Doc#:2100‐AD‐008‐0002
Revision:‐
4mDomeDriveUpgradeInitialProject
DefinitionMeetingNotes
Author(s): Matthew John Evatt 4m Dome Drive Upgrade Initial Project Definition Meeting Notes RevisionHistory
Rev
Date
Approved
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04‐10‐2015
Initiated
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04‐14‐2015
Revised and Released (added post meeting
M.Evatt
notes)
ReasonforChange
Description/Remarks
Name
M.Evatt
2100‐AD‐008‐0002.DOCX Page 2 of 5 4m Dome Drive Upgrade Initial Project Definition Meeting Notes 1. Document Purpose The purpose of this document is to capture the initial meeting held to discuss the goals for a possible project to upgrade the 4m dome drives. 2. Meeting Purpose The purpose of the meeting was to make a first attempt at defining the goals for upgrading the 4m dome drive system. 3. Attendees John Donaldson, Matt Evatt, Ron George, and Shelby Gott. 4. Meeting Notes 4.1. Goals for 4m Dome Drive Upgrade Why is this project being proposed? What problems will be solved by upgrading the dome drive system? 4.1.1. Enable continuous dome movement during telescope tracking Currently, the dome does not move continuously with the telescope while the telescope is tracking. Instead, the control system moves the dome so the telescope field of view is near the east edge of the dome slit, then it stops and waits for the field of view to drift close to the west edge of the dome slit. When the field of view gets close to the west edge, the dome is moved so the field of view is again close to the eastern edge of the slit. 4.1.2. Provide active dome rotation braking Currently, the dome drive system has no brakes. The plan is to replace the existing motors with VFD rated motors containing integral motor brakes. As the system is now, the dome can be rotated by the wind even when the dome drives are off. This behavior presents problems during in several scenarios: 4.1.2.1. Unexpected and uncontrolled rotation during dome work Currently, the dome has to be physically blocked to prevent rotation; while we would continue this practice even after adding brakes to the system, it would add a further layer of safety. 4.1.2.2. Unexpected and uncontrolled rotation during crane work Because the 5 and 50 ton bridge cranes mounted at the top of the dome can only move toward and away from the center of the dome on their radial track, they rely on dome rotation to provide their full coverage area in the building. However, because the dome doesn’t have brakes, the dome has to be physically blocked from moving during critical lifts. Additionally, any crane work that requires rotation of the dome cannot be performed during high winds due to the risk of uncontrolled wind‐induced rotation. 4.1.2.3. Unexpected and uncontrolled rotation any time Any time the dome rotates without being commanded to do so, an unsafe 2100‐AD‐008‐0002.DOCX Page 3 of 5 4m Dome Drive Upgrade Initial Project Definition Meeting Notes condition exists. The potential exists for someone to get caught between rotating and stationary parts of the dome while simply looking at something if the dome begins to move due to high winds. Motor brakes would prevent this. 4.1.3. Provide finer dome rotation control during crane operations The current drive system has only one acceleration, velocity, and deceleration profile; dome speed ramps up to its maximum, holds there for as long as commanded, then ramps down to zero when the rotate command signal is removed. With an upgraded control system, it would be possible to provide multiple motion profiles to the drive. In addition to the tracking profile for use with during observing, motion profiles could be programmed for crane work. Newer cranes typically have two or three speeds; this behavior could be programmed into the dome rotation for use with the cranes. Currently, when the dome rotation is stopped, the dome coasts for several seconds. This makes fine positioning with the crane tricky and potentially unsafe without a skilled operator used to this particular crane. Safety could be increased with customized motion profiles specifically for crane operations. This factor is becoming more important with the combination of the upcoming DESI project and the potential retirement of our most skilled dome crane operator. Additionally, we would like to be able to control the speed down to zero. 4.1.4. Dome More Responsive to E‐Stop Currently, if an E‐Stop button is pressed and the dome is moving at full speed, it can take 10 seconds or so for the dome to stop. With motor brakes and load resistors on the VFDs, the dome could be brought to a stop much more quickly. 4.1.5. Higher Top Speed It is possible that with a modern VFD and motor, the system could run at a somewhat higher speed than is currently possible. This may be advantageous during the DESI survey if it turns out that the telescope has to wait for the dome to move to new positions. 4.2. General Issues that Should Be Addressed If The Project is Performed 4.2.1. Open Communications The communications interface with the new system should be open in the sense of allowing Shelby to write software to command it and change parameters and to receive telemetry from the system (e.g. Modbus over IP or similar). 4.2.2. Evaluation of Existing Gearboxes The existing gearboxes need to be evaluated to see if they need to be replaced. Research says they are rated to deliver an output torque of 15,900 in‐lbs. Assuming a motor speed (and gearbox input speed) of 1750 RPM, this equates to a power rating of about 5.9hp. The existing motors are 3hp. Preliminarily, we should be able to increase the motor size to 5hp if needed to run the system near zero velocity. It should be 2100‐AD‐008‐0002.DOCX Page 4 of 5 4m Dome Drive Upgrade Initial Project Definition Meeting Notes noted that the existing gearboxes have a service factor of I. which means they are designed for up to 10 hours of uniformly loaded operation per day. Because we are apparently have a minimum safety factor of approximately 2.0 to the rated output power of the gearbox now, this service factor is probably conservative. 4.2.3. Wiring Upgrades During troubleshooting of the intermittent dome crane limit induced telescope drive failures, Ron George and the EM group found a junction box in the dome that contains many connections relating to dome limit switches. This junction box and the connections inside it are in poor condition; both the box and the connections are corroded and the connections are not labeled. This junction box needs to be replaced and the wiring needs to be reworked as part of the dome drive upgrade as it is part of the system. 4.2.4. VFD Placement The group discussed placement of upgraded VFDs; it was agreed that placing the VFDs in the screen room where the existing drives are located is not optimal. Instead, the VFDs should probably be placed as close to the motors as possible. This should reduce EMI / RFI emissions from the wiring during observing and CCD readout. 4.3. Schedule Preparations and a portion of this upgrade work could be done without affecting operations, but the actual installation of the motors, drives, and load resistors should be done during a shutdown to accommodate the physical installation and then testing. The drives and motors could be set up on a bench prior to installation which would allow VFD and motor testing functional testing (e.g. how do the drives behave when unequally loaded, etc.), as well as aiding in software writing and testing. Physical installation of the VFDs and load resistors could be performed at any time; those components could be installed and waiting for the motor swap to take place. 5. Additional Information Post Meeting As a result of troubleshooting the intermittent “CRANE STATION ON” problem that causes the telescope drives to trip, Ron G. has found that the problem is most likely coming from the slip rings. Mike H. explained to Ron that it has been impossible to buy the correct size shoes for the 24 volt slip rings for many years. Instead, they use smaller shoes. The wear patterns on these shoes indicate that as they wear, they become very uncentered on the slip ring. As they get worse over time, any runout in the slip rings can start to show up as broken contact. Ron and Mike changed out the shoes after Ron found this issue and it seemed that the intermittent contact problem went away. As a result of this investigation, we are considering adding to the scope of the dome drive upgrade project a subproject to either find the correctly sized slip ring shoes or to purchase raw material and to make them ourselves. 2100‐AD‐008‐0002.DOCX Page 5 of 5