of a variable sheave drive mechanism of the coal feeder the positioner motor will run in the direction it is switched on by the controller 39 until the comparator output signal is back within the deadband of the controller the speed adjustor motor is then turned 34 off 34 and the adjusted coal feeder speed is maintained until the deviation swings out to the deadband limits again thus as the output from the primary controller 35 changes due to an error in bed level the control system changes the coal feeder speed and subsequently the coal feed rate to drive the measured bed level toward the set point level and to more closely match coal consumption rate in the gasifier bed the coal consumption rate will vary with gas demand in addition the bed level is also upset by ash removal and by bed settling lifting resulting from the stirrer 25 helical motion referring now to fig2 the secondary controller 39 will be described in detail the circuit includes comparators 45 and 47 which control the raising and lowering respectively of the coal feeder speed the comparator 45 is connected through a diode 49 and a load resistor 51 to ground potential a.