is mounted rigidly to the spacecraft. Every time the spacecraft moves, or instead every time the IMU detects any change of attitude, the gyro is signaled.
Two motors return the platform to its original orientation. So, as the spacecraft moves around on its path to the moon, the guide’s platform inside remains stable. The guidance platform was aligned on the ground before the flight, and of course, it would occasionally drift during the mission.
So, to compensate for this drift, astronauts would rely on the platform using star sightings. But its use still remained, and it even gave the astronauts information on their attitude in space.
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The data from these arrows are displayed on the flight director attitude indicator, more commonly known as the “eight ball.”
So, it’s the fact that there are three gyros involved in the IMU that makes the lock possible. The three gimbals account for the spacecraft’s three axes of motion. It is the axis of pitch, yaw, and roll.
Because the IMU works in all three directions, any change in the attitude orientation at all is registered on the eight ball. The constant attitude information about these three axes gives the astronaut a very keen sense of exactly where they’re pointing in space. But it’s also something that you can lose.
What is the Phenomenon of “Gimbal Lock”
The gimbal lock phenomenon is one wherein the outer gimbal moves with the spacecraft to a point where it lies parallel to the inner gimbal. At this point, all three gimbal axes would be lined up on a single plane, and none would be able to move around the basic plane to resume a normal orientation.
In short, once the gimbals are lined up, they can’t realign themselves to give you an orientation, and they become locked. It’s the confluence of three angles of the gimbals that create a gimbal lock and move around to get out of that situation.
The Apollo guidance computer was actually designed to prevent gimbal lock by giving the astronauts a warning when they approached that devastating alignment. However, it was possible still to fall into a gimbal lock.
Why Instantaneous Flip of a Gimbal by 180 Degrees Prevents a Gimbal Lock
The confusion arises in terminology. What instrumentation engineers call a gyroscope is an integrating gyroscope or rate gyro. So what they call a gimbal in a gimbal lock isn’t the gimbal used to mount the gyroscope itself.
Here are some basics of the gimbal. The gimbals are used to keep a platform housing three gyroscope levels. A conventional gyroscope will point in the same direction at all times, while a rate gyroscope is used for measuring angular velocity.
The rate gyro will deflect the springs in relation to how fast the frame is spinning. So, the faster the spin, the larger the springs deflect this can, which, via strain gauge or comparable measurements engineering, can be converted into an electrical signal.
Measure the difference in rate to derive the angular acceleration. Have a computer integrate the rate over time to derive the current angular position.
Note the pivoting gimbals for this device, though – they are in the correct configuration of a gimbal lock. So, a rate gyro’s housing can measure changes in a single axis. And if the frame housing the gimbal moves away from the axis, the device won’t measure properly.
So, three-rate gyroscopes are required to measure angular movement in all three angles. These are mounted on a platform that maintains a fixed orientation in space so that the rate gyroscopes can function correctly.
When the rate gyroscope feeds a proportional signal to the instrumentation panel, it also provides this signal to three motors. Those motors are the torque motors in the quote.
They can turn the platform in response to the signal, keeping the platform level. When these gimbals keep the platform level lock, then there is the problem, and the goal is to keep the feedback loop away from turning the gimbals until they lock.
The plan to turn the pitch gimbal 180 degrees would essentially turn the platform upside down, which would reverse two of the three accelerometers – and all of the rate gyros. Specifically, the pitch gyro would now be spinning in opposite orientation after the platform was upside down (counterclockwise instead of clockwise).
This would mean that it reads a 0.05 rpm counterclockwise turn as instead of a 0.05 rpm clockwise turn – and the feedback loop to the motors would similarly be reversed, pulling the platform out of the gimbal lock.
This could be compared to jumping over the discontinuity at 90 degrees for the tangent function and continuing to perform the integration – in theory. In practice, the system never worked.
Apollo 13 and Gimbal Lock
This was what was happening on Apollo 13 when the spacecraft was wildly pitching and yawing around in the wake of the oxygen tank explosion. And that is why you hear in the movie and read in the transcript astronauts yelling about watching the gimbals.
Losing their attitude in space would have made the entire situation on Apollo 13 just that much worse. The simplest way to avoid gimbal lock would have been to add a fourth gimble into the IMU, just giving it another reference point so that the four wouldn’t be able to align the same way.
