Motion Control Resources
Getting More For Less - Technosoft
by John Chandler, Technosoft US Inc.
Technosoft Motion Technology Posted 06/18/2007
By replacing hardware with real-time embedded software, DSP technology has significantly reduced the physical complexity of industrial drives over the past decade. Today, DSP technology is now being harnessed to also minimize, or even replace, motor mounted feedback devices. The result of this newer trend is that motion systems are becoming more robust and less expensive. This article examines several alternative feedback schemes for AC servo motors that are made possible by digital DSP drives.
AC Servo: Performance at a price
The AC Servo motor continues to be the work horse of the motion control industry. However, tapping into its performance has traditionally come at the cost of multiple feedback devices and increased wiring. This is because real-time measurement of Position, Velocity and Electrical Angle, theta, is required to fully harness the capabilities of this machine.
In this scheme, three commutation sensors, S1, S2 and S3, provide an absolute measurement of the rotor’s position to within 60 electrical degrees. Commutation sensors are read during startup so that current can be initially applied in a coarse manner at an appropriate angle. As the motor begins to rotate, the relative position count from encoder channels A and B is captured at a commutation sensor edge. This captured value is arbitrary and is used as an offset to align the signal “theta” with the rotor’s absolute electrical angle. Once aligned, theta is then driven incrementally from channels A+B and provides an exacting measurement of electrical angle.
The signal theta is normally used as an input to drive field transformations. Field transformations act like AC to DC and DC to AC conversions so that motor current can be measured, and voltage can be applied, relative to the rotor’s position, or electrical angle. This transformation approach is known as “Field Oriented Control”. The benefit to FOC is that Position, Velocity and Current can be dynamically controlled in a DC manner independent of the rotor’s electrical angle.
Sensing room for improvement
While the use of commutating encoders remains popular, there are some drawbacks to this approach. Specifically, after starting the motor, the commutation signals are normally not used. If these signals are driven differentially, this means that 6 additional wires are present only for the purpose of starting the motor. The additional wiring adds cost and can reduce reliability, especially in a mechanism where feedback cabling is routinely flexed. A second issue is that not all encoders are provided with commutation signals. For this reason, some AC servo motors are fitted with an incremental encoder plus magnetic Hall sensors to sense rotor position. This alternate approach adds additional component count, cost and volume to the motor.
Putting DSPs to work
As the name implies, Digital Signal Processors are uniquely suited to the task of real time control and signal processing. As DSP based drives continue to evolve, designers are finding new ways to harness their capabilities. In the case of AC Servo control, this means that newer software techniques are now being offered to replace the need for Commutation signals, Encoder A+B signals, and in some cases, both.
A+B Encoder Only Start
The trick to eliminating the need for commutation sensors is to find an alternative method for determining electrical angle at start-up. One such approach is to simply lock the motor into a pole position by ramping current to a high level at a fixed angle for a set period of time. In this case the rotor will naturally jump to and settle in on a preset position. While this approach is simple, the drawback is that it is “open-loop” and it must be imperially qualified in each application. Variation in load friction, inertia and starting angle over time and temperature must be taken into consideration. This approach also causes a significant amount of shaft movement during start up, which is unacceptable in some applications.
Linear Hall sensors
An alternative to eliminating the commutation signals S1, S2, and S3 is to eliminate the encoder A+B+I channels. This approach will also save up to 6 wires. Starting the motor with commutation feedback alone is not a problem; but the challenge then becomes how to synthesize the higher resolution feedback required for precise control of Velocity and Position.
One way to accomplish this task is to substitute linear type Hall devices in place of the digital type normally used. If careful attention is paid to the placement of linear Hall sensors within the motor’s magnetic circuit, two or three phase shifted sinusoidal signals can be generated. A DSP can read these analog inputs (i.e., either 2 at 90 degrees, or 3 at 120 degrees) and use interpolation to determine absolute shaft position to a relative high degree of accuracy.
