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 motor drive supply must be able to source and sink current: During acceleration, current flows into the motor, but at the end of the motion, the motor becomes a generator, delivering current to the driver. |
| The motor drive current handling must be considerable: I measured 25Amp surges. |
| The motor drive must be class D switching type. Switching frequency above hearing frequency: >18kHz. |
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.
Each axis motor is driven by a half-bridge
configuration with dual (4x12V 7Ah battery) supply: Two hefty N-Mosfets
IRFP250 that are driven by a low-side high-side driver IC
IR2110
This IC is capable of driving the bottom Mosfet and the level-shifted top
Mosfet, which gate drive signal rides on the center switching waveform. Two
separate inputs are available for controlling each Mosfet.
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 working of the high and low side Mosfet drive is shown above:
The drive and
position feedback are added together. The output of the opamp drives the
inputs of two comparators, where the resistor network takes care that there
is a small offset between the two comparators inputs. The other inputs of
the comparators are driven by a triangle waveform. (actually it's a sawtooth but
it does not make a difference)
The resulting outputs of the comparators is shown at the right. Moving the
DC voltages up will result in longer on-times of the low side Mosfet and
shorter On times for the Hi-side Mosfet and visa versa. Care must be
taken that the comparator voltages stay within the triangle waveform to
ensure continuous switching waveforms. The dead time between two transitions
will remain constant.
The small signal power supply for the drive electronics is a straightforward +/- 15V
supply. The high power supply is using batteries as they can deliver lots of
current and most important, also can sink lots of current. When switching on the platform drive power section, it is important
that this is done as gently as possible. Since it is a closed loop system,
the platform drive will try to correct the platform position immediately
after switch-on. To do this in a gentle way, the supply of the
half-bridge is done in two steps: After switch-on, after some delay
the half-bridge +/- 24V supply comes up with two 47 Ohm resistors in series
via relay A, and after some further delay the second relay (B) comes in,
connecting the supply directly to the bridge. The two delay circuits that
drive the relays A and B is activated by the platform on-off switch. To be
operational, all end-stop switches must be in closed position. As soon as
one of the end-stop switches is opened, the supply to the half-bridge is
disabled.
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.
There is still some space on the original Motion drive board to fit the
dual DAC's driven
from parallel port. The DAC outputs are amplified and level shifted to get a
DAC signal that varies between -5V (FF) and +5V (00). If no software
filtering is applied, a capacitor in the opamp feedback loop can be used to
slow down the output. The capacitor value can be tweaked to get the right
response. Too little and you'll notice the digital noise and steps. Too much
filtering and the motion response becomes sluggish and delays between
scenery movement and platform movement may become unacceptable.
I used 2.2uF for both C1 values which sets the high frequency roll off around
3Hz.
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.