The new Galil Sine drive amplifiers are a welcome addition to the existing DMC-40x0 and DMC-41x3 line-up of servo and stepper amplifiers - yet the addition of the new amplifiers also brings up a question - "When should I use a sinusoidal drive instead of a trapezoidal drive?". This article will go over the Galil brushless servo drive architecture and highlight what you should know when making an amplifier selection.
Two Loop Architecture
In order to gain a better understanding of servo amplifiers and specifically how the Galil servo amplifiers work, the first thing to do is to understand the Controller/Amplifier architecture. Unlike most single axis drives on the market, Galil uses a split sample rate. The first and highest speed sample rate occurs on the amplifier and is used on the current loop. The D3540 Sinusoidal amplifier runs its current loop at 33 kHz and the D3040 Trapezoidal amplifier runs at 66 kHz (which can be increased to 120 kHz for low inductance applications). The benefits of a high speed current loop are:
Fast response to desired current/velocity command signal
Less destabilizing phase shift on the position loop
Tighter more accurate control - 16bit resolution
High Closed Loop Frequency (3-4 kHz)
The second loop in the system is the position loop. Because of the limitations of real world mechanics, a position loop generally has a closed loop frequency in the range of 20 to 200 Hz. The sample rate required to achieve this is only from 1 kHz to 4 kHz. Note that the DMC-4000 can have a sample rate of up to 16 kHz and can control up to 8 axes allowing all axes to be tightly coupled. General motion control applications run optimally at a 1 kHz position loop update. High performance and high resolution applications can be run at higher rates depending on the required performance.
Separate processors for the Amplifier and Controller allow for this two loop Architecture which allows Galil to be extremely responsive and highly accurate and also perform whatever functions are required in a user's application.
Trapezoidal vs. Sinusoidal Commutation
Trapezoidal commutation is the most cost effective way of controlling a brushless servo motor. It is perfect for higher speed applications and applications where the motor and mechanics will eliminate the torque ripple that occurs during switching current from one phase to the next. Hall sensors are required for Trapezoidal commutation.
Sinusoidal commutation is great for lower speed, direct drive or linear motor applications where the torque ripple of the motor phases needs to be minimized. Since the current to the motor phases are weighted as sine waves, the torque going through the motor is smooth and has minimal ripple. It also allows the mechanics to be simplified because Hall sensors can be eliminated.
Sinusoidal amplifiers rely on an initialization sequence at power-up to provide the correct commutation. This can be done in one of 3 ways on the Galil. The first and most common method is the BX command that uses an algorithm that energizes the phases and determines the brushless angle. Only a small amount of motion (if any) is shown with this method. The second method is to use the BC command that requires Hall sensors to be hooked up. It will move the motor and use the first hall transition as the basis for the commutation. This method is necessary if there is an external force on the motor such as a gravity load. The third method uses the BZ command to drive the motor to the zero degree commutation point which can result in a jump to the closest zero phase. For detailed information on getting started with the Galil Sine Drives, please see Application Note #1501
More info on Galil Sinusoidal Amplifiers
The new AMP-43540 drives four brushless motors operating at up to 8 Amps continuous, 15 Amps peak, 20-80 VDC. The gain settings of the amplifier are user-programmable at 0.4, 0.8, and 1.6 Amp/Volt. The amplifier offers protection for over-voltage, under-voltage, over-current, short-circuit and over temperature. A shunt regulator option is available. For more information, please see:
DMC-40x0 Product Page