Homing and position management
- Stepper motor and homing
- Sensor and switch
- HOME sensor and LIMIT sensor
- Homing commands
- Normal open and Normal close
- Whether light should enter or be blocked upon the sensor detection
- For rotary tables
Stepper motor and homing
When the system powers up, it doesn't know where the motor is currently positioned. It could be pointing to various directions depending on the timing of the last time the system was shut off.
Also, if the stepper motor receives exceeding external force, the step will slip out of alignment (stall). If this happens, the motor will continue to work with offset between the programmed and its physical position.
Therefore, applications that have position or orientation must use sensors to detect a reference position on startup or periodically. This action is called homing.
Sensor and switch
Photointerrupters are often used as home sensors. On the left, a white piece of plastic attached to the slider blocks the photointerrupter's light-emitting and receiving parts. The right side is an example of a rotary table, where the photo interrupter responds to the black screw.
Other devices such as microswitches, or photoelectric sensors are also used for the sensing.
HOME sensor and LIMIT sensor
Each axis of STEP400/STEP800 has HOME connector which can connect sensors or switches. STEP400 has LIMIT sensor inputs in addition to HOME inputs. 5V is supplied to each connector for the sensing power source.
This input is connected directly to the motor driver chip and can be used in conjunction with the driver's homing function. Usually, this connector is used for the home sensor.
LIMIT (Only in STEP400)
Some applications may require two sensors. For example, a slider has a limited operating range, and if it stalls during the operation, it may collide with one of either end. In such cases, installing sensors on both ends of the slider will prevent collisions.
The motor can be set to force-stop when these sensors respond, but these can also be used as simple switch inputs separated from the motor operation. For example, you can connect a push button to one of them and press to send an OSC message.
Collision prevention setting
You can limit the motor rotate direction when HOME or LIMIT sensors are active.
With the command
/setProhibitMotionOnLimitSwyou can prohibit the actuator to move towards
homingDirectionwhen the HOME sensor is active, or the reverse direction towards
homingDirectionwhen the LIMIT sensor is active. With this, you can prevent mechanism from collision.
homingDirectioncan be set from
/setHomingDirectionor from the configTool. This setting is also used for following
The homing command in the STEP400 system is
/homing. This command consists from two commands,
/releaseSw which are inherited from the Motor Driver Chip PowerSTEP01. Let's look closer to those commands.
First, use this command to move towards the home sensor. The motor will decelerate and then stop when the home sensor reacts (if it has been set up as such).
The position where the motor stops is the origin / home position! However, strictly speaking, the
/goUnitl command does not stop immediately, but stop after deceleration, so it's current position has negative offset from the point where the sensor have actually responded. This command slowly moves in the opposite direction from the current position and stops immediately when the sensor reading is no longer positive.
Both commands can be set to reset the current position to zero on the moment when the sensor responds. ->
See this video for these commands in operation.
It is possible to send above two commands over OSC one after another, the
/homing command executes this sequence in single operation. It will automatically complete the home sequence according to the homing direction and homing speed which are pre-configured from the configTool or over OSC commands.
The time-out duration can be set for each of
/releaseSw commands. The controller will halts the actuator movement as the Time-out, if no change in the sensor reading is detected within this time frame. This is to prevent the moving part to be pushed against other mechanical object endlessly, by giving up the homing sequence and stops at the specified timing.
Normal open and Normal close
Let's determine the "sensor reaction" a little more in detail. The pin assignments of HOME and LIMIT connectors are as follows.
|3||5V Power Output|
Each sensor pin on HOME and LIMIT is pulled up to 3.3V. To connect the switch, connect the GND (#1) and the sensor terminal (#2). When the switch is pressed, it is connected to the GND pin and the voltage drops from 3.3V to 0V. When the voltage changes from HIGH level to LOW level (a.k.a. Falling Edge), the sensor is considered to have responded.
Let's take an photo interrupter EE-SX671A as an example, where the connection is as follows:
|Pin number||Function||Sensor pin|
|3||5V Power Output||+|
Whether light should enter or be blocked upon the sensor detection
This is the part you need to consider carefully before ordering a sensor.
In the case of the left picture, the light enters into the sensor at the home position, but in the picture on the right, the light is blocked at the home position.
There are two types of sensors, one that turns on when light enters and one that turns on when light is interrupted. In the case of the above Omron sensor, the action is toggled by connecting "L" and "+" terminals.
The mechanism and sensor must be combined in such a way that the sensor pin goes from HIGH to LOW at the home position.
For rotary tables
In the example on the picture above left, the response position of the home sensor will differ between clockwise and counterclockwise, depending on the size of the hole. The STEP400 can notify both HIGH to LOW and LOW to HIGH changes of the home sensor by OSC messages. The message also includes the rotation direction, so you can align the home position if you write a conditional sequence for each rotation direction.