How to Overcome the Challenges of Designing a Radiation Tolerant Motion Control System on Chip
These fundamental elements must be realized in designing a versatile motion control system for space applications.
There are many challenges to consider when designing a radiation tolerant motion control system on chip (SoC). The first design challenge is in the motion control system itself. Such space applications often involve conceptualizing a versatile part that can support a wide range of motion control applications. This involves partitioning the circuit into realizable functions that can be implemented within the limitations of the intended power and voltage rating of the IC process that are being used for a particular block.
The next challenge is that the process engineer must find or develop a radiation tolerant process technology that can effectively implement the blocks. The circuit design of the motion control circuit blocks is difficult given the ground differences and level shifting required when implementing some analog functions. And the circuit design task is complicated even more due to the unavailability of certain generally available devices that can’t be used due to their radiation intolerance.
Therefore, the circuit designer must find innovative techniques to work around these limitations. Digital code must be written for the controller to implement the necessary algorithms and be proven on the test bench, and the test engineer must prove functionality of the part over the environmental extremes and under radiation exposure.
In this blog we’ll look at seven major challenges when implementing motion control in space applications.
• System Definition and Specification
• Design Partitioning
• Circuit Design
• Process Development and Characterization
• Function Partitioning and Package Design
• Digital IP Block Development
• Radiation Testing
And using Microchip’s LX7720 radiation hardened motor controller with position sensing IC as a working example, we’ll reveal ways to overcome these challenges.
System Definition and Specification
System definition begins by analyzing the applications and their requirements. In a motion control system, there is typically an electric motor that provides the electric-to-mechanical energy conversion and position feedback to monitor the progress of the movement. Motors are typically three phase and either brushless DC (BLDC) motors or stepper motors and are typically powered from the satellite bus power rail, which can range from 22V to 150V. The motor shaft movement can be monitored using an encoder, Hall-Effect sensors or a resolver. Resolvers are also used to monitor rotation of structures such as antennas. If movement is linear as results from an actuator, a Linear Variable Differential Transformer (LVDT) may be used. Since the position information is often used in a motion control system, an integrated position sensing interface with a high-power switch driver is desirable. For many applications, having external motor driving switches is preferred, to optimize the voltage and current requirements of the motor.
These factors defined the basic requirements for our radiation-hardened-by-design motor control IC, the LX7720.
The first step in partitioning a versatile design approach is to look for the common elements in all the different applications. BLDC motors require some type of switching device to sequence and regulate current to the motor coils. Many applications require a pulse width modulated switch in a half bridge configuration. Stepper motors may use a high side, a low side or a half bridge driver and in cases of a bipolar stepper motor, it requires two full bridge outputs. See figure 1.
Figure 1: Two phase stepper motor configurations.
When designing a half bridge driver, it’s common knowledge that an N channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is typically better performing as a switch than an equivalently sized P channel MOSFET; therefore, a size and cost-efficient system will utilize all NMOS power switches and floating high side drivers. All closed loop motor control algorithms require current sensing since the motor torque is proportional to the coil current. A versatile system design will provide floating current sensing that can be configured as power line (high side) sensing, ground current sensing and motor terminal (in-line) current sensing. Current sensing that is referenced to the switched node of a half-bridge configuration presents the challenge of extracting a tiny current sense voltage from a large common mode voltage signal and extremely fast common mode slew rate.
Position sensors such as resolvers or LVDTs consist of a transformer primary driven by an exciter reference. The transformer secondary must be sampled to extract the position information. A closed loop system known as tracking conversion can compensate for a known latency based on the acceleration, speed and position errors. See figure 2.
Figure 2: Resolver and LVDT
The algorithms used to control a Permanent Magnet Synchronous Motor (PMSM) are much different than what is required to control a stepper motor. A versatile system has programmable logic to adapt to the application. A versatile system with consideration for all the fore mentioned attributes using the LX7720 is shown in figure 3.
Figure 3: Block diagram of a motor control system using the LX7720.
