3. Phidget Servo Motor Actuator
- jrenbb
- Alok Batra
- Baseer Khan
Key Concepts
Hardware Setup
In order to run the tutorial, you need to set up the hardware. You can find detailed instructions here.
Write a ThingFacet, Workflow and Device Logic (Message Transformation)
Write a ThingFacet
To abstract the interactions with the phidget servo motor setup in the previous section, we create a PhidgetServoFacet ThingFacet.
As described in writing ThingFacet section there are two kinds of attributes that must be added to a ThingFacet.
- Attributes that store Protocol Specific Parameters
- Attributes that are required to Control a Device (Actuator) OR Receive Data from a Device (Sensor).
For the purpose of this tutorial we have protocol specific parameters as:
- Protocol URL
- Device Interface
- Device Type
We will be controlling the Servo Motor by changing the Angle.
- ServoAngle
<ThingFacet Name="PhidgetServoFacet"> <String Name="ServoProtocolURL" Default="phid://"/> <String Name="DeviceInterfaceIndex"/> <String Name="PhidgetDeviceType"/> <Integer Name="ServoAngle"/> </ThingFacet>
Write a Action with Workflow
The next step in ThingFacet is writing a Action which contains a workflow responsible for making external protocol specific call and store the output in the appropriate ThingFacet attribute. An important decision while writing Action is correct protocol selection. See the /wiki/spaces/TQLDocs/pages/1179871 in the Working with Things section. For Phidget Servo Motor we need the phid:// protocol handler which is part of the TQLEngine.
1. Write an Action name as PhidgetServoAction
<ThingFacet Name="PhidgetServoFacet"> ... <Action Name="PhidgetServoAction" Documentation="Control Servo Motor"> ... </Action> </ThingFacet>
2. Create a Workflow within the Action, with one single continuous running Task and waiting for the Event with its ActionArgument.
<Action Name="PhidgetServoAction">
<Workflow Limit="1" Live="1" Timeout="-1">
<Task name="Main" while="true">
<Event name="Argument" as="ActionArgument"/>
...
</Task>
</Workflow>
</Action>
Here we used three modifiers for this workflow. Limit = "1" means there can be at most one instance of this workflow waiting. Live = "1" means there can be at most one instance of this workflow running. Timeout ="-1" means this workflow will never be timed out. We used a modifier while = "true" with the Task to make the workflow running in a continuous loop, because it needs to run repeatedly, not just once. For more details, refer to workflow modifiers and the lifecycle of a workflow.
The task will be activated by the event handler Event called ActionArgument. "ActionArgument" is the event generated whenever the attribute(s) associated with this Action is modified (See Associate Action with a ThingFacet Attribute). ActionArgument carries all the current values of the ThingFacet attributes, which can be used in the task if needed.
3. Invoke Phid call: Phid supports Get and Post methods. For the Phidget Servo Motor we use method Post to actuate the servo. The parameters required Phid are passed as part of <Message> and <Value>. More details of the phid:// are here in the Working with Things Section.
<Invoke name="ReadValue" waitFor="Argument" post="[%:Event.Argument.ServoProtocolURL.Value:%]"
SerialNumber="[%:Event.Argument.SensorInterfaceIndex.Value:%]"
DeviceType="[%:Event.Argument.PhidgetDeviceType.Value:%]">
<Message>
<Value>
<ServoAngle>
[%:Event.Argument.ServoAngle.Value:%]
</ServoAngle>
</Value>
</Message>
</Invoke>
Here Invoke gets the attribute values that the Event ActionArgument (or Argument) carries. It uses them as the parameter values for the Phid by substitution. For example,
<ServoAngle>[%:Event.Argument.ServoAngle.Value:%]</ServoAngle>
means replacing the value of Message.Value.ServoAngle with the value of Event.Argument.ServoAngle.Value.
Value substitution (rather than assignment) is very common in TQL, given that it is a declarative language. The notation [%: is part of the /wiki/spaces/TEACH/pages/21170773. More details on /wiki/spaces/TEACH/pages/21170773 can be found in the Developer's Guide.
4. Process the message received from the Protocol Handler. It simply returns the Angle that is set on the device.
<Output name="Result" as="ActionResult">
<Value>
<ServoAngle>
[%:[%:@Output:%]Invoke.ReadValue.Message.Value/number():%]
</ServoAngle>
<Log Message="====Phidget Servo Angle ==== [%:[%:@Output:%]Invoke.ReadValue.Message.Value:%]"/>
</Value>
</Output>
Here we used an XPath statement to convert String returned by Phid protocol handler to number. Alternatively, you can use Javascript. An example of using JavaScript for message processing within an Action can be found in another tutorial here.
We used a TP notation "@" in @Outupt. It means substitution should not happen before processing is completed - means should be replaced at the end of the process. More details on /wiki/spaces/TEACH/pages/21170773 can be found in the Developer's Guide.
Associate Action with a ThingFacet Attribute
We will now have to associate a ThingFacet Attribute (ServoAngle) with the Action using the KnownBy modifier. When ServoAngle is "KnownBy" PhidgetServoAction, any ServoAngle value changes will activate the PhidgetServoAction.
<ThingFacet Name="PhidgetServoFacet"> ... <Integer Name="ServoAngle" KnownBy="PhidgetServoAction" /> </ThingFacet>
Combining ThingFacet with a ThingModel
Finally, in order to use ThingFacet we have to combine it with a ThingModel. We define ThingModel to contain only a unique system identifier.
<ThingModel Name="PhidgetServoModel" Combines="PhidgetServoFacet"> <Sid Name="PhidServoId"/> </ThingModel>
PhidgetServoModel ThingModel "combines" with PhidgetServoFacet. PhidgetServoModel will inherit all the attributes from PhidgetServoFacet, in addition to its own attributes. The ThingFacet PhidgetServoFacet hence serves as a reusable component.
More details on the use of "combines" can be found here. Information on Sid cab be found here.
Deploy and test via queries and subscriptions
Export, import and deploy
Once the models are built in a TQLStudio project, you can export the project. URL of the Zip file containing the content of your project will be sent over to your email account. Once you have downloaded the engine – launch the TQLEngine User Interface in the browser and select Import Project. You can copy the URL from Export Project Email and hit import button. Go to ThingSpaces and Deploy the project.
Instantiation
First initialize the device:
<Query>
<DeleteAll>
<PhidgetServoModel>
<PhidServoId ne=""/>
</PhidgetServoModel>
</DeleteAll>
<Save format="version,current">
<PhidgetServoModel>
<ServoProtocolURL>phid://</ServoProtocolURL>
<PhidgetDeviceType>PhidgetAdvancedServo</PhidgetDeviceType>
<DeviceInterfaceIndex>0</DeviceInterfaceIndex>
<ServoAngle>110</ServoAngle>
</PhidgetServoModel>
</Save>
</Query>
Query and subscription
Get Detail Information about PhidgetServoModel
<Query>
<Find format="all">
<PhidgetServoModel>
<PhidServoId ne="" />
</PhidgetServoModel>
</Find>
</Query>
You will see the result from as follows: