INTERFACING MAIN AXIS ENCODERS TO THE CONTROL SYSTEM OF THE GEMINI 8M TELESCOPES

Transcription

1 INTERFACING MAIN AXIS ENCODERS TO THE CONTROL SYSTEM OF THE GEMINI 8M TELESCOPES John Wilkes and Chris Carter ABSTRACT The Gemini Telescopes project is building two eight metre opticavinfrared telescopes, one will be in Hawaii and the other will be in Chile. The Mount Control System is part of this project that, amongst other things, is responsible for moving and tracking the two axes of the telescope. The servos that control the axes are implemented on PMAC: cards which are proprietary VME cards that use the Motorola digital signal processor. The encoding system that is used for position feedback is based around an optical tape encoding system supplied by Heidenhain. The firmware supplied on the PMAC cards is sufficient for most servo applications, however, the outputs from the Heidenhain read heads are not supported by standard PMAC inputs. These signals (two quadrature sinusoids with a period of one tape pitch) require a pitch interpolation algorithm that has to be implemented as user written code on the PMAC card. In addition, the raw Heidenhain signals suffer from repeatable errors due to head misalignment and inequalities in the hardware channels. These errors can be corrected by using a method known has Heydemann compensation, this also has to be implemented as part of the tape interface code. As well as the main tape encoders, there are friction driven incremental encoders, translation sensors and fiducial switches. All of these devices are interfaced to PMAC and the resulting values are combined in additional custom written code, known as the virtual encoder, to produce a single absolute position suitable for feedback to the servo loops. This paper will present an overview of the Gemini project and the place of the mount control system within it. A brief description of the PMAC card and the Heidenhain encoder will be given and then the interpolation, compensation and virtual encoder code will be described. INTRODUCTION The Gemini Telescopes project is an international partnership of the United States, the United Kingdom, Canada, Argentina and Brazil to build two high performance, 8-meter aperture optical/infrared telescopes. One will be on Mauna Kea, Hawaii, and the other on Cerro Pachon, Chile. Both telescopes should be operational by the end of The resulting facilities will exploit the best natural observing conditions to undertake a broad range of astronomical research programs within the national communities of the partner countries. For more information on the project see Author affiliation and information - Royal Greenwich Observatory, Madingley Road, Cambridge, CB3 OEZ, UK John Wilkes: jdw@ askam.ac.uk; WWW: Telephone: +44(0) ; Fax: +44(0) Chris Carter: cjc@ast.cam.ac.uk, WWW: Telephone: +44(0) ; Fax: +44(0) / 1

2 MOUNT CONTROL SYSTEM Overview The mount control system (MCS) is the part of the Gemini telescopes prqject that provides the basic ability to slew and track the telescope. As such, it forms the interface between the mount hardware (amplifiers, encoders, tachos) and the higher level telescope control system (TCS) computer. It also interfaces to a number of secondary systems that are required to provide services. The MCS is intended as an engineering interface to the mount and its subsystems - it is intended that the telescope could be run from here for initial set-up and engineering work. The MCS consists of the following subsystems: MCS SOFIWARE - Written in EPICS (Experimental Physics and Industrial Control System) this software is responsible for interfacing to the higher level computer system and to the user via engineering screens. This software also provides the intelligence and co-ordination required by the various MCS functions. SERVOS SUBSYSTEM - This implements a position servo for both main axes of the telescope (azimuth and elevation). ENCODER SUBSYSTEM - Provides interface to the various devices that are used to determine position feedback for the servo subsystem. This paper describes the operation of parts of this subsystem. INTERLOCK INTERFACE SUBSYSTEM - Provides an interface to the Gemini Interlock System (GIS) that has ultimate control of the power to the telescope drives and brakes. COUNTERWEIGHT SUBSYSTEM - There are four counterweight units mounted on the centre section of the telescope. These allow the elevation axis to be finely balanced in any of the instrument configurations. This subsystem implements the servos that control these units. SERVICE WRAP SUBSYSTEM - The cable wraps that carry power and services across the moving interfaces of the telescope axes are motorised separately from the axes themselves. This subsystem implements the servos that control the wrap drives. MONITORING SUBSYSTEM - Provides a general purpose monitoring function based upon a serial bus (CANbus) with distributed nodes. The MCS consists of a VME enclosure with a MVME 167 CPU running VxWorks and a set of EPICS compatible VME cards that provide the hardware YO function. For more details of the design of the MCS software and hardware see references 1 and 2. PMAC The Programmable Multi-Axis Controller or PMAC is a flexible, motor controller board produced by Delta Tau. The card comes in various formats (VME, PC and stand alone) and is based around a Motorola DSP. The standard card includes the firmware and enough interfacing components to control 4 separate motor axes via incremental encoder feedback. Further functionality (e.g. digital I/O, ADC channels as well as extra encoder counters and DAC channels to provide up to eight motor control channels per card) can be purchased as plug in accessory cards. For more details about PMAC see reference 3. The servo filter programmed into each PMAC motor channel contains a standard PID algorithm followed by a notch filter. It is possible, however, for the user to download a custom filter written in DSP machine code. It is this facility that we used to provide a suitable interface to our position encoding system. Gemini uses a single VME PMAC card (because of the amount of the user written DSP code) for each telescope axis, and a third VME PMAC to control all counterweight and cable wrap axes. Heidenhain Tape Encoder The Heidenhain tape encoder uses a metal tape with 40pm grating which is made by a continuous process. Each head (Gemini uses multiple heads per tape) has a single scanning window which reads the interference patterns of reflected light. These signals are output as two quadrature sinusoids with a period of one tape 712

