Get Service Application proxy for specific service application

Hi 

I am working on automating sharepoint Business Intelliegence site content. I am preparing scripts to create the service applications, web applications, site collection, pass phrase contents, etc usingPowershell 2010.

In this case when i supply a passphrase for the secure store service application, i had to give the service application proxy for the SecureStoreService Application. So i  useGet-SPServiceApplicationProxy command to get the available service application proxys and i am copying the proxy for securestoreapplication and executing the below command to supply apassphrase for secure store service application. 

$MAServiceApplicationProxy="a84fafdf-fb2f-4b24-8368-60bdfbe114e3"
Update-SPSecureStoreMasterKey -ServiceApplicationProxy $MAServiceApplicationProxy -PassPhrase password111.

But, whereas in automation i am not supposed to hardcode the service application proxy. It should be automated.

So i thought of storing the Service Application Proxy for secure Store Service Application alone in a variable and using that in the command.It shuld be like below,

$MAServiceApplicationProxy=Get-SPServiceApplicationProxy [??????-i should pass some parameters to get the proxy for SecureStoreService alone]
Update-SPSecureStoreMasterKey -ServiceApplicationProxy $MAServiceApplicationProxy -PassPhrase password111.

Please help me to get the securestoreservice application proxy alone.

Regards

Antony