In-Situ Calibration: Migrating Control System IP Module Calibration from the Bench to the Storage Ring

The Control System for the Advanced Light Source (ALS) at Lawrence Berkeley National Lab (LBNL) uses in-house designed IndustryPack (IP) modules contained in compact PCI (cPCI) crates with 16-bit analog I/O to control instrumentation. To make the IP modules interchangeable, each module is calibrated for gain and offset compensation. We initially developed a method of verifying and calibrating the IP modules in a lab bench test environment using a PC with LabVIEW. The subsequent discovery that the ADCs have significant drift characteristics over periods of days of installed operation prompted development of an “in-situ” calibration process--one in which the IP modules can be calibrated without removing them from the cPCI crates in the storage ring. This paper discusses the original LabVIEW PC calibration and the migration to the proposed in-situ EPICS control system calibration. INTRODUCTION The ALS Instrumentation Group designed custom IP Modules for the ALS Control System. Housed in cPCI crates, these modules control instruments such as magnet power supplies and beam position monitors (BPMs). Each IP Module consists of two 16-bit analog control channels (DACs) and four 16-bit analog monitor channels (ADCs). To make modules interchangeable and increase channel accuracy, each channel is calibrated for gain and offset compensation. We developed LabVIEW calibration software (IPCal) to calculate calibration constants for each channel and compile this data in a file. The file contains slope, offset, and mean-square-error (MSE) constants for each of the six channels on the IP module. Each IP module stores its calibration file in an onboard EEPROM. The ALS control system database uses the calibration constants to adjust data read from and written to each channel. Originally, the test bench setup was sufficient to perform an initial calibration on each module before installing the modules into cPCI crates in the storage ring. We then discovered that many of the ADC channel offsets drifted several mV over weeks of installed operation. Further investigation revealed that the ADC channels exhibit an exponential drift from their initial offset as illustrated in Figure 1. The ADC (ADS7825U) data sheet specifies a maximum zero offset error of ±10mV with a typical drift of ±2ppm/°C . ALS instrumentation and control requires a maximum drift of 10mV/year. These specifications coupled with the measured drift prompted a recalibration of each installed module. To perform the recalibration, each module would have to be removed during a storage ring shutdown, recalibrated in the test bench environment, and reinstalled during the same or a subsequent shutdown. Moreover, future recalibration, verification, and troubleshooting necessitate access to the installed and operating IP modules.