Technical Description: Control Electronics
Issue 2
75
Part No. 306-0430-102
negative. The effect of the pressure feedback and the integrator reduces the motor speed as the pressure
reaches the level set by the PWM input. The motor speed is regulated by the controllers internal PWM circuit.
The motor controller IC receives position signals from the hall effect sensors on the motor. The internal logic
determines which of the output drivers are active. The output drivers drive six power FETs which switch power
to the motor coils. Only two FETs, one P channel top FET and one N channel bottom FET are on at any time.
The sources of the bottom FETs are fed through a sensing resistor, which feeds a voltage proportional to the
motor current into the controller. If the voltage exceeds 100 mV, equivalent to a 2 A motor current, the controller
uses its internal PWM system to reduce the motor drive current.
The amplifier/integrator U4 power supply is
±
12 V derived from the
±
15 V supplies from the I/O board. And the
motor supply is taken from the +24 V supply via an adjustable 3 A regulator set to a nominal 22.5 V. The Hall
effect sensors are supplied from the 6.25 V reference supply from the controller IC.
The motor controller PCB has two links fitted to enable the sensors to be either 60 degree or 120 degree types
controlling the direction of the motor. The normal configuration of the board has no links fitted.
The controller is also capable of driving a brushed motor by setting the sensor inputs to a fixed configuration
and using only two of the motor driver outputs. This can be most easily achieved by wiring the motor connector
to the required configuration. It is then possible to use the same motor controller and I/O boards in machines
using both brushed and brushless motors without changing the board set up.
Also included on this board is a circuit to oscillate the gutter valve on and off at a pre-selectable frequency of
3 Hz or 6 Hz. Selectable by J4, which can also disable the circuit. The effect of this circuit is to reduce the
solvent consumption caused by sucking back excess air. This valve oscillation only occurs when the machine is
printing, or is ready to print. i.e. EHT is on, therefore it is inhibited when the EHT is off via J5 and J3 pin 1 going
low. When the EHT is on ISO1 shorts out R44 therefore pulling the reference on U7 pin 13 up and enabling a
square wave output from the comparitor U7d.
The Quad Op-Amp U7 forms an astable oscillator which creates a triangular wave output on pin 8 which is fed
into the comparitor to generate the square wave output required to switch Q7 on and off. R 40 is used to adjust
the mark/space ratio of the signal.
There is also a small circuit designed to prevent the feed valve from overheating if left on for long periods.
The valve requires an electrical signal of 24 V for it to switch on, although once it is switched on it only requires
approximately 12 V to remain energised. A hit and drop circuit is used which switches R47 in series with the
feed valve 250 ms after retracting, reducing the voltage flowing across it. This is achieved by Q8 becoming
turned on when the feed valve is switched on by J2 pin 10 going low. 250 ms later C13 charges up allowing the
gate voltage to drop and for Q8 to turn off.
D5 protects the I/O board from damage due to C11 discharging during power down. D2 & D6 are flywheel
diodes which reduce the back EMF generated by the gutter valve, and the feed valve, respectively.
The 430 motor control PCB contains additional ‘hit and hold’ circuits to control the ‘Feed’ and ‘Bleed’ pico valves
mounted within the printhead. These valves require 24 V for about 50 ms for them to open, after which the drive
voltage must reduce to 12 V to hold them open without excessive dissipation. This is achieved with the circuit
shown in Figure 29.
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