Patent Description:
Motor controller circuits frequently include current limiters that are configured to monitor a current in the motor controller and disable or reduce the current when the current exceeds an overcurrent threshold. Such circuits protect the controller, the motor, and the overall system from excessive current that can occur during operation. The specific electrical system for any given application requires a current limit tailored to that specific application, as the amount of current that can be passed through a system without damaging the system varies depending on the specifications of the system itself.

When utilized in spacecraft, and similar applications, every element of the system including the motor controller must undergo a qualifications process to determine if it is capable of operating in conditions present outside of the Earth's atmosphere. Further, any time a component is altered within a system for spacecraft, or similar application, the entire system must be re-qualified. Requalifying a substantially similar motor controller when a current limit requirement is changed can add considerable expense to a project design.

Current limiting circuits are disclosed in <CIT>, <CIT>, <CIT>, <CIT> and <CIT> and in <NPL>.

A motor controller is provided as defined by claim <NUM>.

Also disclosed is a method not forming part of the scope of protection, the method for providing current limiting to a motor controller including: providing a sensed current to a comparator, generating a current threshold using a variable resistance resistor network, and providing the current threshold to the comparator, and comparing the sensed current to the current threshold and outputting a trip signal from the comparator when the sensed current exceeds the current threshold.

Current limiter circuits in motor controllers protect the controller, the motor, and the overall system from excessive current in the event of an overcurrent. Such an overcurrent can arise due to abnormal operations, start-up conditions, failures within the system, or for any similar reason. Each system requires that the particular current limiter circuit be custom tailored to meet the operational parameters and fault protection schemes for that specific system. As a result, traditional current limiter circuits are application specific.

With regard to spacecraft each circuit element is further subjected to a substantial qualifications process to ensure that the element or component is sufficiently operable in extreme conditions, such as those present outside of the Earth's atmosphere. Any change in the design of the system requires the entire system to be re-qualified.

Current limiting circuits for spacecraft, are constructed using analog circuits and circuit elements. Existing analog current limit circuits for digital motor controller designs, however, impose a fixed current limit on the motor controller. The fixed current limit is achieved through the utilization of a fixed voltage divider providing a voltage threshold. Once the circuit is built, and installed, there is no way to alter the current limiting threshold provided by the fixed voltage divider. In certain implementations, however, it is desirable to temporarily allow the current threshold established by the current limiting circuit to be exceeded without entirely disabling current threshold protection.

Spacecraft applications are single or low production applications, meaning that only a single system, or a very limited number of systems, requiring a given current limit circuit will ever be built. As a result of the low production numbers, and the high cost of re-qualifying each component for a new spacecraft, application specific current limit circuits are expensive to produce. However, general current limiter circuits utilizing a digital controller to compare a sensed current value against a threshold, as are known in the art, are unsuitable for some spacecraft applications.

<FIG> schematically illustrates an example programmable current limiting circuit <NUM>, capable of generating a variable current limiting threshold. The current limiting threshold generated by the programmable current limiting circuit <NUM> can be adjusted by a controller during operation without altering or replacing any of the analog current limiter circuit elements.

The programmable current limiting circuit <NUM> includes a comparator <NUM> having a current threshold input <NUM> and a current sense input <NUM>. The comparator <NUM> compares a voltage input received on the current sense input <NUM> against a voltage input received on the current threshold input <NUM> and outputs "high" (a positive voltage) on a comparator output <NUM> when the current sense input <NUM> exceeds the current threshold input <NUM>. The output of the comparator <NUM> is provided to a current trip circuit <NUM> within the motor controller containing the current limit circuit <NUM>. When the output of the comparator <NUM> is high, the current trip circuit <NUM> trips, and an overcurrent condition is prevented. Any known current trip circuit <NUM> can be utilized in conjunction with the system described herein. In the illustrated example, the current sense input <NUM> is a voltage value representative of a sensed current within the motor controller, and the current threshold input <NUM> is a voltage value representative of a current threshold.

Connected to the current threshold input <NUM> of the comparator <NUM> is a resistor network <NUM> configured to provide a voltage to the current threshold input <NUM>. The resistor network <NUM> is connected to a programmable controller <NUM> via multiple pin connections <NUM>. Each of the multiple pin connections <NUM> can be set to output either zero volts or a positive voltage by the programmable controller <NUM>. Each of the multiple pin connections <NUM> is connected to the resistor network <NUM> at different locations within the resistor network <NUM>. The voltage output of the resistor network <NUM> is determined by which of the multiple pin connections <NUM> has a positive voltage at a given time. In this way, the programmable controller <NUM> can set, and alter, the current threshold for the current limiter circuit <NUM> by altering which of the pin connections <NUM> are outputting a positive voltage.

In some examples the programmable controller <NUM> is a Field Programmable Gate Array (FPGA) controller. In alternative examples, any other type of programmable controller qualified for utilization in spacecraft can be utilized to the same effect. In yet further examples, the programmable controller <NUM> is a control element having excess pin outputs within an existing motor controller.

By altering which pin connections <NUM> output a positive voltage, the total voltage seen by the current threshold input <NUM> is controlled by the programmable controller <NUM>, and the current limit threshold can be adjusted. In this way, the controller can adjust the current limit during operations in order to temporarily override or alter the default overcurrent threshold. Utilization of the programmable controller <NUM> further allows a single circuit arrangement, such as the illustrated circuit <NUM>, to be utilized on a common circuit board and included within multiple distinct spacecraft, thereby reducing the costs associated with developing and qualifying a distinct current limiting circuit for each spacecraft.