3 pitch. When used with a suitable compensation system, the Heidenhain system produces errors far below the Gemini specification of 0.4 pm. For more about the Heidenhain encoder and the selection of this system for Gemini see reference 6. ENCODER INTERFACE TO PMAC Overview Figure 1 shows how each of the Heidenhain encoder heads interfaces to the PMAC cards. Other encoding devices (e.g. friction driven encoders, translation sensors and fiducial switches) interface via standard PMAC hardware and software. I* * User-wrinsnazde DSP56ooo I t encoden -- PM4C card (Axis absolute positim available to EPICS) J The tape head electronics consist of amplifiers that buffer the head sinusoids to PMAC ADC channels and a comparator to provide pitch counting signals which interface to PMAC incremental encoder counters. The tape interpolation and compensation code and the virtual encoder code are described below. Interpolation Algorithm To provide a position value from each tape encoder head, we need to interpolate between pitches using the analogue sine and cosine signals. The following algorithm will do this: Normalise the sinusoidal signals so that they lie between -1 and 1. Take the arc-tangent of the sinusoidal amplitude, using both the sine and cosine values to obtain an unambiguous angle, A, in the range 0"-360". Calculate Position using: Scale the output correctly (1 bit = 5 milli-arcseconds on axis). Pitchcounts is taken fxom the PMAC counter that counts tape pitches. A is calculated using a reduced accuracy arctangent function based around two, two-term Taylor series expansions evaluated at different operating points. Heydemann Compensation The majority of the periodic errors present in the Heidenhain measurement will be due to: Lack of quadrature (phase shift between sin and cos signals is not exactly u4 or 90"). The sinusoidal signals having unequal amplitudes. 7/3

4 0 A finite DC offset in either or both of the sinusoidal signals. These errors can be reduced by using a well known compensation technique, developed by Peter L. M. Heydemann4.. This technique was developed for use in interferometers with quadrature detector systems. The Heidenhain tape encoder is, effectively, such a system. In order to obtain the correction parameters required by the Heydemann correction procedure a set of quadrature values, i.e. the sine and cosine values that are used by the arc-tangent function, need to be acquired. These values should be taken at intervals across the tape pitch so that when the two quadrature signals are plotted against one another, they will form an ellipse. Differences between this ellipse and a perfect circle are caused by the factors mentioned above. The equation of the ellipse can be found using a least squares fitting routine. The parameters of this equation can be used to apply corrections to the raw tape head signals in real time in the tape compensation code. To compensate for changes in error along the length of tape (caused by changes in the head position relative to the tape and local obstructions) the tape can be divided into overlapping regions and local compensation parameters can be calculated for each region. A look up table must then be used to ensure that the correct parameters are used for the current tape region. Virtual Encoder The virtual encoder implements one of a series of algorithms which take all the encoding devices as inputs (i.e. all tape encoder heads, friction driven encoders, linear translation sensors and fiducial switches) and provide a single position value to be passed on to the servo filter. The algorithms include, but are not limited to: 0 The average of the outputs of all four azimuth tape encoder heads. 0 The average of the outputs of any two azimuth tape encoder heads. 0 The average of the outputs of both elevation tape encoder heads with correction made for axis translation. 0 the output of any one elevation head with correction made for axis translation. 0 The output of any one of the tape encoder heads. 0 The output of the FDE. It is possible to easily change and extend the algorithms to include new encoders. CONCLUSIONS The MCS is currently in its first implementation stage. The DSP code that implements the tape interpolation and compensation and the virtual encoder is partially written but will not be finally completed until later this year. It is, perhaps, to early to draw conclusions as to the success of the approach we have used and to learn any lessons from it. However, we could not have even considered such approach if it were not for the availability of fast DSP processors like the and flexible hardware implementations such as PMAC. Certainly, we would not have been able to utilise the Heidenhain encoder since, without the use of fast DSP code, it would have proved very difficult to provide a suitable interface. The PMAC card, with its DSP, also allows us flexibility in the type of servo algorithm used. The current design uses the standard PMAC servo filter, but it would be a simple matter (purely a software change) to change the form of this. REFERENCES 1. John Wilkes and Chris Carter, Mount Control System - Control System Design Description. Issue 3, Gemini Telescopes Project, January

5 2. Andy Foster, Mount Control System - Software Design Description. Version 4, Gemini Telescopes Project MCSAJFOS, February PMAC User s Manual, DeItu Tau Datu Systems, December Peter L. M. Heydemann, Determination and correction of quadrature fringe measurement errors in interferometers, Applied Optics Vol. 20 No. 19, pp , K. P. Birch, Optical fringe subdivision with nanometric accuracy, Precision Engineering Vol. 12 No. 4, pp , John Wilkes and Martin Fisher, Selection of a tape encoding system for the main axis of the Gemini telescopes, SPZE Proceedings: Telescope Control Systems ZZ, Hilton Lewis, Vol. 3112, No. 5, July The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WCZR OBL. UK. 7/5