GEM Beamscraper Jaws - ISISComputingGroup/ibex_developers_manual GitHub Wiki
Wiki > The Backend System > Specific Device IOC > Jaws and Slits
The GEM "Beamscraper" jaw set is unique to GEM. The four blades move independently and are all driven by a LinMot. Some calibration is required between the demanded position and the steps sent to the controller. For North/South this calibration is linear, for East/West it is quadratic.
Control under SECI
LinMot motor resolution: 0.01953125 mm/steps (Mistakenly labelled as steps/mm in the VI)
| Jaw | Offset | Read Calibration Curve | Write Calibration Curve |
|---|---|---|---|
| North | 2710 | x + 0.025*x |
x - 0.025*x |
| South | 1530 | x + 0.025*x |
x - 0.025*x |
| East | 2200 | (-0.12038 + 0.06452 * x + 0.02998 * x**2)/0.9 |
(x - 0.1*x) , ((-0.06452 + sqrt(0.06452**2 + 4*0.02998*(0.12038 + x))) / (2*0.02998)) |
| West | 2200 | (-0.12038 + 0.06452 * x + 0.02998 * x**2)/0.9 |
(x - 0.1*x) , ((-0.06452 + sqrt(0.06452**2 + 4*0.02998*(0.12038 + x))) / (2*0.02998)) |
Where there are two equations the first equation calculates the x used in the second equation where there are two.
The motor resolution/offset values are used as below:
| Read/Write | Axis | Equation |
|---|---|---|
| Write | South | OFF + (x / MRES) |
| Write | N/E/W | OFF - (x / MRES) |
| Read | South | (x - OFF) * MRES) |
| Read | N/E/W | (OFF - x) * MRES) |
Where x has been calculated from the calibration curves above.
The setpoints are all set to 5 when home is pressed. There is some tolerance checking, tolerance is set to 1.00, and is based on the presented value.
The limits for the values that the user can set are:
| Axis | Min | Max |
|---|---|---|
| H Gap | 0.2 | 25 |
| V Gap | 0.2 | 40 |
| North | 0.1 | 20 |
| South | 0.1 | 20 |
| West | 0.1 | 12.5 |
| East | 0.1 | 12.5 |
Control under IBEX
The calibration curves under IBEX have been simplified to:
| Jaw | Offset | Read Calibration Curve | Write Calibration Curve |
|---|---|---|---|
| North | 2710 | 1.025*x |
x / 1.025 |
| South | 1530 | 1.025*x |
x / 1.025 |
| East | 2200 | -0.13376 + 0.07169 * x + 0.03331 * x**2 |
(-0.07169 + sqrt(0.07169**2 + 4 * 0.03331 *(0.13376 + x))) / (2*0.03331) |
| West | 2200 | -0.13376 + 0.07169 * x + 0.03331 * x**2 |
(-0.07169 + sqrt(0.07169**2 + 4 * 0.03331 *(0.13376 + x))) / (2*0.03331) |
Note that the linear calibration curve is actually incorrect in the VI as 1 / 1.025 != 0.975. Therefore in IBEX 1.025 and 1 / 1.025 are used for reading and writing respectively.
To do these calibrations an additional soft motor record has been placed between the conventional jaws db and the real motor, such as described here.
The offset / motor resolution in IBEX uses (MRES * x) + OFF when reading, therefore the new offset motor resolution in IBEX are:
| Jaw | New Offset | New Motor Resolution |
|---|---|---|
| South | -SECI_OFF * SECI_MRES |
SECI_MRES |
| N/E/W | SECI_OFF * SECI_MRES |
-SECI_MRES |
Tolerance and limits have been included in IBEX, the homing has not.