Control Servo Design
PROBLEM: PSAS seeks to design a rocket control system that will facilitate guidance of an amateur rocket. This control system uses aerodynamic fins to steer the rocket. The fins must be positioned very accurately, and they are under high aerodynamic loads. To position the fins, a control fin servo is required.
The control servo gearbox and control law must be designed.
REFERENCE DIAGRAM:
(ADD DRAWING OF SERVO)
Next items needed:
- Gear train torque / speed calculations
- Gear power flow sketch
- Gear size calculations
- Gear stress analysis
- Servo motor characteristics
- Position feedback design
- Electronic control circuit design
- Control law
Analysis
Gear-Train Torque / Speed Calculations
The fin actuation requirements call for 520 inch lbs of torque at the fin, actuating through 10 degrees in 0.060 seconds. Our motor produces 316 W (0.424 HP) at 30,000 RPM. For this analysis we will de-rate the spec to 213 W (0.285 HP):
(Google) (520 inch pounds force) * (10 degrees) / (0.060 seconds) = 170.902964 watts
At 80% efficiency, we need to apply 213 watts to the motor.
For our motor, 213 W at 30,000 RPM yields 0.6 inch lbs of torque.
(Google) (213 watts) / ((30 000 / 60) * 360 * (degrees / second)) = 0.60 inch pounds force
The fin speed required is 10 degrees / 0.060 seconds, or 174.5 degrees / second.
We arbitrarily select a servo gear ratio of 150:1.
- 0.60 inch lbs * 150 = 90 inch lbs at the fin.
- Not enough!!
- 0.60 inch lbs * 850 = 510 inch lbs at the fin.
- 30,000 RPM * 360 / 60 / 850 = 211 degrees / second at the fin.
850:1 seems like a reasonable gear ratio, but there is no way to get the motor up to 30,000 RPM in the short time that the fin needs to deflect.
Gear Power Flow Sketch
Gear Size Calculations
Gear Stress Analysis
Servo Motor Characteristics
The servo motor selected is an X12 Vector brushless DC motor. It uses an 8.5 turn octa-wind coil set, and has an integral hall effect sensor. This motor is rated at 316 Watts and 30,000 RPM. Dyno torque vs speed number are not available. Because this motor meets IFMAR RC racing rules, the hall sensor details are standardized.
Hall sensor connector: Six position JST ZH connector, model number ZHR-6. Uses 6 JST 26-28 awg contacts, part number SZH-002T-P0.5
| Pin | Description |
|---|---|
| 1 | Ground potential (black) |
| 2 | Phase C (orange) |
| 3 | Phase B (white) |
| 4 | Phase A (green) |
| 5 | Temp, 10k to ground (blue) |
| 6 | 5V d.c. +/- 10% (red) |
Motor Evaluation
To evaluate the torque/speed characteristics of the servo motor, we created a test jig using an 85 Amp 24 Volt (2k Watts!) speed control. Motor torque was measured using a digital fish scale and an eddy current brake.
The original intent was to characterize the motor performance over a range of RPM to determine the stall torque at the rated power of 213 watts. The problem, however, is that the motor would not produce over 1.8 inch lbs of torque at any speed, and even then the motor was hot enough to smoke. More realistic torque numbers for this motor are in the range of 0.8 inch lbs, with a peak torque near 1.5 inch lbs; however the peak torque is likely way over 213 Watts.
This is a significant issue. Either a gear ratio of 330:1 is needed, or a motor which can supply more torque is required.
As it turns out, an error was made when computing the torque supplied by the motor for a given speed and power. A correct torque figure is around 0.6 inch lbs at 30,000 RPM. A typical motor will deliver significantly more torque (at the same power) at low speed or stall. The chosen motor, however, saturates around 1.0 - 1.5 inch lbs, so a new motor may be required.
Upon finding the results of the first motor dyno tests it was determined that it would be best if we were to attempt to find another candidate motor. Currently we are looking at an 18v replacement drill motor from Dewalt. A link to the motor can be found here: http://www.robotmarketplace.com/products/BP396505-22.html. Additionally a local vendor has been identified. The candidate motor can be purchased from: DeWalt / Delta Porter-Cable Factory Service #027 14811 N E Airport Way Portland, OR 97230 USA 503-255-6556. The units are currently in stock as of 2/8/2012 and listed at the cost of $42.95 per unit. Its model number is 396505-22SV.
Position Feedback Design
Sensor Selection
Position feedback for the control fins is a tough problem considering the space available. The following are some requirements:
- High cycle endurance: > 2 million cycles
- Tight space requirements: < 0.625 inch in one dimension
- Linear response: < 2% non-linearity
- Fast sensor response: Omega_n >= 100 Hz
- Reliable brand: Bournes, etc.
Standard potentiometers are not rated for high actuator cycles (<100,000 cycles), and high quality potentiometers designed for 5-25 million cycles are too large for the space available.
Our present sensor under consideration is an inductive distance sensor from IFI:
- Inductive sensor
- Operating range 0.4...4 mm [nf]
- Analogue output 4...20 mA (linear, gradient: 4.44 mA/mm) *)
- non-flush mountable
- IFK3004A1PKG/US housing
- Metal thread M12 x 1
The sensor is mounted above a cam on the lower arm of the fin control assembly. The cam provides a steel target which can be sensed by the inductive sensor. The cam shape is designed to change the distance between the sensor and cam through 0.6 to 3.9 mm as the arm rotates through +- 17 degrees.
Sensor Evaluation
This sensor shows no measurable phase lag or amplitude change through 50Hz. For a sine wave target distance, a slight distortion exists above 25 Hz; but the general distance vs. voltage relationship remains.
Electronic Control Circuit Design
The servo control electronics are shared with some other projects. Our plan for control of the fins is to use a GFE and a Generic Motor Driver (GMD) .