Journal of the Society of Motion Picture Engineers (1930-1949)

is based on the fact that the torque produced by the motor may be varied by changing the voltage to the motor. Principal elements of the controller consist of the following: a film roll follower mounted on a rotatable arm, a multistep rotary switch actuated by the rotatable arm, a set of resistors having as many adjustable contact bands as there are steps on the rotary switch, a solenoid, a relay, and interconnecting wiring. The film roll follower arm assembly is mounted on the front side of the reel panel as shown in Fig. 1. The remainder of the mechanism is mounted on the back side of the panel together with the torque motor. When used in conjunction with the motor serving to drive the take-up spool or reel, the device functions in the following manner. Sufficient resistance is introduced in series with the motor at the start of the roll during take-up to reduce the film tension to the desired value. Thus the usual initial high film tension is eliminated at the beginning, thus removing this cause of sprocket-hole flutter. The torque controller functions in a manner to maintain film tension relatively constant. As the diameter of the roll of film being wound on the take-up spool increases, the roller on the follower arm is moved farther from the center of the spool, causing the shaft to which the arm is attached to be rotated. The rotary switch contactor arm, mounted on the opposite end of the shaft, is moved gradually across the step contacts of the switch shown in Fig. 2. Sufficient free movement has been provided in the mechanical assembly to prevent rotational eccentricity of the roll of film from causing oscillation of the contact brush across adjacent switch steps. The gradual operation of the rotary switch causes small incremental decreases of resistance in the motor circuit resulting in a gradual increase in motor voltage. This, in turn, due to the electrical characteristics of the torque motor produces an increase in motor torque. As applied herein, torque may be defined as the product of a force multiplied by a moment arm. In terms with which we are concerned, film tension and radius of the roll of film are the force and moment arm members, respectively, of the torque equation. With proper adjustment of the positions of the resistor contact bands, the torque may be controlled so as to maintain the film tension constant within approximately 2 oz. Based on a normal tension of 11 oz, the 2-oz variation is less than 20% deviation. Should one attempt to use 2000-ft rolls with standard 2-in. film cores on apparatus equipped with friction clutch or standard torquemotor take-up drives, the deviation would be on the order of 600%. This deviation in tension would be nearly 400% for 1000-ft rolls under the same conditions. The drag or holdback torque motor functions in the inverse manner from that for take-up. To prevent freewheeling of the roll of film with the motor power off, a friction clutch is incorporated in the mechanical assembly of the motor and reel shaft. Since this available friction is sufficient to provide holdback as the diameter of the roll of film approaches the core, motor power is turned off when the roll is reduced to a predetermined diameter. This is accomplished by means of a relay, shown in Fig. 3, which is energized from the main motor switch that controls the direction of film travel through the apparatus. When the relay is energized, the electrical connections between the last two steps of the rotary switch and their associated resistor contact bands are opened. Then, when the rotary switch arm contacts either of these two steps, the electrical circuit to the motor is opened. Also, when energized, the relay causes additional resistance to be imposed in the electrical circuit to 30 July 1952 Journal of the SMPTE Vol.59