We often get questions from people who are having a hard time getting their turntable to hit a position, so we want to go over the quickest way of positioning a turntable and maybe clear up some of the confusion.

Every machine has an encoder that sends information to Spikemark as counts. In Spikemark, the user can choose to cue that piece of scenery using those counts, or translate them into inches, feet or degrees using a position scale. Inches and feet are great for linear movement, but for a turntable, the best way to represent the encoder counts is by having Spikemark translate them into degrees. Let’s look at how we do that.

Once you’ve connected the turntable machine to Spikemark, take a look at the Positioning Window in the Motor Editing Pane. Make sure that the Position Units is ‘counts’, Position Scale is set to ’1′, and then reset the Position to ’0′.

On the turntable, make a physical mark that lines up with a fixed mark on the stage. Then, using the manual control, move the turntable four complete turns until the mark once again lines up with the fixed mark. Note: You can rotate any number of complete turns- the more turns, the more accurate your result.

Next look at the motor in the cue grid and you will see a big number followed by ‘cts’.

That’s the number of counts that the encoder sent to Spikemark during four full revolutions of your turntable. Now  we need to determine a position scale with a little math. Let’s say you moved 2,983,996.8 counts in 1440 degrees (360 degrees * 4 turns). 2,983,996.8 / 1440 = 2072.22. That’s your position scale. Go back to the Position Window of the Motor Editing Pane, enter your position scale and change the Units to ‘Degrees’.

Now you’re done. If you want to move the turntable one full revolution, just set the target to 360 degrees more (or less) than where it currently is. Half a revolution is 180 degrees, 720 degrees will revolve the turntable twice, etc…

That’s it for the December Spikemark Tip of the Month. If you have an idea for something that you think could use a little explaining just leave us a comment or send me an email at ian@creativeconners.com.

A question that comes up frequently is, “How can I pull a piece of scenery with multiple motors and make sure that they are synchronized?”

It’s a great question.  A common scenario is a full-stage (or mostly full-stage) wagon that needs to track upstage/downstage.  Driving large turntables with multiple motors is another instance where you may want to use multiple motors to act as a single motor.  Using multiple motors in either situation lets you distribute your horsepower more evenly across the load.  Could you do it mechanically?  Sure, but using a piece of cable is a lot easier than building a bunch of mechanical linkages.

The answer is simple.  As long as you are using a variable frequency drive with AC motors, just two-fer the motor into the drive.  A couple of points to be aware of:

1.  You are still limited to the total horsepower of the drive.  A 5HP freq drive will only push 5HP worth of motors (maybe 2 @ 2HP).

2.  You should protect each motor individually from overload current.  If you have two 2HP motors powered by a single 5HP inverter, there’s the chance that one motor will draw 5HP worth of current.  Using a motor starter in series with each motor, that is appropriately rated, will protect each motor from overload.  Though I confess, I have be known to skip this step on quick-hit shows, YMMV.

3.  While you two-fer the motor and brake lines, you still only use one set of limit switches and one encoder.