|Quality Electronic Equipment|
|Technical Details and Dimensions|
|FR15 Series Speed Controllers|
The FR15 series controllers are based on a PIC16C505 8 bit micro-controller, operating at 16MHz, a 45 amp. rated MOSFET and a single industrial quality reversing relay.
Although some cheaper controllers still use Darlington transistors to control the power supplied to the motor, with a high volt drop and loss of top speed, all Electronize controllers use Power MOSFET's. Power MOSFET's are a far more efficient type of transistor that lose only a fraction of the power lost by the old fashioned Darlington type. The FR15 series use MOSFET's chosen for their low 'on resistance' typically only 5.8 mΩ. This means that the MOSFET will only drop about 0.09 volts at a full 15 amp. load. The rating of the MOSFET is 45 amp. continuous (given a very large heat sink) and 270 amp. peak.
In most speed controllers the reversing relay coil is powered from the receiver circuit causing increased load on the small receiver battery. In this controller the relay is powered from the much larger motor battery. To accommodate the 6 to 24 volt range the relay is controlled by the microprocessor in P.W.M. mode to maintain the correct coil voltage over the entire voltage range.
As with all modern controllers, the motor speed is controlled by a P.W.M. (pulse width modulation) system. Power to the motor is pulsed on and off at high speed so that the speed varies with the on to off proportion. The choice of pulse frequency is always a compromise. Both high and low frequencies have advantages and disadvantages and consequently some people prefer high frequency whilst others prefer low. Generally low frequency will give better starting and slow speed control because the larger pulses of torque, produced by the motor, 'kick' the shaft into motion and stop friction bringing it to a stop at low speeds. High frequency systems tend to go off with a rush and are less controllable at low speeds simply because the torque output is so smooth. On the other hand high frequency systems sound less harsh at mid to high speed but do tend to produce a whine or whistle from the motor. High frequency can also give longer running times from the battery because the peak currents are less and so reduce the losses in the battery and motor. For these reasons the FR15 series gives you the choice of high or low frequency mode or you can choose variable mode and have the best of both systems. This mode operates at very low frequency, to get the motor turning and progressively increases the frequency as the speed increases.
All this is achieved by a microprocessor carrying out its program instructions at the rate of four per micro-second. It has a lot to do including the following list:
Input signal timing (to 5 micro-second resolution)
Valid Pulse Recognition
Rolling average calculation
Output frequency calculation and timing
Output pulse timing
Output scaling (from screwdriver speed setting)
Charge pump drive
Relay P.W.M. control
There is no need to adjust the neutral setting on this controller. It is accurately set by the microprocessor program and the frequency of a ceramic resonator. A ceramic resonator operates in the same way as a quartz crystal to provide an accurate measurement of time, in this case within 0.5%. The controller is designed without any complicated start up sequences, instead there are two simple screwdriver settings. One has three positions and selects the operating mode, high, low or variable frequency. The other is a normal potentiometer adjustment and sets the speed range. By this you can set the maximum speed to match the travel of the joystick or wind it down to as little as 25% to provide a realistic scale speed.
The controller has built in radio frequency suppression capacitors on all signal and supply leads to provide adequate resistance to interference effects. The switching speed of the MOSFET's is also controlled so as not to generate radio frequencies that may interfere with the radio receiver. This is the main source of interference in many high frequency controllers. The nominal 5 volt receiver supply, used to power the microprocessor, is reduced and regulated at 4.2 volts to further reduce the risk of interference, removing the effects of varying voltage and protect the microprocessor from any excessive voltages. To protect the MOSFET there are two overload protection systems. A thermistor on the printed circuit board continually measures the temperature and will turn off the output if a pre-set limit is exceeded. Likewise, the current flowing through the MOSFET's is sensed and the output turned off if a pre-set limit (approx. 45 amps at max. temperature) is exceeded.