Pulse control LSIs specialize in motion control, and feature high operational reliability. 

Jerame Chamberlain 
Nippon Pulse

In motion control, acceleration and deceleration are central to the move profiles used to control a motor. Good acceleration and deceleration control ensures quick and accurate positioning, and fast settling without damaging equipment or materials.

For instance, consider a basic tin-can stepper motor tasked with moving a heavy load. If the expectation is that the stepper will be able to generate enough force to move the load at a high speed right out of the gate, it will not be able to perform that movement and will fall out-of-step. This would be akin to trying to start a car’s manual transmission in fifth gear. What happens instead is that car starts in first gear, then slowly works its way up to fifth gear. Similarly, in order to move a heavy load with a small, standard motor, the speed must be ramped up slowly.

Likewise, if a heavy load is moving at a fast speed, inertia prevents it from stopping instantaneously. The load must slow down before coming to a stop from the slower speed. If the deceleration is too quick, it can cause damage to equipment or workpiece due to impact or shock.

There are several types of acceleration/deceleration movements that are commonly used in motion control. Linear acceleration/deceleration is the simplest to generate, but there is a jolt at the beginning and end of any speed changes. Using S-curve acceleration/deceleration is gentler and there is less impact to the load when changing speeds. Linear and S-curve motion profiles can be used in tandem to provide less shock to the load while lessening the demand on the motor. For delicate loads such as semiconductor wafers or electronic chips, S-curve acceleration/deceleration helps mitigate the risk of damage.

What is a pulse control LSI/ASIC chip?
To operate any motor, what’s needed is a device or circuit that produces a speed and direction signal. In many cases, a CPU or FPGA device is used to create movement, because technically these devices can be programmed to generate pulses. However, no serious engineer should consider using these for important motion control applications, unless they are okay with a “good enough” mentality when it comes to their application’s movement.

Another option is to use an LSI (large-scale integration) pulse control IC. A pulse control LSI is a dedicated chip that’s specialized for motor control. Manufacturers have different names for these kinds of controllers – motion control IC, motor control LSI, pulse generator, ASIC chip, etc. – but they are all essentially the same thing. Using a convenient motion control-specific tool like a pulse control LSI makes it easy to design programs.

Choosing a pulse control LSI lets engineers easily write setting data and commands for both linear and S-curve acceleration/deceleration without overloading the CPU. When CPU and engineering resource costs, ease of use, and speed of development are considered, the end user benefits greatly from the specialized motion control option.

Is pulse control with a CPU difficult?
Creating pulse signals only using a CPU has a few drawbacks. For starters, one has to create a program that uses counters or timers to turn the port on and off. It’s easy to control at a constant frequency, but it can be quite troublesome to create a program that features a constantly changing acceleration/deceleration rate. An inexpensive CPU is also easily burdened by processing motion control demands, so it will have a hard time performing other commands simultaneously. This means that a high-performance CPU would be necessary for even basic linear acceleration/deceleration.

As speed increases, the pulse period becomes shorter. There are two general methods of acceleration/deceleration: 1) using a CPU to gradually shorten the pulse period, which is taxing for the CPU, or 2) using the interrupt method of stair-step acceleration/deceleration.

When using a CPU to shorten the pulse period, there are lots of factors to consider: How many pulses should be output in total? At what point does the motor need to start decelerating? If decelerated to the initial speed (FL speed), can it be stopped exactly there? Since the current position cannot be managed by setting only the number of pulses to be outputted, is an absolute value counter also necessary? If the motor and/or controller can be forcibly stopped with a signal from the outside, what should be done with that signal processing?

Developing software for a CPU can also be labor-intensive, and the features and performance of the CPU may determine by how much the pulse cycle can be shortened.

Development of the acceleration/deceleration programs is much easier with a pulse control LSI. There is an easy flow from the time you start writing the program to the completed operation. (See the flow chart below.) Not only can one easily program linear and S-curve acceleration/deceleration, but determining current positioning is easier thanks to the up/down counter values.

Because the CPU is able to leave the LSI unmonitored, the LSI transmits an interrupt signal to the CPU when the operation is completed. An external dedicated terminal can also forcibly stop a pulse control LSI if necessary. The hours required for development are greatly decreased, and high-frequency output pulses are possible regardless of the CPU’s speed because the chips are not burdened by the CPU’s speed or capability limitations. Pulse control LSIs specialize in motion control, so they have high operational reliability.

www.nipponpulse.com