Process Development and Characterization
The motion control electronics can be partitioned into three specific IC process requirements. Since motors in spacecraft can operate from voltage rails up to 150V, a process that can withstand these higher voltages is required.
• Since the MOSFET drivers typically require high currents, a DMOS process is the most effective.
• For the signal processing and logic for the sigma delta modulator, a lower voltage process with higher density and greater bandwidth is needed.
• For the digital companion IC, a very small geometry CMOS process is required.
Processes that are adopted from commercial processes not specifically designed for use under radiation exposure require development of special Process Design Kit (PDK) models that take the effects of radiation exposure into account. This involves exposing devices to radiation and modeling their behavior. When a circuit designer uses these PDK models to design circuitry this considers the anticipated radiation effects and helps make the resulting circuit topologies radiation hardened by design.
Function Partitioning and Package Design
An example of a function that is partitioned between the three different ICs in this system is the floating current sense. The floating current sense uses the wide dynamic range of the high voltage IC to interface to the current sense resistor. There is an initial gain stage implemented in the high voltage process that feeds its output to an instrumentation amplifier implemented in the 5V process. The 5V IC shares the same signal ground with the digital IC. Once level shifted from floating high voltage to a signal ground referenced, the analog signal is sampled using a second order sigma delta modulator implemented in the BiCMOS process. The lower voltage process can implement functions in less space and at higher bandwidth due to the attributes of its smaller geometry. The output of the modulator is a data stream that is voltage compatible with the digital IC. The data stream consumes just one package pin as it is routed between the analog front end (AFE) and the digital IC. See figure 4.
Figure 4: Floating current sense design.
In the digital IC, a specialized IP block performs a sinc3 filter and decimation function that can be done at a speed that could not be supported in the AFE 5V process. This pipeline from sense resistor to the digital control loop takes full advantage of the unique capabilities of each of the ICs it passes through. The high voltage and low voltage analog silicon chips can be co-packaged as a device that appears from the pin out to be a single IC, the LX7720. This co-packaging of chips exploits the advantages of each process.
Digital IP Block Development
The use of a radiation tolerant FPGA or MCU alongside a versatile mixed-signal IC as its companion chip is the essence of our total system solution approach. The digital signal processing of the motor control function can be partitioned into functional blocks to provide the greatest level of IP reuse. Functions can be added or removed to an application depending on what type of control algorithm is needed. Individual blocks can be customized by setting variables. See figure 5. An example of a variable that controls a performance tradeoff is the decimation rate setting in the sinc3 filter IP block. Signals with a higher oversample rate will have higher resolution at the cost tradeoff of longer latency. A CAD design tool such as Microchip’s Libero® SoC allows blocks to be configured and customized.
Figure 5: Decimation Trade-offs on Latency and Resolution
Design flow elements for IP block development:
• Determine block inputs and outputs
• Specify system requirements
• Develop system mathematical model
• Generate HDL code
• Create hierarchal block
• Simulate at top level
• Test using hardware
Radiation tolerance for the LX7720 can be demonstrated by testing for TID to 100 krad total ionizing dose at approximately 50 rad/sec, ELDRS to 50 krad using an enhanced low dose rate of 0.0035 rads/sec and (SEE) single event effects using a fluence of 1 x 108 parts/cm and linear energy transfer of approximately 85 MeV/mg-cm2. Single event latchup (SEL) is measured with the power supply rails adjusted to their maximum voltage levels. Single Event Transients (SET) tests monitor supply rail input currents and regulated output voltages. We also monitor the sensor modulated outputs and driver outputs for glitches and excursions. Cold spared vulnerability to SET is tested with the power rail removed. For Single Event Upset (SEU) we run a scan chain test routine that cyclically monitors the integrity of the latched data. Single Event Functional Interrupts (SEFI) can occur on Power on Reset or UVLO lines that could initiate an erroneous reset.
To learn more, watch our video demonstration of the LX7720 performing motor control with the SAMRH71 microprocessor (MPU). You can also link to more information like datasheets, radiation reports, conference papers and development tool user guides for the LX7720 and other rad hard mixed signal ICs here.