For example, given a 4 pole motor, 2 linear Hall sensors, and x256 interpolation on the DSP, motor shaft position can be calculated to 1:1024, or about 0.3 mechanical degrees. This level of resolution is sufficient for many applications. The use of magnetic Hall sensors in place of optical encoders can offer improved immunity to contamination found in harsh environments. The low frequency nature of linear Hall signals can also simplify cabling and EMC issues. Low frequency feedback permits higher operating speeds that may otherwise be limited by the use of high-resolution incremental encoders.
In some applications, it is even possible to eliminate both sets of feedback signals by further harnessing the processing power of DSPs. Physical feedback devices can be replaced by measuring the terminal quantities of applied voltage and resulting current. In this case, only the motor’s phase leads are connected to the drive. This approach is known as sensorless control.
The trick to sensorless control is to find a mathematical relationship between rotor position, as a variable, and real quantities that can be directly measured.
An estimator is a special class of control filter that repetitively guesses, or estimates the shaft angle based on pre-existing knowledge of the motor’s inductance, resistance, pole count, and magnetic flux distribution. Each time the filter is processed, an estimated value of position is used to calculate what the terminal quantities of applied voltage and measured current should be. The difference between these calculated terminal quantities and those quantities actually measured provides an indication of error in the original guess.
An integrator is then used to drive position error in the guessing process to zero. The integrator operates by adding or subtracting some distance from each new guess, subsequently forcing the calculated terminal quantities to match the measured values. In doing so, the shaft position can be indirectly identified and controlled in real time.
Sensorless lunch is not completely free
In practice, several limitations do exist with this generalized sensorless control approach. Looking at the process as a whole, it functions by observing the influence that the motor’s back emf has upon applied voltage and resulting current. A fundamental limitation here is that when the motor is operating at low speed, insufficient back emf is present to significantly influence these measured values. In other words, position feedback is generally lost at zero speed. For this reason, AC Servo Sensorless control is normally limited to variable speed applications. Only in special cases, where perhaps a motor parameter variation exists as a function of shaft position, can sensorless control be made to operate at zero speed.
A second limitation to this form of sensorless control involves starting the motor. In the absence of hard wired feedback, most starting algorithms involve locking the motor into a pole position and then dragging it up to speed before switching to closed loop control. AC servo motors are generally constructed as low pole count machines. This means that during the dragging process, the load is free to oscillate over a broad mechanical angle and can be difficult to control. If the load friction is too low, or the load inertia is too high, the motor can skip from one pole position to the next when accelerating and eventually stall. This behavior is exactly like that of a step motor dropping pulses when a resonance frequency is encountered.
DSPs ease of implementation
Although the challenges found in sensorless control can be significant, the benefits have been successfully demonstrated in many real world applications. DSPs are at the forefront of expanding the range of practical applications that can be addressed with sensorless control. In competition with conventional ASIC and microprocessor based solutions, DSPs provide the flexibility and increase processing power to tackle computationally demanding applications. The flexibility afforded through software control means that unique solutions can be tailored to specific applications to address the problems of startup and operating range.
Putting it all together
Robust motion system design generally results when key technologies and areas of expertise are combined to form the best system level solution. Special conditions exist in all applications that will, when collectively analyzed, point to the best fit technologies and techniques.
In the case of AC servo motors, an expanding range of feedback possibilities is now offered by DSP based drives. Motion system designers are free to consider a broader range of potential system level solutions as a result. Getting more, for less, is the goal.
Medical applications call for high performance in small spaces. FAULHABER Brushless DC Servomotors use linear Hall feedback to eliminate the volume normally associated with optical encoders. Technosoft’s IBL2403 drive interpolates the analog Hall signals to provide high resolution position feedback.
Sensorless control is ideally suited for applications that require magnetic coupling through an environmental seal. A non-magnetic material can be inserted between the rotor and stator, keeping sensitive electronics away from hazardous environments.