Vari-lites

[timmath]

The only lights I know of that use an absolute encoder are the 7KW and 10KW Razorheads by WWG as used on one of the U2 tours. The 12 bit encoders cost around £300 each so not really suitable for use in most moving heads.

 

AFAIK Diablo and Space Cannon 7k's use optical encoders at >16 bit resolution. being half a degree out makes quite a difference if you are lighting something 600m away.

 

regards

 

Tim Matthews

[Madmac]

AFAIK Diablo and Space Cannon 7k's use optical encoders at >16 bit resolution. being half a degree out makes quite a difference if you are lighting something 600m away.

 

Half a degree is 1 part in 720, a 10 bit encoder gives a resolution that is 1 in 1024

 

I do not know the details of the design of either of these products but would suspect that they use stepper motors that are microstepped. The cost of 2 off 10 bit absolute encoders would be a substantial chunk of the units costs.

 

A standard 1.8 degree stepper gives 200 steps per revolution, microstepping with 128 divisions gives 25600 steps per revolution, slightly less than 15 bit.

 

Most stepper motors can be driven with any number of microsteps but for the drive waveforms to result in enough torque to move the drive shaft due to imperfections in the distribution of the internal magnetic field there is a practical limit. A low cost motor will resolve around 32 to 64 microsteps. High quality, precision steppers can resolve up to 256 to 1024 microsteps, and this includes using custom waveform shapes, but at a substantial price penalty.

 

They may use a technique similar to that of the MAC250 which is a low resolution optical incremental encoder and microstepping. If the microstepping count does not agree with the value that equates to the optical encoder change the unit can recalibrate itself. If the optical encoder detects movement, the lamp being knocked for example,and the stepper is not being moved it can switch off the holding current and wait before returning the fixture to its correct position

 

madmac

[jagardner1984]

Hi there,

 

This thread is very interesting, particularly the section on colour mixing in Moving Lights.

 

A Few questions:

 

1. What is a Stepper?

 

2. What is Open Loop?

 

3. On the VL6c that I have used, the Gobo Wheels make a lot of noise in snap changes, whilst movement, is relatively quiet at all but the fastest of speeds. Are these unique to the unit I was looking at or have other folk come across these "quirks." Whipping VLs around like there's no tomorrow is all very well until all of the audience are looking above their heads at the Moving Lights.

 

James

[MarcT]

Since nobody else has replied:

A Few questions:

 

1. What is a Stepper?

Its a type of electric motor which instead of continuously rotating, is positioned in small fixed "steps". This is achieved by having a number of windings around the axis of the motor, which are switched on and off in turn to create rotation. Stepper motors can start and stop quickly, and can be positioned accurately - making them ideal for use in moving head/mirror fixtures.

 

Here are some basics about stepper motors:

Step Motors Reference Guide

 

2. What is Open Loop?

The terms open loop & closed loop come from control theory.

 

An open loop controller is one which uses a mathematical model of whatever its trying to control, along with some known initial conditions to attempt to give a desired output (which could be voltage, position, temperature, etc). The problem is that no measurement is made of the actual output - and no attempt is made to correct for errors. Therefore the controller can undershoot or overshoot the desired value.

 

A closed loop controller uses "feedback" as well as a mathematical model in the controller. The difference (or error) between the desired output and the actual output is fed-back to the control system, which is designed to minimise the output error.

 

In moving lights, "encoders" are used to determine the actual (or relative) position of the motor shaft for closed loop control.

 

There is an intro to some of the gory mathematics of control theory here:

Introductions to Control Theory and Engineering

 

3. On the VL6c that I have used, the Gobo Wheels make a lot of noise in snap changes, whilst movement, is relatively quiet at all but the fastest of speeds. Are these unique to the unit I was looking at or have other folk come across these "quirks."

I suspect that's just the way it is. The snap change requires a fast acceleration then deceleration of the gobo wheel, and that's invariably going to make noise - all that energy has to go somewhere.

 

Whipping VLs around like there's no tomorrow is all very well until all of the audience are looking above their heads at the Moving Lights.

Yes, I agree. Gratuitous over use of moving lights can become a distraction.

 

Regards,

Marc

[oh lordy]

Hi Mr Steve,

 

One of the ways that he vari moves so quiclky as it always go's the shortest distance to get to the colour or gobo it requires on the vl6 2c fixed gobo colour wheels. You may notice on the VL7 and 6c rotate that it is slower due to having to go back in order, hence you can't go through open white if that is quicker. This is due to ratio of cogs and complex math for it to work it out how many times it has gone through 360 degrees. On the VL7 it did originally!!

 

As for noise at speed this should not occur and may need attention of the luminaire.

 

One thing to remember is to use the correct fixyure for the correct application. certain lights are good at colour fades VL5 others better at quick colour changes VL2c. Others are optically better VL7 but this is not going to move as well, so pick the correct light for the application.

 

One other thing. Most vari lites allow you to put colour in the gobo wheel so if you need quick colour snap you can put colour into the gobo wheel, this said it need to be glued in the other way around in the holder on most instances.

 

 

Oh Lordy

[jagardner1984]

Thanks Marc, that was very helpful

[David Buffham]

As far as colour mixing goes then the VL4, VL5, VL1000, VL2000 wash, VL2416, VL3000 spot, VL3000 wash and VL3500 spot all have CYM mixing, and VL2C and VL7 have their own form of colour mixing.

 

So its actually only the VL6 range & VL2000 spot that don't have any colour mixing capability.

 

In addition to this list of VL fixtures with colour mixing, the VL2500s have just been introduced at LDI which are VL2000s with CYM mixing - how awesome does that sound?

[Don]

Little known fact the VL2c & VL6 range allows the motor to run fowards or backwards depending on the quickest route to the chosen colour. Then newer Series 2000/3000 move away from this type of motor.