I fell in love with cable driven mechanisms a few years ago and put together some of my first mechanical tentacles to celebrate. But only after playing with them did I start to understand the principles that made them work. Today I want to share one of the most important equations to keep in mind when designing any device that involves cables, the capstan equation. Let some caffeine kick in and stick with me over the next few minutes to get a sense of how it works, how it affects the overall friction in your system, and how you can put it to work for you in special cases.
A Quick Refresher: Push-Pull Cable Driven Mechanisms
But first: just what exactly are cable driven mechanisms? It turns out that this term refers to a huge class of mechanisms, so we’ll limit our scope just to push-pull cable actuation systems.
When the Space Shuttle Atlantis rolled to a stop on its final mission in 2011, it was truly the end of an era. Few could deny that the program had become too complex and expensive to keep running, but even still, humanity’s ability to do useful work in low Earth orbit took a serious hit with the retirement of the Shuttle fleet. Worse, there was no indication of when or if another spacecraft would be developed that could truly rival the capabilities of the winged orbiters first conceived in the late 1960s.
While its primary function was to carry large payloads such as satellites into orbit, the Shuttle’s ability to retrieve objects from space and bring them back was arguably just as important. Throughout its storied career, sensitive experiments conducted at the International Space Station or aboard the Orbiter itself were returned gently to Earth thanks to the craft’s unique design. Unlike traditional spacecraft that ended their flight with a rough splashdown in the open ocean, the Shuttle eased itself down to the tarmac like an airplane. Once landed, experiments could be quickly unloaded and transferred to the nearby Space Station Processing Facility where science teams would be waiting to perform further processing or analysis.
For 30 years, the Space Shuttle and its assorted facilities at Kennedy Space Center provided a reliable way to deliver fragile or time-sensitive scientific experiments into the hands of researchers just a few hours after leaving orbit. It was a valuable service that simply didn’t exist before the Shuttle, and one that scientists have been deprived of ever since its retirement.
Until now. With the successful splashdown of the first Cargo Dragon 2 off the coast of Florida, NASA is one step closer to regaining a critical capability it hasn’t had for a decade. While it’s still not quite as convenient as simply rolling the Shuttle into the Orbiter Processing Facility after a mission, the fact that SpaceX can guide their capsule down into the waters near the Space Coast greatly reduces the time required to return experiments to the researchers who designed them.
The United Kingdom is somewhat unique in the world for requiring those households which view broadcast television to purchase a licence for the privilege. Initially coming into being with the Wireless Telegraphy Act in 1923, the licence was required for anyone receiving broadcast radio, before being expanded to cover television in 1946. The funds generated from this endeavour are used as the primary funding for the British Broadcasting Corporation.
Of course, it’s all well and good to require a licence, but without some manner of enforcement, the measure doesn’t have any teeth. Among other measures, the BBC have gone as far as employing special vans to hunt down illegally operating televisions and protect its precious income.
The Van Is Coming For You
To ensure a regular income, the BBC runs enforcement operations under the TV Licencing trade name, the entity which is responsible for administering the system. Records are kept of licences and their expiry dates, and investigations are made into households suspected of owning a television who have not paid the requisite fees. To encourage compliance, TV Licencing regularly sends sternly worded letters to those who have let their licence lapse or have not purchased one. In the event this fails, they may arrange a visit from enforcement officers. These officers aren’t empowered to forcibly enter homes, so in the event a homeowner declines to cooperate with an investigation, TV Licencing will apply for a search warrant. This may be on the basis of evidence such as a satellite dish or antenna spotted on the roof of a dwelling, or a remote spied on a couch cushion through a window.
Alternatively, a search warrant may be granted on the basis of evidence gleaned from a TV detector van. Outfitted with equipment to detect a TV set in use, the vans roam the streets of the United Kingdom, often dispatched to addresses with lapsed or absent TV licences. If the van detects that a set may be operating and receiving broadcast signals, TV Licencing can apply to the court for the requisite warrant to take the investigation further. The vans are almost solely used to support warrant applications; the detection van evidence is rarely if ever used in court to prosecute a licence evader. With a warrant in hand, officers will use direct evidence such as a television found plugged into an aerial to bring an evader to justice through the courts.
The history of storage devices is quite literally a race between the medium and the computing power as the bottleneck of preserving billions of ones and zeros stands in the way of computing nirvana. The most recent player is the Non-Volatile Memory Express (NVMe), something of a hybrid of what has come before.
The first generations of home computers used floppy disk and compact cassette-based storage, but gradually, larger and faster storage became important as personal computers grew in capabilities. By the 1990s hard drive-based storage had become commonplace, allowing many megabytes and ultimately gigabytes of data to be stored. This would drive up the need for a faster link between storage and the rest of the system, which up to that point had largely used the ATA interface in Programmed Input-Output (PIO) mode.
This led to the use of DMA-based transfers (UDMA interface, also called Ultra ATA and Parallel ATA), along with DMA-based SCSI interfaces over on the Apple and mostly server side of the computer fence. Ultimately Parallel ATA became Serial ATA (SATA) and Parallel SCSI became Serial Attached SCSI (SAS), with SATA being used primarily in laptops and desktop systems until the arrival of NVMe along with solid-state storage.
All of these interfaces were designed to keep up with the attached storage devices, yet NVMe is a bit of an odd duck considering the way it is integrated in the system. NVMe is also different for not being bound to a single interface or connector, which can be confusing. Who can keep M.2 and U.2 apart, let alone which protocol the interface speaks, be it SATA or NVMe?
Let’s take an in-depth look at the wonderful and wacky world of NVMe, shall we?
While searching for a connector recently, I revisited an old project of mine called the Serial Squid. This was to have been my first open-source hardware design. After completing the entire design, PCB, BOM, and preparing for a crowd-funded campaign, I eventually gave up for reasons discussed below, I’ve always thought of this as a failure, but on further reflection I see it in a new light. There were some good lessons learned along the path to abandonment.
Once upon a time, bailing out of a plane involved popping open the roof or door, and hopping out with your parachute, hoping that you’d maintained enough altitude to slow down before you hit the ground. As flying speeds increased and aircraft designs changed, such escape became largely impossible.
Ejector seats were the solution to this problem, with the first models entering service in the late 1940s. Around this time, the United Kingdom began development of a new fleet of bombers, intended to deliver its nuclear deterrent threat over the coming decades. The Vickers Valiant, the Handley Page Victor, and the Avro Vulcan were all selected to make up the force, entering service in 1955 through 1957 respectively. Each bomber featured ejector seats for the pilot and co-pilot, who sat at the front of the aircraft. The remaining three crew members who sat further back in the fuselage were provided with an escape hatch in the rear section of the aircraft with which to bail out in the event of an emergency.
An invisibility cloak may seem like science fiction, but despite that, many scientists and engineers have put much time into developing the concept, pushing it closer to reality. A device which detects the nature of its surroundings and changes its own properties to blend in may be complex, but a multitude of examples in the animal world show that it’s not impossible to achieve.