P11565: ITT Mirror Steering System
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Electrical Control System Design Documents

Table of Contents

System Modeling

Matlab's Simulink modeling software was used to model the system design and tune the PID controller. It includes the mechanical system as the plant, each buffer stage as a gain, the voice coils as a current to force transfer function, the voltage to current Voltage Controlled Current Source (vccs) as an attenuation and the feed back as well as the PID. The matlab model shows that the system is feasible all together, and it is possible to come close to and possibly meet specs (CN3) with the control electronics.

With the tuning shown below, the system gives a positional result as shown below while not railing any current or voltage rails at steady state. Also, no integration was necessary for controlling the device. Due to the noise injected by a derivator, however, a small integrator will most likely be added to keep steady state error small.

PD Controller

The control electronics are designed to reduce electrical noise and hysteresis of the voice coil while providing a current for voice coil motion The input command signal and sensor feedback network output are summed together in a buffer stage before entering the PD controller. The PD controller will be tuned to reduce the settling time to less than 80ms from the time of an applied voltage to the system (CN3). With current values, the settling time is about 72 ms. The rise time is about 22 ms.

The PD outputs to a non inverting summing amplifier stage that provides the input to the vccs stage. The stage starts with a buffer between the control system and the trans-conductance amp that also adds a feedback from the end of the stage. The OPA547 then provides a current based on the input voltage, acting as the VCCS. The resistor on top, in series sets the current for the load, which is a 25 ohm resistor for simulator purposes. The actual load would be two voice coils in series which would be modeled as a 25 ohm resistance in series with an 860uH inductor. With this load, a 121mA current can be developed for a 10V input command signal as shown on the output graph. The -100mV is the input to the PD controller and the 8V is the output from it.

OP177 was chosen to be used for the basic op-amp in the design because of the high voltage rails possible and the low noise it has at DC. It was also chosen based on Customer knowledge of and experience with the part. OPA547 was chosen for its high output current and excellent control over a wide range of voltages. It has a maximum rated output current of 500mA, more than enough for our desired design.

Power Regulator

The Power regulator in the circuit above is used to step down the ±24 volt supply and regulate a constant ±15V for the control circuit positive and negative powers. The LM337K was chosen for power regulation due to its ability to regulate negative voltage with low noise and the availability of a sister chip that regulated positive voltage. As can be seen below, when the regulators are run over an AC sweep, the negative regulator had the larger voltage spike at about 500Hz, though the spike is only 150mV. The regulators output ±15.02V at DC and the circuit is using real resistance and capacitance values.

Pictures and Design Files

Electronics Design Files
Matlab model PD Controller Power Regulator
Simulink Model

Simulink Model

Simulink Results

Simulink Results

PD Controller

PD Controller

Output

Output

Power Regulator

Power Regulator

Regulator Output

Regulator Output

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