||The platform drive
consists of a servo system with platform position feedback: The position
feedback signal is compared with the drive signal. The difference between
the actual position and the input drive value is amplified and used to drive
the motor system in the direction necessary to reduce or eliminate the
The complete circuit therefore consists of a differential amplifier, and a power buffer that must be able to drive the motor in both directions.
During the design phase I had a number of considerations for the circuit:
The Pitch and Roll drive circuit is shown below. It consists of a differential input circuit that provides the error signal, a sawtooth generator that determines the switching frequency, 4 comparators that make the PWM (Pulse Width Modulation) signals, 2 driver IC's that make the PWM signals suitable for driving the MOSFETS.
The high-side Mosfet drive has it's own floating supply. You could make use of the bootstrap trick as shown in the IC spec, but I decided for a separate floating supply via a small transformer, as I had read somewhere that the bootstrap application could have some problems, especially during start-up. With the solution shown above, the Mosfet drive always has correct gate drive swing, regardless the switching conditions.
The IRFP250 Mosfets are selected because they have rather fast inner body diodes, that carry part of the inductive current.
The driving signals for the IC are made such that there is some dead-time between the switch-off of one Mosfet and switch-on of the other. Without sufficient dead time, the two Mosfets could conduct simultaneously, shorting out the supply, thereby killing both Fets.
For the interface, I have used the parallel port DAC circuit below. Two latch IC's connected to the 8 bits from parallel port, and two control signals will latch the byte in either pitch or roll IC. The R/2R network will transform the 8 bits into a DC voltage, the value of which depends on the binary value of the 8 bits. An Op-Amp will buffer and transform the DC voltage in a -5 to +5V DC voltage.
The two DAC outputs are connected to the roll and pitch motion inputs of the servo drive. The input gain needed to be adjusted. Updated servo schematics below.
Later update: For improved speed and vibration capabilities, the values for R1 and R2 have been changed to 33k. Also C1 value have been reduced to 47nF.
Some pictures of the motion platform drive electronics. The heatsink does not need to be this big. Wiring is done via solder, transformer wire and thick wire for the high-current section. The layout of the IR2110 to the Mosfets must be done with care. As always with these kind of switch mode power circuits, keep small signal ground and power ground separate, and keep gate-drive current loops and supply decoupling loops small. There is no layout for this board, as all wiring is done with isolated wires at bottom side, but the new 3 channel motion drive has more details on layout, so that design could also be used for the 2DoF platform.