Automatic control of crystal growth

In an apparatus for growing crystals comprising:

• a crucible for holding crystal material;
• heaters operatively associated with said crucible to render said material molten;
• a pull rod for pulling a single crystal from the molten material;
• motors for rotating and raising the pull rod at rates related to the temperature of the molten material to produce a crystal having an expected radius;
• a load cell for measuring the weight of said crystal, said cell producing an output which includes an error signal when the radius of the crystal varies from the expected value, said error signal including a component which is a differential of radius variation with respect to time;
• an electrical control loop connected between the load cell and said heaters, said loop including a comparator for comparing the output of said cell with voltage from a voltage source to produce a comparator output;
• and
• a correction circuit connected to the control loop at a point between the load cell and said heaters, the improvement wherein said circuit comprises:
• a differentiation network responsive to the load cell output for producing an output signal which includes a component which is a first differential with respect to time, and means joining said differentiation network output signal to said control loop for adding to the electrical control loop the output from the differentiation network to compensate for said differential of radius variation with time in the load cell output, to reduce said differential of radius variation with time and eliminate instability in said loop.