System Features

The FCP electromagnets feature continuously adjustable air gaps, allowing rapid air gap changes to suit individual experiments. They have water-cooled coils and precision yokes made of magnetically soft, ultra pure steel, assuring precise pole cap alignment as well as excellent field homogeneity and stability. The H-yoke configurations are oriented at a 45° angle for optimum air gap accessibility. The electromagnets also incorporate compact size with steel eyebolts, permitting convenient bench top mounting to optical tables.

The FCP magnet power supplies provide stable current regulation and true, bipolar power output. The bipolar output allows rapid uniform magnetic field ramping and field reversal, avoiding discontinuities that occur during zero crossover when using unipolar power supplies. The bipolar output also eliminates the need for current reversal contactors or relays. The magnet power supplies are CE compliant and include features such as automatic power output disable upon failure detection, operation as a current or voltage source, and operation in either manual or remote mode.

Unlike other commercial solutions, the Lake Shore FCP system incorporates closed-loop field control (also called feedback control) capability directly into the Model 475 DSP gaussmeter, eliminating the need for a separate computer with redundant software control. The Model 475 includes the electronics and firmware to perform closed-loop, digital PI field control of the electromagnet and magnet power supply.

Control Features

The Model 475 attempts to keep the magnetic field density at exactly the user-entered control setpoint, expressed in gauss, tesla, oersted, or ampere/meter. To do this, the Model 475 uses feedback from the gaussmeter Hall probe to calculate and actively adjust the control (analog) output. In order to maximize control stability, the Model 475 updates the analog output every 33 ms. The result is an internal PI controller providing peak-to-peak field stability of 0.5 G*.

When the setpoint ramp is enabled, the instrument will start ramping from the current field reading, rather than the current setpoint, based on the user settable P and I settings. In addition, the 475 can be configured to ramp the control setpoint from the present field reading to a new value by using a smooth linear transition in field rather than the step response PI control.

Open-loop field control is also possible by using the Model 475 analog output in manual output, meaning feedback is ignored and the analog output stays at the manual user setting. This method makes the magnet power supply operate in constant current mode.

The Model 475 also incorporates user settable control slope limits and analog output voltage limits. These software limits assure that the magnet power supply is not damaged if the field control system is improperly tuned or begins to oscillate.

*Measured at 0 G and 1000 G

Following are examples of the various display configurations and setup screens:

The display configured to turn the field control On/Off



The display configured to enter the field control setpoint



The display configured to enter the field control ‘P’ value



The display configured to enter the field control ‘I’ value



The display configured to enter the field control ramp rate



The display configured to set the field control slope limit



The display configured to show the field control setpoint and current field value, when field control is active



Gaussmeter Display

The Model 475 has a two-line by 20 character vacuum fluorescent display. During normal operation, the display is used to report the current filed value and field control setpoint. When setting instrument field control parameters, the display provides prompts for turning the control on/off, entering the control setpoint, entering the control P and I values, entering a ramp rate, and entering the control slope limit. When field control is not active, the display can be configured to show features such as maximum and minimum DC field values, RMS field value, frequency, or probe temperature.


The instrument has a 22-position keypad with individual keys assigned to frequently used features. Menus are reserved for less frequently used setup operations. The keypad can be locked out to prevent unintended changes of instrument setup.

Computer Interface

Two computer interfaces are included with the Model 475, serial RS-232C and parallel IEEE-488. Both allow setup of all instrument parameters and read-back of measured values. The maximum reading rate over the interface is nominally 30 readings per second during field control. LabVIEW™ driver for the Model 475 is provided. Application software is not included with the FCP.