Stepper Motor Valves

Stepper motors are used to actuate four of the valves used to control the target: the transfer line needle valve, the separator throttle valve, the Run valve, and the Bypass valve. This section describes the use of these valves and the ``expert'' GUI that may be used to set up their control parameters.

Figure 11: Control GUI for a stepper-motor driven valve, IPStepper_2.adl.

Two pieces of electronics are necessary to make the stepper motor work, a control circuit and a driver circuit. The IP-Stepper is an industry pack mounted on the VME carrier board and is used as the controller for two valves. Thus there are two IP-Steppers on the carrier board. The controller sends two signals to the drive circuit: a series of TTL pulses at a specified frequency, and a TTL-compatible direction bit. The drive circuit amplifies and redirects the TTL pulses in a specific order to the six windings of the stepper motor. Based on the direction bit, the order will cause the motor to rotate in a clockwise or counterclockwise fashion. The drive circuit is designed by the Target Group and is located in a separate enclosure from the controller. The driver also allows for manual control of the stepper motor using a pair of panel mounted switches and an internal 60 Hz pulser.

The four stepper motor valves each has a rotary optical encoder mounted either to the motor shaft or to the valve stem itself. The encoder is read by a display unit (US Digital ED2), and has a resolution of 0.0025 revolutions. The motors themselves only have a resolution of 0.005 revolutions. The ED2 units (there are two of them) are read into the control computer over serial lines.

For the most part the valves can be operated without any knowledge that a stepper motor is used to drive the valve. However, an ``expert'' GUI exists for the valves that can be used to modify the stepper control parameters. Figure 11 show one such screen, for the Run and Bypass valves. A similar GUI is available for the other two valves. Both are accessible from the ``Instrument Expert Controls'' menu at the top of the Expert GUI.

The following table describes most of this GUI's features.

1. Enable/Disable buttons allow the user to disable computer control of the valve;
2. Input mode: NO = PID, YES = Manual;
3. Pos. Rdbck is the current encoder value, RST button resets the RS-232 communication protocall for the ED-2 display unit;
4. Motor Busy light will go GREEN when the valve moves; RED when stationary;
5. A ``CW/CCW'' indicator to the left of the BUSY light will indicate motor direction;
6. ``Conversion'' is the number of motor pulses necessary for one valve revolution, about 200 for each valve;
7. Normal/Reverse buttons allow the user to reverse the definition of OPEN and CLOSE for a valve (eg if a valve goes in the wrong direction, try toggling the Normal/Reverse buttons). This feature is necessary because some motors face in the opposite direction of the valve stem;
8. Frequency is the frequency of pulses produced by the IP-stepper controller. While not critical, a value of 50 works well;