Instruction Manual

Overspeed Limiter

Manual written by

Jon Hagen

8-9-99

Arecibo Observatory

National Astronomy and Ionosphere Center (NAIC)

Electronics Department

Arecibo, Puerto Rico, 00612

Table of Contents

1. Introduction 3

2. Description 3

3. Operating Instructions 4

4. Electrical Connections 6

5. Description of circuitry 6

6 Encoder 7

7. Precision and accuracy considerations 8

Appendix 1. Memo by Luis Murray

Appendix 2. Schematic Diagrams

Appendix 3. Encoder data sheets

file: c:\hagen\wpfiles\overlim.wp6 jbh 8-9-99

Dome and Carriage House Overspeed Limiters

1. Introduction

Identical overspeed limiters are fitted to the Gregorian Dome and the carriage house. These are safety devices, designed to produce an emergency stop in the abnormal event that the velocity exceeds the limit imposed by the drive systems installed by Vertex GMBH. This extra level of safety is deemed necessary because neither the dome nor the carriage house is counterbalanced against gravity; if ever released from 20 degrees (a height of 26 feet with respect to 0 degrees), they would reach the devastating velocity of 2gh = 41 ft/sec (28 miles per hour). Like locomotives on a cog railway, the dome and the carriage house are kept from rolling downhill by pinions that engage a rack gear. The torque on these pinions is supplied either by the motors, (when the drive systems are active) or by parking brakes (when the drive systems are turned off). The parking brakes, which are built into the motors, are "normally on" They are applied by springs and are released by electromagnets. This protects the system in the event of sudden and total power failure. However, it is not hard to imagine a software or hardware failure in the Vertex systems that could leave the motor windings with zero current while the brakes are released. The overspeed limiters are designed for this situation. Their function is to apply the brakes quickly via the Vertex emergency stop loop (a simple hardware circuit). "Quickly" means in less than about 1/4 second, while the kinetic energy is still small enough to be absorbed by the tuny parking brakes.

2. Description

Each limiter consists of an incremental encoder which is driven by an 18-tooth pinion engaging the rack gear and which is electrically connected to its own control box. Speed is measured by counting the the elapsed time (ticks from an internal clock) between four encoder pulses, which is the time for 0.188" of travel along the elevation rail. Whenever the elapsed time is less than a fixed threshold, a relay opens and breaks the Vertex system's emergency stop loop. The threshold speed is set at 1.5 x slew speed = 1.5 (2.5 deg/min) = 3.75 deg/min = .0625 deg/sec. Slew speed corresponds to a velocity of 3.792"/sec along the rack gear, so the time threshold is

(0.188/3.792)(1/1.5) = 33 msec. The relay has a second pair of contacts that turn on an "overspeed" indicator lamp inside the control box. This overspeed status is also transmitted via optical fiber to the control room, indicating that an engineer must go up to the platform to check the overspeed limiter and the drive system and, when satisfied that everything is in order, push a reset button in the control box to re-enable the drive system. After six months of use, there has been only one overspeed trip event - coincident with severe arcing in the 430 MHz slotted waveguide and probably not an actual overspeed occurrence.

Sealed lead-acid batteries prove each overspeed limiter with back-up power; automatic shutdown in the event of prime power failure remains the responsibility of the Vertex system. AC power is keeps the batteries charged. In the event of power failure (ac and battery), the overspeed limiter triggers as if overspeed had occurred. A second latched indicator lamp indicates, however, that battery failure, not overspeed, was the triggering event.

3. Operating Instructions

When the dome or the carriage house has been immobilized by the tripping of its respective overspeed limiter, the limiter will remain tripped until manually reset via a "reset" button on the panel of its control box. One of two indicator lamps will be lit: "overspeed" or "battery failure". Pressing "reset" will turn off the lamp and enable the system (by eliminating the interruption of the emergency stop loop). The drive system should be checked carefully before resetting the unit; don't assume the tripping has been a false alarm. The "battery failure" lamp indicates there has been a simultaneous failure of both ac power and battery power. When the "battery test" button is pressed, the battery test meter should indicate about 80% of full scale.

Another button on the panel of the control box is marked "emergency stop". Pressing this button has the same effect as overspeed; the unit will trip. This can be used to verify that the unit can indeed interrupt the emergency stop loop. But a better test is a real overspeed test. Since the Vertex drive system cannot be commanded to 1.5 x slew speed (slew speed is its speed limit), a toggle switch on the panel of the control box cuts the trigger threshold in half, to 0.75 x slew speed. Periodically, the unit should be tested by selecting this 0.75 position and then driving downhill (from an already low position) faster than 0.75 x slew speed. The unit should trip, causing an emergency stop. If the unit fails to trip, check to see whether the encoder is bad. Use the dummy encoder unit (see schematic diagram). This unit cab be plugged into the control box in place of the actual encoder. The dummy encoder has an internal oscillator. When the speed knob is brought up 1.5 x slew (or 0.75 x slew) the overspeed limiter should trigger.

