Navigator Training: Day Two

By Richard Cadena

On day two of Navigator training, we learned about some of the features and functions of the software. In some ways it seems to be a lot like programming an automated lighting console and in other ways it’s not. The biggest difference is that if you foul up a lighting cue, no one gets hurt. But if you mess up an automation
cue it can crash and burn in ways you never even thought of.

For example, suppose some guy named Bob was flying a load. (Bob is not Dana Bartholomew, just in case you were reading my mind.) This load weighs 500 pounds and Bob has programmed the console to accelerate it in an upward direction at 4 feet per second squared. When he presses the Go button, a force is applied to the line and load in excess of the weight of the load. How much force? Glad you asked.

Sir Isaac Newton taught us that the force exerted by an object is the mass of the object times the acceleration of the object. If a load weighs 500 pounds, that means that gravity is exerting a force of 500 pounds on it. My high school physics teacher taught us that the acceleration of gravity is 32 feet per second squared. (I think he learned that from one of his contemporaries, Galileo.) Therefore, a 500 pound object has a mass of 15.625 slugs (500 pounds ÷ 32 feet per second squared = 15.625 pound-feet per second squared or 15.625 slugs). In order to accelerate a 15.625 slug mass to a velocity of 4 feet per second squared over a period of one second, it would take a force of 62.5 pounds (15.625 x 4 = 62.5). So the line would experience a combined force of 562.5 pounds (500 pounds + 62.5 pounds = 562.5 pounds).

Now suppose that Bob were to accelerate the same load to a velocity of 43 feet per second in half a second. The force applied to the line would be 1,843.75 pounds. That a lot of force.

But if that 500 pound load was dropping that was falling at 43 feet per second and Bob hit the E-stop, then the power would be cut off to the winch and the brakes would be applied. Remember that the brakes have a 10:1
safety factor. Suppose the brakes were to stop the load in 0.1 second. How much force would that place on the system? Try 7,218 pounds.

For this reason, the winches that FTSI designs and builds are made to slip slightly so that they brake over a longer period of time. Still, Bob has to be extremely careful about what he does at all times. According to Dana, OSHA regulations allow for a maximum force for fall protection of 1800 pounds, and most companies limit it to 900 pounds.

But that’s not the only concern with falling loads. Once Bob was testing a rig and he wanted to know quickly a load could accelerate from the top of a rig to the ground. After a bit of testing he programmed the Navigator system to accelerate in the down direction at a very fast rate. When he hit the Go button, the line came off of the drum “like a can of silly string.” It seems he forgot to take into account that the acceleration of gravity is only 32 feet per second so the Navigator system drove the winch faster than the load could fall.

So Dana advised us to mind our maximum accelerations as well as our decelerations in the increasing and decreasing positions. It is possible to decelerate a load faster than 32 feet per second squared but it requires an
additional line on the load connected to a take up winch. (There must be a name for that kind of line, but as I said in yesterday’s blog, I’m not a rigger.)

There are a lot of safety features designed into the Navigator system. It uses distributed processing, meaning that instead of having one central processor or computer, each device in the system has a processor. When
a component is plugged into the network it becomes part of the whole. If that component goes down, it won’t bring down the entire network; instead, another part of the system takes over.

Thankfully Navigator doesn’t rely on the Windows operating system or it might decide it needs to upgrade in the middle of a cue. It runs on a QNX platform, which is an operating system designed for real-time operation on distributed systems. We used laptops to run a terminal that was networked to a server running Navigator. In order to talk to the server all of the work stations were connected through a network switch. Otherwise we would have needed a crossover cable to connect directly to the server.

There are many more safety features in the system; nevertheless, programming and operating it requires thought, care, and knowledge. In addition, Dana offered these pearls of wisdom based on years of running automated systems in the field as well as working with Bob:

Dana’s Pearls of Wisdom for Running an Automation System:

1. Pay attention – Once you learn a show it’s very easy to forget to pay close attention. Most problems encountered in the field are the result of operator error.
2. Understand the dangers involved.
3. Make sure you’re running what you expect to be running. Dana recounted stories of how Bob was looking the wrong way because he thought he was moving one load but instead he accidentally moved another.
4. Inspect and maintain the system regularly!
5. Train properly on the system.
6. Listen to the performers. It’s easy to disregard the comments of the talent, thinking they are prima donnas, but often when they feel something irregular in the travel of a system they are correctly identifying a potential problem before it becomes a problem.
7. Disable the system when you walk away from the controller. Dana recalled how Bob once leaned against a set piece and accidentally triggered a move. Had the E-stop been engaged that never would have happened.

This is the second of a five-part blog documenting the Fisher Technical Services Inc. Navigator automation software class. Visit http://www.prolightingspace.com/profiles/blog/list
to read all of the posts.

Dana Moore is head over heels for the Navigator system.