General Motion Stages:

General stages operate in one of two modes depending on the stage's function. The first type, a rotary stage, is capable of continuous movement in either direction. This type of stage is positioned relative to a home position. This position is determined in each case by using an optical slotted switch which provides the function of a fiducial. Once the stage has been 'homed' the targeted position is achieved by counting the encoder pulses. The second type of stage is a constrained motion stage. This type of stage is constrained by forward and reverse limits. Each stage has three separate limits for both forward and reverse motion(Fig. 2). The first limit that the stage encounters is the software limit. The routines moving the stage have a maximum travel limit at which the software stops further motion the that direction. In case something goes wrong, the stage next encounters the primary hardware limit switch. This switch is connected to the motor controller and signals it to ramp down the motor to a stop. It also disables further motion in that direction. The final hardware limit switch, the secondary limit, is wired in series with the motor power lead and shuts off power to the motor when encountered. It also is connected to a digital input bit on the Galil motor controller. This lets the upper levels of software know the stage has traveled into the final limit.

In normal operation, the stage should never make it to any of the three limits. If something goes wrong each limit should cause the stage motion to stop. If a moving stage makes it to either the software limit or the primary limit the stage can be moved only in the opposite direction to back out of the limit. In the case of the secondary limit however, the motor power is held off by the limit and the only way to back out of this limit is to manually turn the shaft of the motor!

There are variations among the stages in the type and number of encoder(s) used. See below for a discussion.

---

Single-loop and Dual-loop encoders.

1. The first type, single-loop encoding, is the normal way of running a servo motor. The motor encoder is connected directly to the controller and we position to the motor encoder reading. single-loop encoding is used on the Guide TV Focus, Guide TV Filter Wheel, CCD Focus, CCD Translation, Grating Slide, and Slitmask stages.

2. The other type of encoding that we use is dual-loop encoding. In this scheme, the load encoder is wired (via the EL-1272 interconnect box) into the motor encoder's input and the motor encoder is wired into the auxiliary input. This allows the stage to servo on the load encoder giving finer control of the positioning of the stage. In DEIMOS, we are using Gurley Precision 835 series encoders on the grating tilt stages. These are mounted on axis on the tilt stages and because of physical restraints, the encoders are mounted to different sides on the different tilt stages. Thus, the stage interconnect box on tilt #4 is wired 'normally' but tilt #3 is wired in 'reverse'. To accommodate the opposite rotation of the encoder relative to the servo motor encoder, the 'A' and 'B' phases of the encoder are exchanged at the terminal strip inside the stage interconnect boxes (EL-1272REV.)

Fig. 1

Renishaw encoders as the main encoders in the three collimator stages, Filter Wheel, Slitmask Wheel, and Aperture Wheel stages. The Collimator stages use model RGH22Y .1 micron encoders while the other stages use the model RGH22D 5 micron encoder. The Renishaw units are linear encoders that read a tape strip. In our application the tape is applied to the moving portion of the collimator actuators and to the edge of the moving wheel of the three wheel stages. The read heads are then aligned for optimum performance. The heads have a two colored LED that is used for alignment. When the LED is green the encoder has locked onto the tape. If the indicator turns red the readings are no longer guaranteed. This doesn't necessarily mean the the encoder has lost it's position but it may have and it should be adjusted.

Encoder Homing.

To set the zero point on our incremental encoders, we use a homing scheme that consists of moving the stage to the Home flag at the normal running speed of the stage and looking for a transition of the fiducial signal. At this point, the controller ramps down to a stop.

 

Fig. 3

The direction of movement is then reversed and the motor runs at a very slow speed back to the edge of the flag. When the fiducial signal changes state the the controller assigns this as the zero point, that is, encoder counter buffer is set to zero. At this point, with the zero point of the stage set, the stage can be moved to any desired position. Note however, if the stage is not homed before moving it, it can be run into the primary limit switches because the position of the stage is not known and the software limits may or may not be encountered before reaching the primary limits. More important, if the stage is moving with sufficient speed it can reach the primary limits start ramping down and possibly travel to into the secondary limit. If this happens, the only way to move the stage is to manually turn the shaft of the servo motor!

In stages where it is possible, the blade or flag is made to be just over one half of the stage travel. In this case, the control electronics can tell at power up which direction to move to find the edge of the flag. For instance, when the power comes up and a particular stage looks at the signal from its fiducial, it will know which direction to move based on whether the signal is high, (the blade is blocking the beam) or low. This, of course, means that the blade must extend beyond the secondary limits so that the stage can't get into a condition where the fiducial gives a false reading.

Page last updated: May 8, 2002