While illustrated herein as including four pin connections <NUM> for simplicity, it is understood that existing controllers <NUM>, such as FPGA's, can include significantly more available pin connections <NUM>. Each additional pin connection <NUM> between the programmable controller <NUM> and the resistor network <NUM> increases the number of possible voltage thresholds and decreases a voltage difference between each possible threshold and the next possible threshold. Increasing the additional possible voltage thresholds is referred to as increasing the voltage resolution of the overvoltage threshold.

In some examples, such as the illustrated example of <FIG>, the variable voltage set by the programmable controller <NUM> is controlled by a controller <NUM>, distinct from the motor controller. In alternative examples the programmable controller <NUM> is incorporated into the motor controller, and the motor controller can control the outputs of each of the pin connections <NUM>. A voltage source <NUM> is connected to, and powers, the programmable controller <NUM>. Similarly, a voltage source <NUM> is connected at the output of the resistor network <NUM> and operates in conjunction with the voltage provided by the resistor network <NUM> to provide the desired current threshold to the current threshold input <NUM>. In alternative examples, a single voltage source can be connected in place of the illustrated two voltage sources <NUM>, <NUM>.

With continued reference to <FIG>, and with like numerals indicating like elements, <FIG> schematically illustrates a detailed example implementation of the programmable current limiting circuit <NUM> illustrated in <FIG>. In the example of <FIG>, the resistor network <NUM> is an equal value resistance ladder network having multiple resistance sections <NUM>, or rungs. The sections <NUM> are arranged in series with each other. Each section <NUM> of the resistance ladder is connected to a distinct pin connection <NUM>. The sections <NUM> include multiple resistors <NUM>, with each of the resistors <NUM> having the same resistance value as each other of the resistors <NUM> within the resistor network <NUM>, within manufacturing tolerances.

By utilizing approximately identical resistors <NUM> within the resistor network <NUM>, only a single type of resistor needs to undergo the spacecraft qualification process, and all resistors <NUM> utilized within the resistor network <NUM> can be sourced from a single resistor batch. The illustrated resistor network <NUM> includes four sections <NUM>, alternative examples can include any number of sections <NUM> In some examples, the single type of resistor <NUM> can be resistors <NUM> of equivalent values, but sourced from any number of sources. Resistors of the same resistance, within manufacturing tolerances, are referred to herein as equivalent. In alternative examples, all the resistors <NUM> utilized to create the resistor network <NUM> originate from a single resistor batch. In both examples, the resistors <NUM> have approximately identical resistance values, within manufacturing tolerances.

In a practical implementation, each of the pin connections <NUM> can be set to output either <NUM> volts or a positive voltage by the controller <NUM>. In one example, the positive output voltage of the pin connections <NUM> is <NUM> volts. By setting the voltage of any given pin connection <NUM> to a positive voltage, the programmable controller <NUM> provides a voltage to the corresponding section <NUM> of the resistance ladder in the resistor network <NUM>. The voltages then promulgate through the resistor network <NUM> which acts as a voltage divider in conjunction with the connected voltage source <NUM>. The output of the resistor network <NUM> is provided to the current threshold input <NUM>, and sets the overcurrent threshold of the comparator <NUM>.

Further connecting each of the current threshold input <NUM> and the current sense input <NUM> to the comparator <NUM> are a pair of conditioning resistors <NUM>. In some examples, the conditioning resistors <NUM> can be equivalent to the resistors <NUM> in the resistor network <NUM>. In alternative examples, the conditioning resistors <NUM> can be any other resistance, and are not equivalent.

In yet a further example, the above described current limiter is incorporated into a common "current limiter" circuit board. The common current limiter circuit board can be utilized in multiple different spacecraft, without requiring the re-design or re-qualification of the current limiter circuit. By way of example, the resistor network <NUM> and the comparator <NUM> could be incorporated on a single circuit board and connected to an existing programmable controller within a spacecraft. The resolution of the variable resistor network <NUM> is dependent on the number of available pin connections <NUM> from the existing programmable controller, and the existing programmable controller can be programmed to apply any desired voltage threshold within the resistance resolution of the variable resistor network <NUM>.

Claim 1:
A motor controller configured for use in a spacecraft comprising:
a current limiting circuit including a comparator (<NUM>) having a sensed current input (<NUM>), a current threshold input (<NUM>), and an overprotection trip output (<NUM>), and characterized by: the current limiting circuit further including a variable resistance resistor network (<NUM>) having a plurality of sections (<NUM>) of equivalent resistors arranged as a resistor ladder circuit and wherein every resistor in the resistor ladder circuit has identical resistance values within manufacturing tolerance, an output of the variable resistance network being connected to the current threshold input; and
a programmable controller (<NUM>) having a plurality of pin connections (<NUM>), each of said plurality of pin connections connected to a distinct node within said variable resistance resistor network, and each of said plurality of pin connections being operable to provide one of a zero volt output and a positive voltage output, the programmable controller being operable to alter whether each of the plurality of pin connections is providing a zero voltage output or a positive voltage output to adjust a current limit during operation in order to temporarily override and/or alter a default overcurrent threshold.