The overspeed limiter is not inherently fail safe. The tests described above should be made every few months. Meanwhile, however, each unit sends status words to the control room whenever its system (dome or carriage house) is moving downhill. This status can be monitored automatically to produce a warning when the status word indicates a measured speed different from the speed reported by the Vertex system.

A status word is sent for every 0.188" of downhill travel. No status words are normally sent when the system is stopped (or traveling uphill). If, however, the unit is tripped, a status word is sent every 13.3 msec. The status word has 8 serial bits, transmitted at 1200 baud. It is preceded by one start bit. The start bit is followed by a 6-bit field (LSB first) that indicates measured speed. The 7th bit is the overspeed flag . The 8th bit indicates that the ac power is alive (i.e. that nobody has inadvertently disconnected the ac power).

If bit 7 is a "one", indicating that the unit has tripped, all the other bits will also be "ones". In this case, the only information being sent is that the unit has been tripped and must be reset.

Normally, bit 7 will be a "zero". In this case, the data word is valid and the speed can be calculated as follows:

Note: The 6-bit speed data saturates at 63; longer elapsed times will all be returned

as 63 as shown above on the graph below.

The serial status words from the overspeed limiters should be constantly compared with speed data from the Vertex system to verify proper operation.

4. Electrical connections to overspeed limiter control box:

1. Vertex Emergency Off loop (2 wires)

2. 120 VAC power cord

3. One optical fiber optic to the control room (status information)

5. Description of Circuitry

See the attached 3-page schematic. The limiter has an internal 1200 Hz clock. It counts the number of clock pulses that occur during an interval 1 ms shorter than the time needed for the encoder to advance downhill by n counts. We've set n = 4 so that at slew speed, the expected count at slew speed will be 1200[(4/600)/.134 -.001] = 58.5. In order to trip at 1.5 x slew speed, the count threshold is set at 58.5/1.5 = 39.

Page 1 of the schematic shows the input conditioner and divider. The encoder's quadrature signals, A and B are fed to the conditioner. The conditioner produces a signal A' which, for constant downhill motion, is the same as A. The conditioned signal, however, has hysteresis. If the encoder is moving back and forth across an A transition, A' will go hi and remain high while B is hi or A' will go lo and remain low while B is lo. During uphill motion A' will have no transitions. Presetting the divider to 11 results in a division factor of 15-11=4.

Referring to page 2 of the schematic, clock pulses are counted by an 8-bit binary counter. At the end of the count gate, this counter is preset to 89. If, during the next clock gate, 39 or more clock pulses occur, the MSB of the counter will go hi. The counter will never overflow; when it reaches its terminal count (TC) it gates itself off and the count will remain at the terminal count until the end of the gate. At the end of the count gate, the MSB of the counter is loaded into the overspeed flip flop. If this MSB is a one, an OR gate at the D input keeps the flip-flop hi until it is manually reset.

6. Encoder

The encoder is the Dynapar HS35-0060-70115 Hollowshaft Encoder. This $305 unit fits over a 0.500" diameter shaft. Its body is "tethered" by a slightly flexible mounting bracket, simplifying the mechanical alignment. This model has 600 counts per revolution. (Many other count options are available; the most popular are 60, 600, and 1024). 800-621-8276 Dalec (distributor) Dave Nelson, Appl. Eng. One or two encoders should be kept on hand as replacement parts.

7. Precision and accuracy considerations

If the Gregorian is somehow abruptly released near 20 degrees it will accelerate downhill at a rate estimated to be about 4ft/sec2 . It takes only .08 sec for the velocity to increase by an amount equal to the slew velocity (.316 ft/sec). The only brakes on the Gregorian are small parking brakes. These brakes are not sufficient to stop the Gregorian except from velocities below, say, four times the slew velocity. It is therefore desirable that the overspeed limiter have a response time of .08 sec or less and necessary that the response time be no longer than, say 0.24 sec.

At slew rate, the encoder pinions are turning at 0.134 rev/sec. In .08 sec they therefore turn only .0115(.08)= .0107 rev = .0673 radian. The encoder must therefore produce at least

1/.01072 = 93 pulses per revolution. It seems reasonable to order encoders with 600 pulses per revolution. This would allow us to read the velocity as often as every .014 sec.

While it should be easy to find suitable encoders, the necessary fast response will make the system susceptible to imperfections in the rack gear and the limiters mounting hardware. The joints between sections of rack gear, for example, may momentarily increase the encoder rate and trigger the limiter. These imperfections will determine the final sensitivity of the limiter. The threshold will have to be set at a value for which there is no more than , say, one or two false detections per year. This may require that the integration time (response time) be longer than .08 seconds.