||As mentioned in the platform description, 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.
The value of the 220k resistors from the position feedback potmeters together with the 180k feedback resistors determine total loop-gain, and need to be tweaked to get optimal platform motion response. You can do this by inserting a step pulse on the drive input, and check how the platform reacts. My final value was 220k increased to 820k for reasonable response, without overshoot. In case of oscillation, small capacitors may be needed across the 180k feedback resistors. Note that this tweaking needs to be done with all weight load added to the platform. I used three 20 liter water bottles to simulate my weight. I did not do any frequency analysis on the platform, but it was reasonable fast: A large +30 to -30 degrees movement is accomplished in about 400msec. Small signal frequencies up to about 10 Hz are definitely possible.