Part III: Nuts & Bolts

This is an unfinished draft for a book I wrote back in 1999 on the topic of motion control. Tomorrow I will publish the final part, just as it was written and with lots of gaps in the text (I told you it was unfinished).

Table of Contents

  1. Introduction to stepper motors
  2. Camera heads
  3. Linear type of rigs
  4. Radial type of rigs
  5. Turntables, modelmovers and motionbases
  6. Cameras and camera motors
  7. Track and platforms
  8. RP4
  9. Kuper Control
  10. Lynx Robotics
  11. Mark Roberts

Introduction to stepper motors

There is a large array of motion control rigs built and used over the past years. Many of them was built for a certain purpose and production. But there are also commercial available motion control rigs to buy today.

Most of them have stepper motors attached to all rotating shafts on the camera, on the camera head, dolly and boom arm. Stepper motors take their instructions from electrical step and direction pulses delivered by the computer. With each pulse, the stepper motor moves one or more increment. The increments through which the rotor of each stepper motor can turn can be quite small, a fraction of a degree, in which case the movements of the rig also will be minute. Once the movement of the camera have been programmed into the computer, these movements can be repeated, over and over with great precision.

The heart of most motion control system are a pulse generator card. Whose job is to spit out step and direction. The step and direction pulses are fed to a amplifier that try to amplify the signal with as much power as possible.

The direction pulse tell the stepper motor which direction it should turn. One step pulse makes the motor to turn one step. Most stepper motors need 2 000 pulses to turn one revolution. So to get the motor to turn one revolutions per second, the system need to send 2 000 pulses per second. Or 14 000 pulses per second to turn seven revolutions per second.

But there’s a limit how much power that can be applied. A stepper motors power can be defined as:

Power = Torque × Revolutions Per Minute

Torque is the force that a motor can apply on its turning shaft. And RPM the speed it turn its shaft. The stepper motor have a dynamic range of operation:

Stepper Motor Power Curve

The motor got pretty much the same torque from lower speeds up to 40 000 revolutions per second where it go steeply down and finally stalls around 80 000 RPMs.

The same is true for how much torque and force already are applied mechanically on the motors shaft from the rig itself. A motor can never go from 0 to 40 000 RPM immediately, it need time to get up to speed. But when it is already up to speed then speeding the motor up and down are no problem at all.

Basically the same rules applies when a dolly-grip pushes a dolly in front of him. He needs time and force to overcome the friction.

The motor most likely to stall often are the motors controlling boom and dolly movement. Because they have plenty of mass of rig to move around. Or when an operator try to change direction of a motor without speeding both up and down. This is a problem when motion control rigs have to operate in real time. The problem are solved by letting the motor accelerate during even increasing of velocity, these are called ramps.

To do moves that start at a high speed the rig needs pre and post roll for doing ramps up and down. Usually the rig backs up beyond the start of the move and then speeds up and moves forward during pre-roll and are hopefully up to speed when it hits the moves starting point. How much ramp a motor need, depends on the size of it and what it control.

But the ramps can also be built into the move, if the move itself allow smooth acceleration and deceleration in the beginning and end of the move. And it is possible to create a move which will shoot faster than it will jog by building smooth accelerations into the move during programming.

Camera heads

[To be written]

Linear type of rigs

[To be written]

Radial type of rigs

[To be written]

Turntables, modelmovers and motionbases

[To be written]

Cameras and camera motors

[To be written]

Track and platforms

[To be written]

RP4

[To be written]

Kuper Control

[To be written]

Lynx Robotics

[To be written]

Mark Roberts

[To be written]

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