Apollo 11 Gimbal Lock Event
A well-known gimbal lock event happened in the Apollo 11 mission. A set of gimbals were used on an inertial measurement unit (IMU). The engineers working on this mission were aware of the gimbal lock problem but had declined to use the fourth gimbal. And some of the reasons behind this decision are obvious from the following quotes below:
- “The advantages of the redundant gimbal seem to be outweighed by the equipment simplicity, size advantages, and corresponding implied reliability of the direct three degrees of freedom unit.”-
Quoted from David Hoag, Apollo Lunar Surface Journal
They favored an alternative solution by utilizing an indicator triggered when near to 85 degrees pitch.
- “Near that point, in a closed stabilization loop, the torque motors could theoretically be commanded to flip the gimbal 180 degrees instantaneously. Instead, in the Lunar Module, the computer flashed a ‘gimbal lock’ warning at 70 degrees and froze the IMU at 85 degrees.”
Quoted from Paul Fjeld, taken from Apollo Lunar Surface Journal
So, rather than attempt to drive the gimbals faster than they could go, the system just gave up and froze the platform. From this point on, the spacecraft would have to be manually moved away from the gimbal lock position. Furthermore, the platform would have to be manually realigned using the stars as a reference.
Here’s Why Michael Collins Whish He Have Had a Fourth Gimball For Christmas
After the LM had landed, Mike Collins aboard the Command Module joked, “How about sending me a fourth gimbal for Christmas?” And this is why Michael Collins asked for Christmas jokingly when he was orbiting the moon on Apollo 11.
From his orbital vantage point in the command module, it calls us to try to find the lunar module eagle on the surface, but every time he gets close to the gimbal lock, he has to stop. And that’s why he wished he’d had a fourth gimble onboard.
So I hope that clears up “gimbal lock,” and I hope you enjoyed this short article. Check out this article that reveals the inside of the Apollo Saturn V rocket and its significant components. See for yourself these fantastic drawings. You will be amazed.
Learn about the daring Apollo astronauts and their inspiring journey through the Apollo Program, from tragedy to triumph, by reading our article here.
FAQ
Gimbal lock is a phenomenon that can occur when the three gimbals used to stabilize an object in space, such as a spacecraft or aircraft, align in a way that results in the loss of one degree of freedom of movement. This can cause the vehicle to lose control of its attitude and orientation in space, which can be dangerous or even catastrophic.
A Gimbal lock occurs when two of the three gimbals that control the orientation of an object align on the same plane, causing the third gimbal to lose its ability to control motion in one direction. This can happen when the object experiences sudden changes in acceleration or when the gimbals are not properly calibrated. Other factors that can contribute to gimbal lock include issues with the vehicle’s software or the failure of the gimbal system’s mechanical components.
The consequences of a gimbal lock can be severe, as it can cause the vehicle to lose control of its orientation in space. This can be particularly dangerous during space missions, where precise control of the spacecraft is essential. To mitigate the risk of gimbal lock, engineers and designers use a variety of techniques, including using redundant systems and designing software to detect and correct gimbal lock if it occurs.
Gimbal lock has been a concern during many space missions, including the Apollo missions to the Moon. It has also been a concern for military aircraft and other vehicles that rely on gimbal systems for control and stabilization.
Engineers and designers work to prevent gimbal lock by using redundant systems, calibrating the gimbals properly, and designing software that can detect and correct gimbal lock if it occurs. Other strategies may include using different sets of gimbals for different phases of the mission or using other stabilization methods, such as reaction wheels or thrusters.
Software plays a critical role in preventing and correcting gimbal locks, as it can detect when the gimbals are in danger of aligning on the same plane and initiate corrective actions. This technology has evolved over time, with newer spacecraft using increasingly sophisticated software systems that can detect and correct gimbal locks more quickly and effectively.
There are several potential solutions or new technologies that could help to eliminate or reduce the risk of gimbal locks in the future. One promising approach is to use advanced control systems, such as adaptive or nonlinear control, that can anticipate and compensate for gimbal lock before it occurs. Another approach is to use alternative stabilization methods, such as magnetic bearings or electrostatic gyroscopes, which are not susceptible to gimbal lock.
Engineers and designers face a variety of challenges when working with complex systems in space, including issues related to reliability, durability, and environmental conditions. To overcome these challenges, they use advanced materials and manufacturing techniques, conduct extensive testing and simulation, and work closely with mission planners and other stakeholders.